Paleontological Research, vol. 9, no. 2, pp. 143–168, June 30, 2005
6 by the Palaeontological Society of Japan
The shell structure of the Recent Patellogastropoda
(Mollusca: Gastropoda)
TAKESHI FUCHIGAMI1 AND TAKENORI SASAKI2
1Department of Earth and Planetary Sciences, Faculty of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
2The University Museum, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (sasaki@um.u-tokyo.ac.jp)
Received March 25, 2005; Revised manuscript accepted March 28, 2005
Abstract. The shell microstructure of 44 species belonging to 19 genera and 5 families of Patellogastropoda was observed by scanning electron microscopy on the basis of material mainly from the Northwest
Pacific. As a result, 17 microstructures of prismatic, crossed, and lamellar structures were recognized. The
comparison among species revealed 20 shell structure groups which are defined by microstructures and shell
layer arrangement. The relations between taxa and shell structural composition indicate that the Recent
patellogastropods generally have distinctive and stable shell structures at the genus level. This high level
of consistency provides a firm basis for the application of shell structural characters to identify fossil patellogastropods. However, the evolutionary process of microstructures and homology across different shell
layers are mostly ambiguous in the absence of robust phylogeny and undoubted positional criteria for
comparison. More studies from phylogenetic, ontogenetic and mineralogical viewpoints should be undertaken to discuss the process of shell structure diversification in patellogastropods.
Key words: Shell microstructure, new definition, taxonomic occurrence, Patellogastropoda, Recent
Introduction
Studies of shell microstructure are particularly important because they provide information for the taxonomic and phylogenetic analysis of molluscs including fossil taxa. Detailed comprehensive works have
been published for various shell-bearing molluscs, especially bivalves (e.g., Taylor et al., 1969, 1973; Taylor,
1973; Carter, 1990a, b), gastropods (Carter and Hall,
1990; Bandel, 1990a), and cephalopods (e.g., Bandel,
1990b; Kulicki, 1996). However, studies of shell structure in gastropods are limited to a relatively small
number of specific taxonomic groups (e.g., Taylor and
Reid, 1990 for Littorinidae; Bandel and Geldmacher,
1996 for Patella; Sasaki, 2001 for Neritoidea; Kiel,
2004 for gastropods from vents/seeps) and their taxonomic coverage is insufficient compared to other major conchiferan molluscs. Therefore, the gastropod
shell structure is not well understood in general overview even at present.
One of the difficulties in systematic investigations of
fossil gastropods is the low preservative potential of
soft parts. Meanwhile, gastropod higher taxa are defined primarily by the anatomy of soft tissue in the
Recent groups. Phylogenetic analyses on Recent gas-
tropods using soft-part characters have repeatedly
corroborated many cases in which macroscopic teleoconch morphology is useless in determining higher
systematic position (Haszprunar, 1988; Ponder and
Lindberg, 1997). Limpets, including patelliform gastropods and also monoplacophorans, are particularly
notorious as a case of multiple convergence. Several
different major taxa are inferred to have evolved
similar-looking limpet-form shells. Among them patellogastropods are one of the outstanding groups
comprising limpet-shaped species exclusively.
Patellogastropods share many apomorphic characters anatomically, and their monophyly is strongly
supported (Ponder and Lindberg, 1997; Sasaki, 1998).
Recent phylogenetic studies revealed that the patellogastropods constitute a well-defined independent clade
(Eogastropoda), being clearly separated from the
rest of the gastropods (Orthogastropoda) (Ponder
and Lindberg, 1997; Sasaki, 1998). When the shell
is perfectly intact, the patellogastropods can be distinguished from other gastropods by (1) a conical
shell with an anteriorly positioned apex (except lepetid Propilidiinae having a posteriorly situated apex:
Lindberg, 1998: 649), (2) a thick horseshoe-shaped
muscle scar of the shell muscle with constricted outline
144
Takeshi Fuchigami and Takenori Sasaki
and a thin scar of the pallial retractor muscle, (3) a
symmetrically little coiled protoconch (Sasaki, 1998),
and (4) a characteristic scar in the apex of the teleoconch after the protoconch is detached (cf. Sasaki,
1998: figure 21g, h). Unless these characters are observable, it is not possible to detect the systematic position of limpets in question based on shell morphology only. In such a case, the shell structure characters
must be useful, if their original structures are well
preserved.
The systematization of patellogastropod shell
structure was first established by MacClintock (1967).
He investigated shell structures of 120 Recent and
fossil patellogastropod species by optical microscopy
and proposed 9 microstructures and 17 shell structure
groups. He also clarified a high degree of consistency
between taxonomic categories and shell structure
groups. Lindberg (1988a) further generalized the significance of shell structural characters in patellogastropod systematics, and this methodology has been
broadly applied to the identification of fossil patellogastropods by Lindberg and Hickman (1986), Lindberg (1988a, b), Lindberg and Marincovich (1988),
Lindberg and Squires (1990), Kase (1994), Kase
and Shigeta (1996), Lindberg and Hedegaard (1996),
and Hedegaard et al. (1997). However, the existing
knowledge on patellogastropod shell structure still
largely depends on the descriptions of MacClintock
(1967) at the level of optical microscopy, although
recent standards in studies on calcified hard tissue
essentially require SEM-level information. Hence,
the reinvestigation of patellogastropod shell structure at finer resolution has been an unquestionable
primary subject in the studies of molluscan shell microstructure. In this study, we attempted to provide
detailed SEM-level descriptions to advance the
knowledge on patellogastropod shell structure.
Materials and methods
The materials used in this study include 44 species
belonging to 19 genera of 5 families. They were collected alive mainly around Japan and some nonJapanese species, especially type species of several
genera, were also included for comparison (Table 1).
Immediately after capture, the soft parts were removed from the shell. The shell of each species was
broken in approximately radial or transverse direction, and pieces of shell fragments were treated with
bleach for 24 hours and etched with 3% acetic acid
for three seconds. After cleaning with an ultrasonic
cleaner, they were coated with platinum – palladium
to a thickness of 400 Å. The three-dimensional shell
microstructures were observed in their radial and
commarginal sections and inner shell surface with
scanning electron microscopes (SEM, HITACHI S2400S in Department of Earth and Planetary Science,
University of Tokyo and HITACHI S-4500 in University Museum, University of Tokyo). All specimens
observed with SEM are registered and preserved in
the Department of the Historical Geology and Paleontology, University Museum, University of Tokyo
(UMUT) (Table 1).
The descriptive terminology used in this study basically follows MacClintock (1967) and Carter et al.
(1990). The shell layers are divided into outer and inner layers by a myostracum which corresponds to the
insertion of muscles on the shell. The position of each
shell layer is indicated by its arrangement relative to
the myostracum (abbreviated as M). In this nomenclature outer shell layers are described as Mþ1, Mþ2,
Mþ3, etc. from the inner to the outer side, and likewise inner layers as M1, M2, M3, etc. from the
outer to the inner side. Each layer reclines at small
angles with the inner surface, and its distribution is
visible as a concentric ring in ventral view (Figure 1).
Some layers with identical microstructure were described separately as ‘‘concentric’’ or ‘‘radial’’ layers
under the criterion of its first-order unit arrangement.
