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Reproductive ecology of the deep-sea protobranch bivalves Yoldiella ecaudata, Yoldiella
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sabrina, and Yoldiella valettei (Yoldiidae) from the Southern Ocean
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Adam J. Reed1, James P. Morris1, Katrin Linse2, Sven Thatje1
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1 University
of Southampton, Ocean and Earth Science, National Oceanography Centre
Southampton, European Way, Southampton, SO14 3ZH, UK
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British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley
Road, Cambridge, CB3 0ET, UK
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Corresponding Author: Adam J. Reed
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email: ajr104@soton.ac.uk
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Abstract
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The protobranch bivalves of the Southern Ocean are poorly understood ecologically, despite
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their high abundances in soft sediments from the shelf to the deep sea. The subclass has a
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long evolutionary history pre-dating the formation of the polar front, and knowledge of
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their reproductive biology is key to understanding better the successful radiation into the
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Southern Ocean, and within deep-sea basins. In this study we investigate the reproductive
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biology of three deep-water protobranchs for the first time; Yoldiella ecaudata from 500 m
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in the Amundsen Sea; Y. sabrina from between 200 and 4730 m in the Amundsen Sea,
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Scotia Sea, and South Atlantic; and Y. valettei from 1000 m in the Scotia Sea. All three
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species demonstrate evidence of lecithotrophic larval development with maximum egg size
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of 130.4 µm, 187.9 µm, and 120.6 µm in Y. ecaudata, Y. sabrina, and Y. valettei, respectively,
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further supported by prodissoconch I measurements. There is evidence for simultaneous
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hermaphroditism in Y. valettei. Asynchronous oocyte development within specimens of Y.
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ecaudata and Y. valettei is described, and also between populations of Y. sabrina separated
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by depth. The reproductive characteristics, comparable to those of North Atlantic deep-sea
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protobranch species, are discussed in the context of the cold-stenothermal conditions
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prevailing on the deep-Antarctic continental shelf and deep sea. The requirement for
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reclassification of this complex subclass is also discussed in relation to observed soft
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anatomy and shell characteristics.
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Introduction
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The Protobranchia are a subclass of small deposit feeding bivalves containing over 600
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species globally, and represent one of the oldest clades of bivalve with fossil records dating
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back to the early Ordovician ~500 Ma (Allen 1978). They share in common highly specialised
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characteristics for their preferred fine sediment environment, including a strong gill
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structure solely used for respiration, long hindguts for feeding in food impoverished
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environments, and adapted feeding palps to consume large quantities of sediment (see
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review by Zardus 2002). Most commonly found in the deep-sea, protobranchs are also well
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represented in shallow water environments, and are of interest because of their long
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evolutionary history (Allen 1978; Zardus 2002), potentially long life spans (Turekian et al.
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1975; Nolan and Clarke 1993; Gage 1994; Peck and Bullough 1993; Peck et al. 2000), and
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diverse ecology.
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Despite representing between 50 and 90% of bivalve species on the continental slope and
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abyssal plain, respectively, deep-sea protobranch bivalves are ecologically poorly
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understood. They are considered an important macrofaunal taxon in the deep sea for their
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sediment turnover and biomass (Sanders et al. 1965; Allen 2008), and yet their taxonomy
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remains largely unclear, with many synonyms as a result of previous misidentifications of
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new species (Verrill and Bush 1898; Knudson 1970). Allen and Hannah (1986), who state
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that names used within the group are ‘a confusing array of regional uses that are not
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collated’, reorganised the group omitting erroneous names, and subsequent studies have
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outlined the defining characteristics of different genera (Allen and Sanders 1973; 1982,
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1996; Allen and Hannah 1989; Allen et al. 1995). More recently, genetic tools have been
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used to analyse widespread distributions of species revealing contrasting levels of genetic
3
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differentiation and homogeneity among different species, often relating to bathymetry and
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geography (Chase et al. 1998; Etter et al. 2005, 2011; Zardus et al. 2006).