To avoid confusion, the four terms regarding ‘‘shell
structure’’ were used in the following sense in this
study. (1) Microstructure – The morphology of crystal
units and their mode of aggregation. (2) Shell layer –
A sheet-like component consisting of single microstructure. (3) Shell structure – The total composition of
microstructures and shell layers constituting the shell.
(4) Shell structure group – The group of species having
the identical order of shell layer arrangement and
microstructure. In the descriptions the dip angle indicates the angle between growth axis of crystal units
and inner shell surface, unless otherwise mentioned.
The identification of crystal forms of calcium carbonate (aragonite or calcite) using X-ray diffraction
was not carried out in this study. Feigl stain (see Carter and Ambrose, 1989 for details), which can distinguish aragonite from calcite by staining in black, was
preliminarily tested for some species but generally this
method yielded poor results. Most shallow-water species are originally pigmented a dark color on their
inner surfaces, and such a dark background color
hindered a clear identification of a series of tightly
stratified thin shell layers. Therefore, determination of
crystal forms for each microstructure of the respective
species is a subject for future study and was not resolved in this study.
Shell structure of Patellogastropoda
Table 1.
Species
Lottiidae
Niveotectura pallida (Gould)
Niveotectura pallida (Gould)
Yayoiacmea oyamai (Habe)
Patelloida saccharina lanx (Reeve)
Patelloida pygmaea (Dunker)
Patelloida pygmaea (Dunker)
Lottia gigantea Sowerby
Lottia cassis (Eschscholtz)
Lottia sp. cf. borealis (Lindberg)
Lottia dorsuosa (Gould)
Lottia dorsuosa (Gould)
Lottia langfordi (Habe)
Lottia kogamogai Sasaki & Okutani
Lottia tenuisculpta Sasaki & Okutani
Lottia lindbergi Sasaki & Okutani
Lottia lindbergi Sasaki & Okutani
Nipponacmea schrenckii (Lischke)
Nipponacmea gloriosa (Habe)
Nipponacmea boninensis (Asakura & Nishihama)
Nipponacmea concinna (Lischke)
Nipponacmea fuscoviridis (Teramachi)
Nipponacmea radula (Kira)
Nipponacmea radula (Kira)
Nipponacmea nigrans (Kira)
Nipponacmea teramachii (Kira)
Nipponacmea habei Sasaki & Okutani
Tectura emydia (Dall)
Tectura emydia (Dall)
Tectura virginea (Müller)
Tectura virginea (Müller)
Atalacmea fragilis (Sowerby)
Atalacmea fragilis (Sowerby)
‘‘Lottia’’ scabra (Gould)
Discurria insessa (Hinds)
Patellidae
Patella vulgata Linnaeus
Scutellastra flexuosa (Quoy & Gaimard)
Scutellastra flexuosa (Quoy & Gaimard)
Scutellastra optima (Pilsbry)
Nacellidae
Nacella deaurata (Gmelin)
Cellana toreuma (Reeve)
Cellana enneagona (Reeve)
Cellana nigrolineata (Reeve)
Cellana testudinaria (Linnaeus)
Cellana grata (Gould)
Cellana mazatlandica (Sowerby)
Lepetidae
Lepeta caeca pacifica Moskalev
Cryptobranchia kuragiensis (Yokoyama)
Cryptobranchia kuragiensis (Yokoyama)
Limalepeta lima (Dall, 1918)
Sagamilepeta sagamiensis (Kuroda & Habe)
Sagamilepeta sagamiensis (Kuroda & Habe)
Acmaeidae
Acmaea mitra (Rathke)
Pectinodonta rhyssa Dall
Pectinodonta orientalis Schepman
Bathyacmea secunda Okutani, Fujikura & Sasaki
145
Material examined in this study
Locality
Register Number
Shiofukiiwa, Miyako, Iwate, Japan
Benten, Matsumae, Hokkaido, Japan
Banda, Tateyama, Chiba, Japan
Mitsuishi, Manazuru, Kanagawa, Japan
Araihama, Misaki, Kanagawa, Japan
Makurazaki, Kagoshima, Japan
California, USA
Nemuro, Hokkaido, Japan
Nemuro, Hokkaido, Japan
Araihama, Misaki, Kanagawa, Japan
Mitsuishi, Manazuru, Kanagawa, Japan
Mitsuishi, Manazuru, Kanagawa, Japan
Mitsuishi, Manazuru, Kanagawa, Japan
Mitsuishi, Manazuru, Kanagawa, Japan
Nagahama, Kanagawa, Japan
Araihama, Misaki, Kanagawa, Japan
Mitsuishi, Manazuru, Kanagawa, Japan
Mitsuishi, Manazuru, Kanagawa, Japan
Chichijima Island, Ogasawara Islands, Japan
Mitsuishi, Manazuru, Kanagawa, Japan
Mitsuishi, Manazuru, Kanagawa, Japan
Goto Islands, Nagasaki, Japan
Kuwabara, Tokuyama, Yamaguchi, Japan
Mitsuishi, Manazuru, Kanagawa, Japan
Mitsuishi, Manazuru, Kanagawa, Japan
Nojima, Otsuchi, Iwate, Japan
Akkeshi, Hokkaido, Japan
Nemuro, Hokkaido, Japan
Algeciras, South Spain
Off Roscoff, France
Lyle Bay, Wellington, New Zealand
Evans Bay, Wellington, New Zealand
California, USA
California, USA
RM28482, 28483
RM28484
RM28485–28488
RM28489–28492
RM28493
RM28494
RM28495–28496
RM28497–28500
RM28501–28504
RM28505–28506
RM28507
RM28508–28510
RM28511–28512
RM28513–28514
RM28515
RM28516
RM28517–28520
RM28521–28524
RM28525–28526
RM28527–28529
RM28530–28532
RM28533
RM28534–28535
RM28536
RM28537
RM28538–28540
RM28541–28542
RM28543
RM28544
RM28545
RM28546
RM28547–28548
RM28549–28551
RM28552–28553
Ile d’Oleron, Charente Maritime, France
Makurazaki, Kagoshima, Japan
Banda, Tateyama, Chiba, Japan
Yokoatejima Island, Amami Islands, Japan
RM28554–28557
RM28558
RM28559–28561
RM28562–28565
Fuego Island, Argentina
Makurazaki, Kagoshima, Japan
Chichijima Island, Ogasawara Islands, Japan
Araihama, Misaki, Kanagawa, Japan
Iriomote Island, Okinawa, Japan
Araihama, Misaki, Kanagawa, Japan
Ogasawara Islands, Japan
RM28566–28568
RM28569–28570
RM28571–28572
RM28573–28576
RM28577–28578
RM28579–28580
RM28581–28582
Daikokujima Islet, Akkeshi, Hokkaido, Japan
Suehirocho, Wakkanai, Hokkaido, Japan
Asamushi, Aomori, Japan
Off Kushiro, Hokkaido, Japan, 50–100 m deep
Off Hota, Chiba, Japan, ‘‘200–300’’ m deep
Off Kanaya, Chiba, Japan, 160–180 m deep
RM28583–28584
RM28585
RM28586
RM28587–28590
RM28591
RM28592–28593
California, USA
Kii Channel, Japan
Tosa Basin, Japan, 1034–1036 m deep
Izena Hole, Okinawa Trough, 1340 m deep
RM28594
RM28595–28596
RM28597–28600
RM28601–28603
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Takeshi Fuchigami and Takenori Sasaki
Figure 1. Descriptive terminology of shell layers. Layer
distribution is shown in sagittal section (left) and on inner surface (right).
Descriptions
SEM-level observations on 44 species in this study
revealed 17 microstructures of prismatic, laminar and
crossed structures (Figure 2). Their morphological
definition and taxonomic and layer distribution are
described below for each microstructure.
Prismatic structure
Definition.—‘‘Mutually parallel, elongate, adjacent
structural units that do not interdigitate strongly along
their mutual boundaries’’ (Carter, 1990c).
Taxonomic and layer distribution.—Outer layers
of most taxa of Lottiidae, Nacellidae and Acmaeidae
(Figure 2).
Remarks.—Many kinds of prismatic structures have
been proposed in molluscan shell. They are generally
categorized into simple prismatic structure, fibrous
prismatic structure, composite prismatic structure and
spherulitic prismatic structure (Carter, 1990c). In
Patellogastropoda, a variety of prismatic structures
have been described under the names of simple prismatic structure, spherulitic prismatic structure, fibrous
prismatic structure, etc. (MacClintock, 1967; Lindberg, 1998; Carter, 1990c, etc.). They were classified
and redefined as follows.
Simple prismatic structure type-A (SP-A)
(Figures 3A–D, 8A, 17A)
Definition.—First-order prisms ornamented with
sharp-edged straight keels parallel to their growth
axis, elongated in the radial direction; width of prism
almost constant with growth. Second-order units arranged parallel to the first-order units, triangular or
blade-shaped in cross-section, visible as aggregations
of triangular lines on inner shell surface.
Dip angle.—Roughly perpendicular in first-order
units.
Size.—5–20 mm wide in first-order units.
Taxonomic and layer distribution.—Outermost
layer of Cellana (Figure 2).
Remarks.—Complex prismatic structure (cp) in
Groups 12 and 13 of MacClintock (1967) corresponds
to SP-A. According to MacClintock (1967: p. 15),
‘‘cp’’ was defined as ‘‘regularly or irregularly shaped
first-order prisms which, in turn, contain parallel
or fan-shaped aggregates of fibrils or second-order
prisms.’’ Prisms that contain parallel aggregates of fibrils may be compared to this structure or irregular
spherulitic structure type-B (ISP-B) (described below). Although he mentioned differences in ‘‘cp’’ of
groups 12 and 13 (MacClintock, 1967: 57–83), it was
treated as a single structure. However, because there
are discretely distinguishable differences under SEM,
SP-A and ISP-B were separated in this study. Hedegaard et al. (1997) and Lindberg (1998) treated complex (cp) and simple (sp) prismatic structures of MacClintock (1967) as calcitic homogeneous structure, but
these layers consist of prismatic crystal units.
Simple prismatic structure type-B (SP-B)
(Figures 3E–F, 8B, 17B)
Definition.—Single prism of first-order units smooth
without ornamentation, hexangular in cross section,
slightly curved, reclined against inner surface, elongated in radial direction; width of prism almost
constant with growth. Second-order units fibrous, arranged oblique to the axis of the first-order units.
Dip angle.—Ca. 60 degrees in first-order, roughly
perpendicular (30 degrees to growth axis) in secondorder units.
Size.—10–60 mm wide in first-order units, 0.5 mm in
diameter in second-order units.
Taxonomic and layer distribution.—Mþ2 layer of
Tectura virginea (Figure 2).
Remarks.—This structure has never been reported
in patellogastropods in detail. MacClintock (1967) described this structure as ‘‘fibrillar prismatic (fp)’’ in
Mþ2 or Mþ3 layer of some lottiid limpets. Fibrouslike structures in T. virginea and other lottiids, however, can be clearly separated by second-order mor-
Shell structure of Patellogastropoda
147
Figure 2. Layer distribution of microstructures in the species examined in this study. The type species of genera are denoted by asterisks. Bar (-) indicates the absence of shell layer. Abbreviations: CCCL, cone complex crossed lamellar structure; CP-ISP, prismatic
structure with minor irregular spherulitic prismatic structure; FP-ISP, fibrous prismatic structure with minor irregular spherulitic prismatic
structure; ICCF, irregular complex crossed foliated structure; ICCL, irregular complex crossed lamellar structure; ICF, irregular crossed
foliated structure; IFF, irregular fibrous foliated structure; ISP-A, irregular spherulitic prismatic structure type-A; ISP-B, irregular spherulitic prismatic structure type-B; NCP, Niveotectura-type composite prismatic structure; RFF, regular fibrous foliated structure; SF, semifoliated structure; SP-A, simple prismatic structure type-A; SP-B, simple prismatic structure type-B; cCF, concentric crossed foliated
structure; cCL, concentric crossed lamellar structure; cRF, concentric regularly foliated structure; rCF, radial crossed foliated structure;
rCL, radial crossed lamellar structure.
phology. Thus, MacClintock’s ‘‘fibrillar prismatic’’ is
divided into SP-B, fibrous prismatic structure with
minor irregular spherulitic prismatic structure (FPISP) and composite prismatic structure with minor irregular spherulitic prismatic structure (CP-ISP) (described blow).
Fibrous prismatic structure with minor irregular
spherulitic prismatic structure (FP-ISP)
(Figures 4A–D, 8C)
Definition.—First-order prisms fibrous, much longer
than wide, widened and branched mainly near their
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Takeshi Fuchigami and Takenori Sasaki
Figure 3. Scanning electron micrographs of prismatic structures (1). A–D. Simple prismatic structure type-A (SP-A). A. Oblique
view of radial section. The left top of the figure is the apical side of the shell. B. Cross section of first-order unit. C–D. Ventral view on
inner shell surface. E–F. Simple prismatic structure type-B (SP-B). E. Radial section. F. Dorsal view of hexagonal cross section of firstorder prisms. A, C. Cellana toreuma (A. UMUT RM28569; C. UMUT RM28570). B. Cellana enneagona (UMUT RM28571). D. Cellana
nigrolineata (UMUT RM28573). E–F. Tectura virginea (UMUT RM28545).
origin, keeping their width almost constant along their
length; surfaces of well-matured prisms ornamented
with oblique lines, but immature ones smooth; growth
axes of these fibrils slightly curved, arranged in radial
direction; cross sections irregular. Unlike simple prismatic structures, second-order units absent.
Dip angle.—80 degrees.
Size.—1–2 mm wide.
Taxonomic and layer distribution.—Yayoiacmea
oyamai (Mþ2 layer) (Figure 2).
Remarks.—This layer is similar to composite
prismatic structure with minor irregular spherulitic
Shell structure of Patellogastropoda
149
Figure 4. Scanning electron micrographs of prismatic structures (2). A–D. Fibrous prismatic structure with minor irregular spherulitic prismatic structure (FP-ISP). A. Radial section exhibiting oblique sculpture on prisms. The left top of the figure shows apical side of
the shell. B. Commarginal section near shell margin. C. Enlarged view of Figure 4B, D. Oblique view of cross section near shell margin. A–
D. Yayoiacmea oyamai (A. UMUT RM28485; B–C. UMUT RM28486; D. UMUT28488).
prismatic structure (CP-ISP) (see below). However,
prisms of FP-ISP are narrower, dip at larger degrees,
branch more frequently, and their surface has different ornamentation.