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Reproduction of deep-sea protobranchs has been studied in few species, and in most detail
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from those collected in the Rockall Trough in the North Atlantic, where a research station at
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~2900m was sampled extensively between 1973 and 1983 (Lightfoot et al. 1979; Tyler et al.
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1992), but also the Gay Head-Bermuda transect (Scheltema and Williams 2009). These
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studies identified seasonality in Ledella pustulosa, Yoldiella jeffreysi, Nucula delphinodonta,
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and N. annulata while asynchronous reproduction prevailed in Malletia cuneata, Ennucula
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similis, E. granulosa, Deminucula atacellana, and Brevinucula verrilli, representing both
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lecithotrophy and direct larval development. To date, this represents the most detailed
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reproductive
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chemosynthetic environments.
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Protobranch bivalves have a unique, short dispersing pericalymma larva, in which the larval
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shell develops beneath an external layer of cells and cilia (the test) to help movement
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(Zardus and Morse 1998; Zardus and Martel 2001). This distinct shell development results in
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a smoother but distinctly pitted prodissoconch I (first larval shell) and absent prodissoconch
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II (second larval shell) (Ockelmann 1965). To date, known larval development in
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protobranchs can be pelagic lecithotropthic or direct larval development with parental care,
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commonly inferred by the larval shell and oocyte size (Ockelmann 1965; Scheltema and
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Williams 2009), with prodissoconch and oocyte size above 135 and 90µm, respectively,
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suggesting lecithotrophic development over planktotrophic development (Ockelmann
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1965).
biology
known
within
deep-sea
protobranch
bivalves
from
non-
4
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In the Antarctic, the protobranch bivalves are well represented between 1 and 5000m but
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research has predominantly focussed on the ecology of the comparatively large and shallow
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water protobranch Yoldia eightsii (Davenport 1988; Nolan and Clarke 1993; Peck and
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Bullough 1993; Peck et al. 2004). Despite frequent sampling, even the reproduction of
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Yoldia eightsii is still poorly understood and is demonstrative of the difficulties associated
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with time constraints for seasonal sampling in the Antarctic. The Southern Ocean
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protobranchs Yoldiella sabrina (Hedley 1916), Yoldiella ecaudata (Pelseneer 1903), and
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Yoldiella valettei (Lamy 1906) are commonly found from the Antarctic shelf to the deep-sea
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basins (200-4000m+) and have wide distributions (Gofas 2012) that are likely to be circum-
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Antarctic, although locally distinct morphotypes have been identified (Reed et al. 2013a).
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Additionally, Yoldiella sabrina has a known distribution outside of the Polar Front into the
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South Atlantic (Linse et al. 2007).
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This study, for the first time, describes the reproduction of deep-water Southern Ocean
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bivalves Yoldiella ecaudata, Y. sabrina, and Y. valettei collected from the South Atlantic,
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Amundsen Sea, and Scotia Sea. This knowledge will enhance our understanding of the
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benthic ecology of protobranchs in the cold-stenothermal environments of the deep sea
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and Southern Ocean, and provide an improved ecological framework for future studies on
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biogeography and population connectivity in the Southern Ocean. The taxonomic status of
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these species is also discussed in the context of anatomical observations made through
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histology and comparisons with other genera.
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Materials and Methods
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Sample collection
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Yoldiella valettei, Y. ecaudata, and Y. sabrina were collected during a number of research
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expeditions to the Weddell Sea, Scotia Arc, Antarctic Peninsula, and Amundsen Sea between
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2005 and 2008; ANDEEP III 2005 (Fahrbach 2005); BIOPEARL 2006 (Linse 2006); BIOPEARL II
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2008 (Enderlein and Larter 2008) (Table 1). Benthic samples were collected primarily using
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an epibenthic sledge (EBS) (Brandt and Barthel 1995; Brenke 2005) or Agassiz trawl (AGT),
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and the deployment methods and conditions are described in the relating reports. Samples
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containing the target protobranch species were taken in depths ranging from 192m to
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4900m. Specimens collected by EBS were fixed in either 96% ethanol or 4% buffered
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formaldehyde immediately after coming on deck and later sorted, while specimens
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collected by AGT were sorted alive and then fixed in 96% ethanol or 4% buffered formalin.