Irregular spherulitic prismatic structure type-A
(ISP-A)
(Figures 5A–D, 8D, 17C)
Definition.—First-order prisms slightly curved,
oriented in the radial direction, demarcated with
somewhat obscure boundaries, widened with crystal
growth, composed of very small blade-shaped secondorder units arranged in a radiating form.
Dip angle.—Roughly perpendicular (ca. 80 degrees
with layer boundary) in first-order units.
Size.—5 mm in diameter in first-order units, 0.2 mm
wide in second-order units.
Taxonomic and layer distribution.—Outermost
layer of most species of Lottiinae, Nacella, Cryptobranchia and Acmaeidae (Figure 2).
Remarks.—‘‘Simple prismatic structure’’ of MacClintock (1967) in Discurria, Nacella, Acmaea and
Cryptobranchia should be identified as irregular
spherulitic prismatic structure because second-order
units are diverging from their origin.
Irregular spherulitic prismatic structure type-B
(ISP-B)
(Figures 5E–F, 9A, 17D)
Definition.—First-order units slightly curved, variable from pillow-like to fan-shaped in cross section,
ornamented with longitudinal creases, oriented in the
radial direction, gradually increasing their width at
initial growth stage, later maintaining nearly constant
width toward growth surface. Second-order units fibrous, diverse from axis of first-order units.
Dip angle.—60–80 degrees in first-order units.
Size.—5–20 mm wide in first-order units, 0.5 mm in
diameter in second-order units.
Taxonomic and layer distribution.—Mþ2 layer of
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Takeshi Fuchigami and Takenori Sasaki
Figure 5. Scanning electron micrographs of prismatic structures (3). A–D. Irregular spherulitic prismatic structure type-A (ISP-A).
A. Radial section. Left top of the figure is apical side of the shell. B. Commarginal section near shell margin. C. Commarginal section seen
obliquely from ventral side. D. First-order units on growing surface. E–F. Irregular spherulitic prismatic structure type-B (ISP-B). E.
Commarginal section. F. Ventral view on inner shell surface. The right side of the figure shows the shell margin. A–B. Nipponacmea gloriosa (A. UMUT RM28524; B. UMUT RM28523). C. Lottia dorsuosa (UMUT RM28506). D. Nipponacmea schrenckii (UMUT
RM28520). E. Patelloida saccharina lanx (UMUT RM28492). F. Patelloida pygmaea (UMUT RM28494).
Patelloida (Figure 2).
Remarks.—Complex prismatic structure (cp) in
Group 2 of MacClintock (1967) is redefined as ISP-B,
since second-order units are diverging from their origin. This microstructure resembles composite pris-
matic structure with minor irregular spherulitic prismatic structure (CP-ISP) in the presence of creases on
the surface of prisms. But the first-order units are
much larger and the second-order units are more
clearly visible in this microstructure.
Shell structure of Patellogastropoda
151
Figure 6. Scanning electron micrographs of prismatic structures (4). A–F. Composite prismatic structure with minor irregular
spherulitic prismatic structure (CP-ISP). A–D. Commarginal section. Bottoms of figures show ventral sides of the shells. Arrows indicate
the direction of crystal growth. E. Commarginal sections seen slightly from ventral side. F. Ventral view on inner shell surface. A, D. Lottia
sp. cf. borealis (A. UMUT RM28503; D. UMUT RM28501). B. Nipponacmea habei (UMUT RM28539). C. Nipponacmea concinna
(UMUT RM28527). E. Nipponacmea gloriosa (UMUT RM28522). F. Nipponacmea nigrans (UMUT RM28536).
Composite prismatic structure with minor irregular
spherulitic prismatic structure (CP-ISP)
(Figures 6A–F, 9B, 17E)
Definition.—First-order prisms fibrous, ornamented
with divaricating creases, much longer than wide,
widened and branched only near their origin, keeping
their width almost constant along their length; growth
axes of these fibrils straight in radial direction; their
cross sections diamond- to ginkgo-leaf-shaped. Second-order units absent unlike simple prismatic structures.
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Takeshi Fuchigami and Takenori Sasaki
Figure 7. Scanning electron micrographs of prismatic structures (5). A–D. Niveotectura-type composite prismatic structure (NCP).
A–B. Commarginal section. C. Radial section. This structure resembles concentric crossed lamellar structure (cCL) in radial section. D.
Ventral view on inner shell surface. A–D. Niveotectura pallida (A. UMUT RM28483; B. UMUT RM28484; C–D. UMUT RM28482)
Dip angle.—30–45 degrees in first-order units.
Size.—2–4 mm wide in first-order units.
Taxonomic and layer distribution.—Mþ2 or Mþ3
layer of most taxa of Lottiinae (Figure 2).
Remarks.—‘‘Fibrillar structure’’ of MacClintock
(1967) was revealed to include simple prismatic structure type-B (SP-B), fibrous prismatic structure with
minor irregular spherulitic prismatic structure (FPISP) and this structure (CP-ISP) in this study.
Niveotectura-type composite prismatic structure
(NCP)
(Figures 7A–D, 9B, 17F)
Definition.—First-order prisms of variable size,
roughly rectangular, composed of a vertical stack of
divaricating platy second-order units; their growth
axes straight; boundaries with adjacent units interdigitated in ventral view but not in vertical view; their
width constant with growth. Second-order units ori-
entated in radial direction, appearing as parallel radial
lines on the inner surface. Third-order units fibrous,
arranged in parallel.
Dip angle.—Vertical in first-order units, ca. 60 degrees in second-order units.
Size.—50–100 mm wide in first-order units, ca. 2 mm
wide in third-order units.
Taxonomic and layer distribution.—Mþ2 layer of
Niveotectura pallida (Figure 2).
Remarks.—This structure has never been described
in detail. MacClintock (1967) identified this layer as
‘‘fibrillar prismatic’’ and ‘‘simple prismatic’’ layers in
Acmaea pallida (Niveotectura pallida in the current
systematics). In radial section, this layer can be confused with concentric crossed lamellar structure (cCL)
(Figure 7C), but the width of first-order units are
variable unlike those of cCL. This microstructure is
similar to ‘‘irregular crossed lamellar structure’’ of
Carter (1990c) in second-order unit morphology.
However, it should be grouped as prismatic structure,
Shell structure of Patellogastropoda
153
Figure 8. Schematic illustrations of prismatic structures (1). A. Simple prismatic structure type-A (SP-A). B. Simple prismatic
structure type-B (SP-B). C. Fibrous prismatic structure with minor irregular spherulitic prismatic structure (FP-ISP). D. Irregular spherulitic prismatic structure type-A (ISP-A). a. Inner surface. b. Commarginal vertical section. c. Radial section. d. Oblique three-dimensional
view. White and gray arrows indicate the directions of crystal growth and accretionary shell growth, respectively.
because first-order prisms are mutually parallel and
have clear boundaries.
and Acmaeidae, and a few species of Lottiidae (Figure
2).