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Differences in fixing/sorting procedure were the consequence of different science priorities.
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Histology
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Histology was done according to the protocol of previous studies (Tyler et al. 1992; Reed et
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al. 2013b). Specimens were dissected from their shell or decalcified in rapid decalcifying
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solution (with HCL). Where possible, specimens were photographed before and after shell
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removal for identification of anatomy post processing. Soft anatomy and internal shell
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characteristics was used to confirm identification of species.
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Whole animals were used for histology to avoid damaging gonads in dissection (see Higgs et
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al. 2009). Samples were dehydrated in graded isopropanol and cleared in histoclear
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(CellPath). Tissue was set in wax and serial sectioning of tissue was done at 6µm where
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possible, to ensure the gonad was cut.
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A total of 37 Yoldiella sabrina from 500m in the Amundsen Sea, 4730m in the South Atlantic
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and from 200-3500m in the Scotia Sea; 34 Y. valettei from the 1000m in the Scotia Sea; and
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15 Y. ecaudata from 500m in the Amundsen Sea, were used for histology, representing the
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size range of each species where possible. Microphotographs were taken using a Nikon
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D5000 mounted on a microscope and Feret diameter was used to measure egg size using
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SigmaScan Pro 4 software.
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Prodissoconch size
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Prodissoconch length of ten Y. ecaudata and eleven Y. valettei were measured by scanning
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electron microscopy (SEM) of the umbo region of the shell. Prodissoconch sizes of Y. sabrina
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were not measured as only adult specimens with eroded umbones were available for this
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study. Specimens for SEM were mounted onto stubs, coated with 5µm gold layer and
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analysed using a Leo 1450 VP SEM. The maximum length and height of the prodissoconch I
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(PI) were measured using sub-adult shells in good condition. Shells were positioned so that
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the prodissoconch was lying flat to the SEM beam. Length measurements were taken tilting
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the shell along the mid-axis until the maximum length was obtained.
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To retain consistency when measuring the maximum length, the line of measurement was
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always kept parallel with the hinge line. Maximum height was always perpendicular to the
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length. The PI has been previously found to be directly linked to size of offspring,
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representing the size of shell secreted by the larval stage, and so linked to egg size and
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nutritional content (Ockelmann 1965; Goodsell and Eversole 1992).
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Results
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Reproductive ecology
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Yoldiella sabrina
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In total 25 animals were identified with active gonads and 12 with no identifiable gonads.
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The gonad occupied an area dorsal laterally on both the left and right external to the
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digestive gland and internally around the digestive tissue and gut loop (Fig. 1, 2). Oocytes
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and testis were observed beside the intestinal loop wall and there was no evidence of
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hermaphroditism (Fig. 2).
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The smallest female was found at 3.3 mm shell length from the Scotia Sea with
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previtellogenic oocytes observed internally around the digestive gland (Fig. 2a, b). Small
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vitellogenic oocytes were observed at 5.5 mm shell length with up to 22 oocytes in
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specimens from Scotia Sea (Fig. 3a), the largest measuring 55.7 µm in diameter. No more
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than ten large vitellogenic oocytes were measured in specimens of 9.1 mm shell length from
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the Scotia Sea and the largest female at 10.3 mm from the South Atlantic (Fig. 3a). The
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largest eggs measured 187.9 µm and 174.0 µm respectively. Only one female with small
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vitellogenic oocytes was observed in the Amundsen Sea at 5.8mm shell length.
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Males were observed from 3 – 13 mm shell length, identified by extensive testis surrounding
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the digestive glands, and hind gut (Fig. 2c). The right dorsal-ventral gonad surrounding the
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hind gut appeared less extensive however, than the left dorsal ventral gonad (Fig. 2c).