Laminar structure
Regularly foliated structure (RF)
Definition.—‘‘Rods, laths, blades or tablets comprise sheets which are oriented parallel or nearly parallel to the depositional surface’’ (Carter, 1990c).
Taxonomic and layer distribution.—Mainly outer
layers, especially Mþ2 layer, of Nacellidae, Lepetidae
(Figures 10A–D, 12A, 17G)
Definition.—First-order units uniformly thin and
flat sheets formed at equal thickness; their growth
front nearly straight, finely serrated; their surface
almost smooth occasionally with faint striation.
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Takeshi Fuchigami and Takenori Sasaki
Figure 9. Schematic illustrations of prismatic structures (2). A. Irregular spherulitic prismatic structure type-B (ISP-B). B. Composite prismatic structure with minor irregular spherulitic prismatic structure (CP-ISP). C. Niveotectura-type composite prismatic structure
(NCP). a. Inner surface. b. Commarginal vertical section. c. Radial section. d. Oblique three-dimensional view. White and gray arrows
indicate the directions of crystal growth and accretionary shell growth, respectively.
Second-order units blade-like, extremely thin, mutually parallel.
Dip angle.—Smaller than 10 degrees in first-order
units.
Size.—0.5–1 mm thick in first-order units, 0.5 mm
wide in second-order units.
Taxonomic and layer distribution.—Mþ1 or Mþ2
layer of Nacellidae and Acmaea mitra (Figure 2).
Remarks.—The second-order blades grow commarginally from posterior to anterior midline. (Figure
10B). Their arrangement is slightly variable individually and not completely symmetrical relative to sagittal
axis.
Semi-foliated structure (SF)
(Figures 10E–H, 12B, 17H)
Definition.—First-order units sheet-like, not composed of elongate blades but flakes defined by several
growth fronts, sculptured by rough growth lines, waved
Shell structure of Patellogastropoda
with irregular thickness; their boundaries roughly
parallel, wavy in radial and commarginal sections; unfilled holes often visible on growing plane in ventral
view. Growth fronts of second-order units forming
angles of 45, 90 or 135 degrees.
Dip angle.—Roughly parallel in first-order units.
Size.—1–2 mm thick in first-order units of most
species, but 10–15 mm thick in Bathyacmaea secunda.
Taxonomic and layer distribution.—Mþ2 layer of
Lepetidae, and Mþ2 or M1, Mþ1 and Mþ3 layers of
part of Acmaeidae (Figure 2).
Remarks.—MacClintock (1967) and Lindberg and
Hedegaard (1996) treated this structure as foliated
or regularly foliated structure, but it was clearly distinguished from regularly foliated structure (RF) in
thickness, surface sculpture and growth directions of
second-order units. This structure resembles semifoliated structure of Carter (1990c) reported in
oysters, brachiopods, bryozoans and corals. In Patellogastropoda, this structure is distributed only in
subtidal and bathyal species. Its occurrence may be
related to deep environments.
Regular fibrous foliated structure (RFF)
(Figures 11A–B, 12C, 17I)
Definition.—Blade-shaped units elongated invariably in radial direction with constant size; their
surfaces smooth; cross sections roughly rectangular.
Dip angle.—20–30 degrees.
Size.—1 mm wide and 0.5 mm thick.
Taxonomic and layer distribution.—Mþ2 layer of
Lottia langfordi (Figure 2).
Remarks.—This structure resembles lath-type fibrous prismatic structure of Carter (1990c). However,
it should be included in laminar structure, because the
morphology of the growing front of crystal units is of
foliated type, and their dip angles are small.
Irregular fibrous foliated structure (IFF)
(Figures 11C–D, 12D, 13)
Definition.—Blade-shaped units gradually changing
their growth orientations from radial to commarginal
on outer to inner side, or from margin to center in
ventral view, keeping equal size with growth; their
surfaces smooth.
Dip angle.—Very small.
Size.—0.5–2 mm thick.
Taxonomic and layer distribution.—Mþ2 layers of
‘‘Lottia’’ scabra (Figure 2).
Remarks.—Modified foliated structure (mf) of
MacClintock (1967) was recognized as irregular fi-
155
brous foliated structure (IFF) and irregular complex
crossed foliated structure (ICCF). IFF was termed
in connection with regular fibrous foliated structure
(RFF), because these structures have the identical
crystal units of 1–2 mm wide.
Crossed structure
Definition.—‘‘Microstructures showing two or more
clearly non-horizontal dip directions of their elongate
structural units relative to the depositional surface’’
(Carter, 1990c).
Taxonomic and layer distribution.—Various layers
of all species examined.
Crossed lamellar structure (CL)
(Figures 14A, 16A)
Definition.—Thick first-order lamella arranged
in radial or commarginal directions. Second-order lamellae sheet-like, alternating at equal angle to growth
surfaces, but in opposite directions. Third-order lamellae arranged parallel to one another.
Dip angle.—Roughly vertical in first-order units,
20–40 degrees in second-order units.
Size.—5–20 mm thick in first-order units, 0.5 mm
wide in third-order units.
Taxonomic and layer distribution.—Mainly M1
and/or Mþ1 layers of all species examined except Nacella deaurata (Figure 2).
Remarks.—This structure is very common in the
shells of patellogastropods. In the inner layers, this
structure often has pseudolayers that are insertions of
thin prismatic layers parallel to the inner surface (see
MacClintock, 1967; p. 52). This structure can often be
confused with cone complex crossed lamellar structure
(CCCL) in that some first-order lamellae are occasionally wavy with variable thickness on the inner
surface. However, adjacent first-order lamellae of CL
do not interdigitate, unlike CCCL.
Cone complex crossed lamellar structure (CCCL)
(Figures 14B–D, 16B 17J)
Definition.—First-order lamellae irregularly columnar, vertically oriented, strongly interdigitating along
lateral boundary, appearing as radial circles in ventral
view. Second-order lamella cone-like or conical spiral
structure; apical angle of cone ranging from 30 to 45
degrees. Third-order lamellae spicular plates radiating
from cone apex.
Dip angle.—Generally vertical in first-order units.
Size.—80–100 mm in diameter in first-order units,
1 mm thick in second-order units.
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Takeshi Fuchigami and Takenori Sasaki
Shell structure of Patellogastropoda
157
Figure 11. Scanning electron micrographs of laminar structures (2). A–B. Regular fibrous foliated structure (RFF). A. Inner shell
surface. B. Commarginal section. C–D. Irregular fibrous foliated structure (IFF). Inner shell surface. A. Lottia langfordi (UMUT
RM28508). B–D. ‘‘Lottia’’ scabra (UMUT RM28550).
Taxonomic and layer distribution.—M2 layer of
Cellana mazatlandica (Figure 2).
Remarks.—Following Carter (1990c), complex
crossed lamellar structure of MacClintock (1967) was
renamed cone complex crossed lamellar structure
(CCCL).
Irregular complex crossed lamellar structure (ICCL)
(Figures 14E–H, 16C, 17K)
Definition.—As in irregular complex crossed foliated structure (ICCF), patches of first-order units
arranged irregularly, appearing as non-parallel lines
on any vertical section. Second-order units parallelarranged lamellae; ones in adjacent first-order units
having different growth directions. Third-order lamellae thin, arranged in parallel.