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Spermatogonia and spermatocytes were observed, but no accumulations of spermatozoans
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were visible in the specimens from any location. Males were well represented in specimens
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studied from the Scotia Sea, Amundsen Sea, and South Atlantic, with 16 males to 8 females
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in all areas. A relationship of two males to every female, or higher, was also observed within
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each area studied.
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Yoldiella ecaudata
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Nine specimens of Y. ecaudata were identified has having active gonads and in six no
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reproductive tissue was found. The gonad is situated dorsal-laterally, external to the
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digestive gland and around the hind gut (left and right), and internally around the digestive
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gland (Fig. 4a digestive gland; 4b for gonad). Testis or oocytes were observed in areas
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otherwise occupied by the digestive gland, which appeared reduced in specimens with well-
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developed gonad; there is no evidence of hermaphroditism
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Female Y. ecaudata were found between 2.2 and 2.8 mm shell length with the largest
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oocyte measured 130.4 µm in a female of 2.8 mm shell length (Fig. 3b). Up to 13 eggs were
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counted in a single female of 2.6 mm shell length. Only large vitellogenic oocytes were
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observed in females although the size range measured varied within and between
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individuals (Fig. 3b). Only three male Y. ecaudata were observed between 2.4 and 2.7 mm
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shell length, but no evidence of spermatazoan accumulation.
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Yoldiella valettei
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In total, 21 specimens from the Scotia Sea were identified with active gonads. The gonad
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occupied an area dorso-laterally on both the left and right sides of the body, external to the
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digestive gland, and internally around digestive tissue and hind gut loop (Fig. 5). The gonad
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extended dorsally to join the left and right portions of gonad (Fig. 5a). In males, the testis
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extended anteriorly under the mantle epithelium (Fig. 5b).
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Females were found between 2.2 and 3.2 mm shell length with vitellogenic oocytes
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observed in the gonad. The largest oocyte measured 120.6 µm diameter and up to 56
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oocytes were counted in a single individual (3.1 mm) (Fig. 3c). In one specimen of 3.1 mm
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shell length, both testis and vitellogenic oocytes were observed with spermatozoa
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accumulation in the lumen (Fig. 5a). Testis in the hermaphrodite was only observed dorso-
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laterally on the right side, external to the digestive gland and hind gut loop. Male Y. valettei
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were found between 2.3 and 3.2 mm shell length with well-developed testis often
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enveloping the bivalve, externally to the digestive glands and along the mantle cavity
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epithelium (Fig. 5b). Spermatogonia and spermatocytes were observed and accumulations
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of spermatozoans were visible in some specimens. There was no evidence of protandry or
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protogyny.
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Prodissoconch measurements
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Prodissoconch I was identified by a characteristic pitted surface. Both Y. valettei and Y.
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ecaudata showed similar prodissoconch lengths of 184.4 µm ±7.2 SD (n=11) and 191.6 µm ±
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4.0 SD (n=10), respectively (Fig. 6), and no prodissoconch II was observed in any specimens.
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Discussion
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Reproductive biology of deep-sea protobranch bivalves is not well known; species with
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known reproduction include the nuculanid Ledella pustulosa and yoldiid Yoldiella jeffreysi
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from 2900 m in the Rockall Trough, North Atlantic (Lightfoot et al. 1979; Tyler et al. 1992).
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Gonad morphology and anatomy in all three studied Southern Ocean species are similar to
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that described in Tyler et al (1992); the gonad is found external to the digestive gland and
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hind gut loop, but histology also revealed oocytes and testis formation internal to the
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digestive gland, and dorsally above the stomach.
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Although not intended as a taxonomic revision, the deep-water protobranchs Y. valettei, Y.
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ecaudata, and Y. sabrina appear most morphologically and anatomically similar to deep-sea
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species of the genera Yoldiella (Allen et al. 1995), Ledella (Allen and Hannah 1989) and
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Portlandia (Killeen and Turner 2009), respectively. These observations highlight the
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requirement for a comprehensive morphological and molecular taxonomic revision of the
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current Yoldiidae and related small sized protobranch families. The soft anatomy of Y.