Dip angle.—Over 45 degrees in second-order units.
Size.—0.5 mm thick and 5–10 mm wide in secondorder units, ca. 1 mm wide in third-order units.
Taxonomic and layer distribution.—Outermost
layer of Lepetidae excluding Cryptobranchia (Figure
2).
Remarks.—This structure was newly described in
Patellogastropoda.
U Figure 10. Scanning electron micrographs of laminar structures (1). A–D. Regularly foliated structure (RF). A. Vertical section. B–
D. Inner shell surface. B. Folia facing from left and right sides near anterior midline of the shell. Arrows indicate the direction of crystal
growth. C–D. Enlarged view of serrated growing edge of folia. E–H. Semi-foliated structure (SF). E–F. Vertical section. G–H. Inner shell
surface with rough growth lines and polygonal growth fronts. A. Cellana testudinaria (UMUT RM28578). B. Cellana nigrolineata (UMUT
RM28573). C. Cellana toreuma (UMUT RM28570). D. Acmaea mitra (UMUT RM28594). E. Lepeta caeca pacifica (UMUT RM28583). F.
Pectinodonta orientalis (UMUT RM28597). G. Limalepeta lima (RM28588). H. Bathyacmaea secunda (UMUT RM28601).
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Takeshi Fuchigami and Takenori Sasaki
Figure 12. Schematic illustrations of laminar structures. A. Regularly foliated structure (RF). B. Semi-foliated structure (SF). C.
Regular fibrous foliated structure (RFF). D. Irregular fibrous foliated structure (IFF). a. Inner surface. b. Commarginal vertical section. c.
Radial section. d. Oblique three-dimensional view. White and gray arrows indicate the directions of crystal growth and accretionary shell
growth, respectively.
Figure 13. Diagram showing the distribution of irregular
fibrous foliated structure (IFF). White arrows indicate the
growth directions of crystal units. A. Three-dimensional view in
the whole shell. Shell layers except for IFF layer are omitted. B.
The shift in growth directions of crystal units from outer to inner
side. Their change is actually gradual.
V Figure 14. Scanning electron micrographs of crossed structures (1). A. Crossed lamellar structure (CL). Vertical cross section perpendicular to first-order lamellae. B–D. Cone complex
crossed lamellar structure (CCCL). B. Oblique section. C. Inner
shell surface with part of cone-like second-order lamellae removed. D. Intact inner shell surface with third-order lamellae
radiated in a concentric form. E–H. Irregular complex crossed
lamellar structure (ICCL). E. Vertical section. F–H. Inner shell
surface showing patchy arrangement of crystal aggregation. A.
Patelloida saccharina lanx (UMUT RM28489). B–D. Cellana
mazatlandica (UMUT28582). E, F, H. Lepeta caeca pacifica (E.
UMUT RM28583; F, H. UMUT RM28584). G. Sagamilepeta sagamiensis (RM 28591).
Shell structure of Patellogastropoda
159
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Takeshi Fuchigami and Takenori Sasaki
Figure 15. Scanning electron micrographs of crossed structures (2). A–B. Crossed foliated structure (CF). Inner shell surface. A.
Three adjacent first-order units. Arrows indicate alternating direction of crystal growth. B. Enlarged view of second-order units arranged
in the same direction. C–D. Irregular crossed foliated structure (ICF). C. Radial section and inner surface. This structure resembles cone
complex crossed lamellar structure (CCCL) in radial section. D. Commarginal section showing mutually parallel folia. E–F. Irregular
complex crossed foliated structure (ICCF). Inner shell surface. A–B. Patella vulgata (UMUT RM28555). C. Pectinodonta rhyssa (UMUT
RM28595). D. Pectinodonta orientalis (UMUT RM28598). E–F. ‘‘Lottia’’ scabra (UMUT RM28551).
Crossed foliated structure (CF)
(Figures 15A–B, 16D, 17L)
Definition.—First-order units arranged in radial or
commarginal directions. Second-order units regularly
foliated dipping at equal angle to growth surfaces but
in opposite directions in adjacent first-order units.
Third-order units thin, lamellae, margined with sharp
edges, arranged parallel.
Dip angle.—Ca. 10 degrees in second-order units.
Shell structure of Patellogastropoda
Size.—Ca. 40 mm thick in first-order units, 0.3 mm
thick in second-order units, 1–2 mm wide in thirdorder units.
Taxonomic and layer distribution.—M2, Mþ2 and
Mþ3 layers of Patella (Figure 2).
Remarks.—This structure resembles crossed lamellar structure (CL); but the second-order units of
this structure have a smaller dip angle and the thirdorder units are wider than those of CL.
Irregular crossed foliated structure (ICF)
(Figures 15C–D, 16E)
Definition.—First-order units, patch-like, irregular
in shape. Second-order structures arranged in radial
directions; units in adjacent first-order units having
opposite dip directions. Third-order units thin, bladelike.
Dip angle.—Ca. 10–20 degrees in second-order
units.
Size.—Ca. 40–60 mm wide in first-order units, 1 mm
thick in second-order units.
Taxonomic and layer distribution.—M1 layer of
Pectinodonta (Figure 2).
Remarks.—This structure was newly described in
patellogastropods. Its name was given following the
scheme of irregular crossed lamellar structure (ICL)
of Carter (1990c). This structure resembles composite prismatic structure (CP), but the first-order structure does not take a form of a prism. It is also similar
to radial crossed lamellar structure (rCL) or cone
complex crossed lamellar structure (CCCL) in radial
section, but the arrangement of crystal units in ICF is
parallel in commarginal section, unlike rCL and
CCCL.
Irregular complex crossed foliated structure (ICCF)
(Figures 15E–F, 16F)
Definition.—Unlike CF, the first-order structures
patch-like, irregular in shape. Second-order units regularly foliated, arranged in parallel, having different
growth directions in each first-order unit like ICCL.
Third-order units thin, blade-like.
Dip angle.—Ca. 10 degrees in second-order units.
Size.—Ca. 0.5 mm thick and 2–7 mm wide in thirdorder units.
Taxonomic and layer distribution.—Mþ2 layer of
Scutellastra and M1 layer of Nacella (Figure 2).
Remarks.—Crossed foliated structure with irregular
crystal unit morphology and growth direction is redefined as irregular complex crossed foliated structure
(ICCF). ‘‘Irregular’’ implies a random form of the
161
first-order units, and ‘‘complex’’ means the arrangement of folia is not unidirectional. Irregularly foliated
structure (ifo) of MacClintock (1967) in Nacella is
identified as the same structure.
Discussion
Shell layers
The number of shell layers is variable among the
patellogastropods. Most species have five layers, viz.
three outer layers, myostracum and single inner layer
(Figure 2), whereas Niveotectura and Patelloida, Patella, Nacella and ‘‘Lottia’’ scabra have only four layers, viz. two outer layers, myostracum and single inner
layer. Only limited taxa (Bathyacmaea and Atalacmea) have six layers, viz. four outer layers, myostracum and single inner layer or three outer layers,
myostracum and two inner layers.
There is no doubt about the homology of the myostracum across the entire taxa of patellogastropods.