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ecaudata appears most similar to the nuculanid Ledella pustulosa with a characteristically
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large stomach and extensive hind gut coil on both the left and right sides of the body, while
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the shell is short and compact, rostrate with strong concentric sculpture and strong chevron
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teeth (Allen and Hannah 1989). Sharma et al. (2013) regarded Y. ecaudata as Ledella
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ecaudata in a recent molecular study, although without a taxonomic diagnosis. Gonad
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morphology and oocyte size is also similar with L. pustulosa having a maximum oocyte size
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of 120 µm (Tyler et al. 1992) while Y. ecaudata have maximum oocyte size of 130.4µm
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(Table 1). Likewise, the similar Southern Ocean Y. valettei and North Atlantic Y. jeffreysi have
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ovate and fragile shell structure with very fine concentric sculpture, and maximum oocyte
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sizes of 120.6 µm and 120 µm respectively. While only representing a ‘snapshot’ of the
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reproduction in these species, these comparisons of maximum oocyte size with regularly
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sampled material from the North Atlantic may infer that the largest oocytes observed are
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fully developed. Prodissoconch measurements of Y. valettei and Y. ecaudata together with
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the oocyte sizes between 85 and 140 µm suggest a pelagic lecithotrophic development
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(Ockelmann 1965), as found in other deep-sea protobranch bivalves (Tyler et al. 1992;
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Scheltema and Williams 2009).
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Yoldiella sabrina share more morphological characters with the genus Portlandia (Yoldiidae
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although high levels of plasticity may cross over with characteristics of Yoldiella spp.; the
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shell is oblong with an extended hinge margin, a distinct lunule and escutcheon, and a single
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hind gut loop to the right of the body, (Verrill and Bush 1897, Allen and Hannah 1986,
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Killeen and Turner 1999). Reproductively, over a range of depths and across biogeographic
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ranges, we find a higher proportion of males, which contrasts to Y. ecaudata and Y. valettei
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observed having one male to two females (Table 1). A maximum oocyte size for Y. sabrina of
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187.9 µm is larger than observed in Y. ecaudata and Y. valettei and is close to the predicted
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range for direct larval development (Ockelmann 1965). The mean prodissoconch I
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measurements of Y. sabrina from published data suggest a length of approximately 200 µm
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with a barely discernible prodissoconch II (Hain and Arnaud 1992), which would, however,
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support lecithotrophic larval development.
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Using an underlying knowledge of deep-sea protobranch life history published to date,
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evidence of seasonal or continuous reproduction can be identified from our samples,
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despite only having a snapshot of their reproductive cycle. Lecithotrophic larval
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development could be uncoupled from seasonality of food supply in the Antarctic benthos
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as found in the asteroid Porania sp. (Bosch and Pearse 1990). Protobranch bivalves are
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specially adapted to living in food-impoverished conditions (Allen 2008) reducing the effect
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of food limitation into the deep-sea (Gage and Tyler 1991). Cold Antarctic deep-sea
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conditions can store high levels of organic matter in sediments, acting as an annual food
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supply to deposit feeding fauna (Mincks et al. 2005; Glover et al. 2008), further reducing any
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effect of seasonality. This study found variation in oocyte size within individuals of Y.
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ecaudata and Y. valettei, which may be representative of asynchronous oocyte
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development within individual specimens, as found for Y. jeffreysi in the North Atlantic
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(Tyler et al. 1992). The smaller egg size range within Y. ecaudata could also represent an
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earlier stage of oocyte development at smaller sizes. Oocyte development is often slow in
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Southern Ocean and deep-sea molluscs taking between 12 and 24 months (Picken 1979,
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1980; Richardson 1979; Brey and Hain 1992; Higgs et al. 2009; Reed et al. 2013b) and
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multiple cohorts may exist within individuals. The smaller eggs seen in some specimens may
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represent a second cohort of eggs being produced simultaneously to the current cohort. The
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differences observed in egg size and depth in Y. sabrina could also indicate a difference in
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seasonality of reproduction with depth, or an earlier stage of oocyte development with
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275
smaller shell size. However, the bathymetric and subtle ecological differences between
276
samples may also have a role in the timing and tempo of reproduction at the benthos.