It is formed at the insertions of homologous shell
muscles (including pedal and pallial retractor muscles)
on the interior shell surface. Therefore, it is regarded
as an unmistakable landmark among shell layers.
There are some tendencies in the distribution of
shell microstructures throughout patellogastropods.
Most typically, prismatic layers are always deposited
on the outer layer, especially on the outermost layer.
It is also a common characteristic that many species
share paired crossed lamellar layers (CL) across the
myostracum. These layers may be viewed as homologues in congruent position.
In some cases, homology of shell layers may be
suggested by comparable order of layer arrangement,
even though the total number of shell layers is different. For example, Atalacmea fragilis has an identical
layer arrangement with other Lottiinae, except only
for M+1 having radial crossed lamellar structure
(rCL) (Figure 2). Therefore, it is considered that the
shell structures of A. fragilis and other Lottiinae have
derived from a common pattern through insertion or
deletion of a single layer. In this case, Mþi layer of
other Lottiinae should be homologous to Mþ(iþ1)
layer of A. fragilis. In the Lepetidae, the comparison
among the species examined suggests that the outermost layer of irregular spherulitic prismatic structure
type-A (ISP-A) of Cryptobranchia kuragiensis may
correspond to that of irregular complex crossed lamellar structure (ICCL) in other species of the Lepetidae (Figure 2).
In the remaining shell layers, homology is difficult to
establish under positional criteria. More accurate relations among different shell layers may be suggested
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Takeshi Fuchigami and Takenori Sasaki
Figure 16. Schematic illustrations of crossed structures. A. Crossed lamellar structure (CL). B. Cone complex crossed lamellar
structure (CCCL). C. Irregular complex crossed lamellar structure (ICCL). D. Crossed foliated structure (CF). E. Irregular crossed foliated structure (ICF). F. Irregular complex crossed foliated structure (ICCF). a. Inner surface. b. Commarginal vertical section. c. Radial
section. d. Oblique three-dimensional view. White and gray arrows indicate the directions of crystal growth and accretionary shell growth,
respectively.
Shell structure of Patellogastropoda
163
Figure 17. Isolated crystal units of various microstructures. A. Simple prismatic structure type-A (SP-A). B. Simple prismatic structure type-B (SP-B) C. Irregular spherulitic prismatic structure type-A (ISP-A). D. Irregular spherulitic prismatic structure type-B (ISP-B)
E. Composite prismatic structure with minor irregular spherulitic prismatic structure (CP-ISP). F. Niveotectura-type composite prismatic
structure (NCP). G. Regularly foliated structure (RF). H. Semi-foliated structure (SF). I. Regular fibrous foliated structure (RFF). J. Cone
complex crossed lamellar structure (CCCL). K. Irregular complex crossed lamellar structure (ICCL). L. Crossed foliated structure (CF).
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Takeshi Fuchigami and Takenori Sasaki
Shell structure of Patellogastropoda
by the sequence of shell layer appearance during ontogeny from larval to adult shells and also by the distinction between two crystal forms of calcium carbonates (aragonite and calcite) in each shell layer.
Taxonomic distribution of microstructures
The microstructures described above are not universally distributed throughout the Patellogastropoda.
The comparison in taxonomic order (Figure 2) reveals
that most microstructures tend to be restricted to particular taxonomic groups at genus to family levels.
(1) Prismatic structures: In Patellogastropoda, a
majority of species in Lottiidae, Nacellidae and Acmaeidae have the prismatic structure in their outer
shell layers. Simple prismatic structure type-A (SP-A)
and Irregular spherulitic prismatic structure type-B
(ISP-B) characteristically occur in Cellana and Patelloida, respectively. Fibrous prismatic structure typeA (FP-A) is common to most taxa of Lottiinae. Irregular spherulitic prismatic structure type-A (ISP-A) is
widely shared by Lottiinae and Acmaeidae in the
outermost layer.
Simple prismatic structure type-B (SP-B), fibrous
prismatic structure with minor irregular spherulitic
prismatic structure (FP-ISP) and two types of composite prismatic structure (CP-ISP, NCP) are specific
to part of patellogastropods among gastropods. Although ‘‘fibrous prismatic structure’’ is commonly
found in bivalves (Carter, 1990a, b), their surface
sculpture of first-order units is strikingly different
from that of patellogastropods. ‘‘Composite prismatic
structure’’ also has been documented in other gastropods or bivalves, but they are identified as ‘‘denticular
or non-denticular composite prismatic structure’’
(Carter, 1990c) unlike those in patellogastropods.
(2) Laminar structures: Nacreous structure is distributed in the Vetigastropoda and Cephalopoda as
columnar nacreous structure and in Bivalvia as sheet
nacreous structure (Carter, 1990c; Hedegaard, 1997).
All patellogastropod taxa lack this structure. On the
other hand, Nacellidae, Acmaeidae, Lepetidae and a
few species of Lottiidae have four kinds of foliated
structures (RF, SF, RFF and IFF). These microstructures are mainly distributed in Mþ2 layers except
for Bathyacmaea (M1, Mþ1 and Mþ3) and Nacella
(Mþ1). Regularly foliated structure (RF) is limited
165
to Nacellidae and Acmaea mitra, and semi-foliated
structure (SF) is shared by Lepetidae and Pectinodontinae. RF and SF are also formed in bivalves
(Carter, 1990a, b) but not in other gastropods. Regular fibrous foliated structure (RFF) and irregular fibrous foliated structure (IFF) are found only in Lottia
langfordi and ‘‘Lottia’’ scabra.
(3) Crossed structures: Various forms of crossed
lamellar structure (CL) are common to most taxa of
gastropods. In Patellogastropoda, all genera excluding
Nacella have this structure mainly on both sides of the
myostracum. Irregular complex crossed lamellar
structure (ICCL) is shared by most taxa of Lepetidae
and generally possessed by gastropods and bivalves.
Irregular complex crossed foliated structure (ICCF)
is restricted to part of bivalves and patellogastropods
(Carter, 1990c; this study). Crossed foliated (CF),
irregular crossed foliated (ICF), and cone complex
crossed lamellar (CCCL) structures are restricted to
Patella, Pectinodonta, and Cellana mazatlandica, respectively within Patellogastropoda.
Shell structure groups
The high diversification at microstructural level
in contrast to simple macroscopic morphology is
a noticeable characteristic of patellogastropod shells.
MacClintock (1967) categorized patellogastropods into
17 groups by combination of microstructures. Following his concept, 20 groups were recognized in 44 species in this study (Figures 2, 18).
The comparison between MacClintock (1967)
and this study revealed 7 new shell structure groups
(Groups D, E, F, N, Q, R and S). MacClintock’s
(1967) Groups 1 and 15 can be subdivided into a few
more groups as a result of detailed redefinition of microstructures by means of SEM. By contrast, the distinction between Groups 12 and 13 was not verified in
this study. The identical results were obtained between Groups 3 and H, 4 and I, 8 and J, 9 and K, 11
and L, and 16 and T. Groups Q, R, and S were newly
recognized in the species that were not examined
by MacClintock (1967). Species belonging to MacClintock’s (1967) Groups 5–7, 10 and 17 were not examined in this study.
Group 1 of MacClintock (1967) included many different supraspecific taxa. However, as a result of de-
U Figure 18. Comparison of shell structure groups of MacClintock (1967) and this study. Shell-layer thickness is approximately proportional to actual thickness except for very thin layers. Broken arrows indicate possible correspondence between different species in the
same genus. See the captions of Figure 2 for abbreviations used in this study. Abbreviations used by MacClintock (1967): ccf, concentric
crossed foliated; ccl, concentric crossed lamellar; cp, complex prismatic; fi, fibrillar; fo, foliated; ifo, irregular foliated; itf, irregular tabulate
foliated; mf, modified foliated; rcf, radial crossed foliated; rcl, radial crossed lamellar; sp, simple prismatic; xcl, complex crossed lamellar.
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Takeshi Fuchigami and Takenori Sasaki
tailed observations, it was divided into four groups,
Groups A (includes Lottia, Nipponacmea and Tectura), B (Yayoiacmea and Lottia), C (Tectura) and D
(Niveotectura). The distinction of these groups resulted from the division of MacClintock’s (1967) ‘‘fibrillar prismatic (fp)’’ in the Mþ2 layer into simple
prismatic structure type-B (SP-B), fibrous prismatic
structure with minor irregular spherulitic prismatic
(FP-ISP), and composite prismatic structure with minor irregular spherulitic prismatic (CP-ISP). Niveotectura pallida (as Acmaea pallida in MacClintock,
1967) of Group F was a member of Group 1 of MacClintock (1967). This grouping was refined by the new
observations on the outermost layer of N. pallida.
Group 15 was divided into Groups O and P based
on the differences in foliated structures (RF and SF).
The units of regularly foliated structure (RF) area
composed of thin blades with smooth surface and
equal thickness. On the other hand, the foliated units
of semi-foliated structure (SF) consist of flakes with
rough surface and irregular thickness.
The separation of Group 13 and Group 12 by the
presence of irregular tabulated foliated structure (itf)
by MacClintock (1967) was not confirmed even under
SEM-level observation. Accordingly, these two groups
were incorporated into Group M. Based on MacClintock’s (1967) description, Carter (1990c) regarded the structure (itf) as calcitic semi-nacreous, but
it may be transitional layer from concentric crossed
lamellar (ccl, Mþ1) to regularly foliated structure (fol,
Mþ3).
Systematic implications
MacClintock (1967) stated that ‘‘there is a close
relationship’’ between shell structure groups and classification in patellogastropods. In the current systematics, however, his shell structure groups and taxonomic groups have some major conflicts especially in
the Acmaeidae, Lottiidae and Patellidae. For example, Group 1 of MacClintock (1967) has many genera
belonging to the Acmaeidae, Lottiidae and Patellidae
(MacClintock, 1967: table 5).
The results of this study, however, revealed that revised shell structure groups are more consistently correlated with taxa in the up-to-date systematics based
on anatomical characters. Most genera examined have
specific and conservative shell structure, and there is
almost no conflict, with a few exceptions (Figure 2).
At the family level, all members of Patellidae secrete
either crossed foliated structure (CF) or irregular
complex crossed foliated structure (ICCF), and members of the Nacellidae can be identified by regularly
foliated structure (RF) without exception. The species
of the Lepetidae possess semi-foliated structure (SF)
and irregular complex crossed lamellar structure
(ICCL), except for Cryptobranchia kuragiensis. The
Acmaeidae is characterized by an outermost spherulitic prismatic (SpP) layer and an underlying semifoliated (SF) layer in general. In the Lottiidae most
species are typified by an outer layer of spherulitic
prismatic structure (SpP) and a subjacent layer of fibrous prismatic structure (FP-A, FP-B) except species
of Patelloida and Niveotectura.
All the above results imply that shell structures
can be used as characters to identify systematic position of fossil species mostly without any information
on soft parts. Thus it is possible to trace the chronological distribution of patellogastropods by applying
the taxon-shell structure correlations in the Recent
species to fossil patellogastropods.
On the other hand, some groups do not conform to
a one-to-one relationship between shell structure and
taxonomic groups. The inconsistency is found in the
case in which species belonging to the same genus
have different shell structures. Lottia cassis or Lottia
kogamogai versus other species of Lottia is an example of such cases (Figure 2). Species currently assigned
to the genus ‘‘Tectura’’ have composite prismatic
structure with minor irregular spherulitic prismatic
structure (CP-ISP), but its type species, T. virginea,
lacks it. The taxonomic relationships of these taxa
may need reconsideration by further phylogenetic
studies.
The Acmaeidae and Lepetidae are the groups that
have been founded on limited anatomical characters
compared to the rest of the patellogastropods. For
example, the Acmaeidae has been defined mostly by
a single shell microstructural character, i.e., the possession of foliated structure (Lindberg, 1988a, 1998;
Sasaki, 1998). Recently Nakano and Ozawa (2004)
proposed to transfer Niveotectura pallida from the
Lottiidae to the Acmaeidae based on the results of
molecular analysis. This new assignment is not consistent with the conventional definition of the family by
the possession of foliated structure. Currently it is
not possible to evaluate the validity of this systematic
change from the anatomical viewpoint, because the
animal of the type species of Acmaea (A. mitra) has
never been studied, except for its radular morphology
(Lindberg, 1981). These less clearly defined genera
and families should be further investigated for all
available characters.
Although many authors tried to reveal phylogenetic
relationships of patellogastropod limpets (Dall, 1876;
Lindberg, 1988a; McLean, 1990; Hodgson, 1995; Lindberg, 1998; Sasaki, 1998; Koufopanou et al., 1999;
Shell structure of Patellogastropoda
Harasewych and McArthur, 2000; Nakano and Ozawa,
2004), no complete consensus has been achieved yet.
Therefore, the evolutionary scenario of shell microstructure is difficult to present here. A robust phylogenetic hypothesis is required to discuss the homology
and evolutionary process of microstructural characters
for further implications.
Acknowledgements
We are very grateful to Joseph G. Carter, University of North Carolina at Chapel Hill for identification of various microstructures, suggestions on
research technique and literature, and for critical
reading of the manuscript. We also thank Klaus
Bandel, Geological-Paleontological Institute and
Museum, Hamburg University for comments and
improvement of the manuscript. This study was conducted as joint work of the authors under the direction
of Kazushige Tanabe, Department of Earth and Planetary Sciences, University of Tokyo. We deeply thank
him for his education and encouragement. The materials were kindly provided by Yasufumi Arima,
Amami-Oshima Island (Scutellastra optima), Rachel
Collin, Smithsonian Tropical Research Institute,
Panama (Atalacmea fragilis), Ronald Janssen, Senckenberg Museum, Germany (Tectura virginea), Yasuhiro Kuwahara, Hokkaido Abashiri Fisheries Experimental Station (Limalepeta lima), Dr. Takashi
Okutani, Japan Agency for Marine-Earth Science and
Technology (species from California and Bathyacmaea secunda), Hiroshi Saito, National Science
Museum, Tokyo (Lepeta caeca pacifica), and the late
Eiji Tsuchida (Pectinodonta orientalis). This study was
supported by Grants-in-Aid from the Japan Society of
the Promotion of Science (No. 15740309, No.
15340175).
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