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Additionally, biogeographic related plasticity in morphology and growth in the target species
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have already been identified, and should not be underestimated in the Southern Ocean
279
(Reed et al. 2013a).
280
Pelagic lecithotrophic larval development not only has the advantage of being uncoupled
281
with primary productivity, but also offers the potential for long periods of dispersal,
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essential for maintaining a homogenous circum-Antarctic distribution. Bosch and Pearse
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(1990) determined the development period of the demersal and pelagic lecithotrophic
284
asteroids Porania sp. and Acodontaster hodgsoni to be 78 and 106 days, respectively, while
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deep-sea Atlantic protobranchs have been modelled to disperse between 237 and 749 km
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(McClain et al. 2012). Furthermore, a study into the metabolic rates and biomass of larvae
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and eggs in Antarctic echinoderms suggested larval survival over extended periods in the
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Antarctic, in contrast to much shorter periods in similar temperate species (Shilling and
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Manahan 1994). Although untested in the field, the large egg size of the Antarctic
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protobranch bivalves may provide the energetic requirements for long periods of
291
development, facilitating a wide dispersal of these species in the Southern Ocean. Deep-
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Antarctic waters have complex currents driven by cold bottom water formation in the
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Weddell Sea and the flooding of warmer waters from the Pacific. Together with the
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Antarctic circum-polar current, continental counter currents, and large scale eddy formation
295
characteristic of the Weddell and Scotia Seas, larval dispersal has great potential if
296
suspension and survival can be maintained in the pelagic environment (for review see Thatje
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2012).
13
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The occurrence of hermaphroditism is rare in deep-sea protobranchs (Zardus 2002)
299
although not unusual in Southern Ocean molluscs, and bivalves, globally. Simultaneous and
300
sequential hermaphroditism has been described in Laternula elliptica (Bosch and Pearse
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1988), Thracia meridonalis (Sartori and Domaneschi 2005), Mysella charcoti and M. narchii
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(Passos and Domaneschi 2009), Lissarca notorcadensis (Prezant et al. 1992) and Lissarca
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miliaris (Reed et al. 2013b). In the deep-sea context, simultaneous hermaphroditism could
304
contribute to the successful radiation of Y. valettei, increasing the likelihood of successful
305
fertilisation in areas of low species abundance. Simultaneous hermaphroditism observed in
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Y. valettei may be key to a successful species radiation throughout the Southern Ocean.
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From an evolutionary perspective, simultaneous hermaphroditism may have enabled fast
308
recolonisation of the Antarctic continental shelf during ice retreat in glacial-interglacial
309
cycles of the Late-Cenozoic. Past glacial maximums are modelled to have grounded at the
310
edges of the continental shelf seen today (Huybrechts 2002), which may have forced many
311
benthic species into the adjacent deep-sea. Evidence however, suggests that ice advance
312
was unlikely to be synchronous around the Southern Ocean, and temporary benthic shelters
313
may have facilitated species survival on the shelf (Thatje et al. 2005, 2008, Convey et al.
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2009). Simultaneous hermaphroditism, as observed in Y. valettei, would allow for species to
315
rapidly colonise new areas by increasing the chance of both sexes being available for
316
fertilisation, or even the possibility of self-fertilisation. Single specimens could be all that is
317
needed to found new populations in areas of high ice impact, and may have facilitated their
318
likely radiation from deep-sea basins beyond the Polar Front, a hypothesis, which should be
319
tested in future molecular studies. Hermaphroditism in deep-sea protobranchs may be
320
underestimated globally through a lack of life history studies, but may have implications to
321
understanding their ecological success over wide geographic and bathymetric ranges.
14
322
Conclusion
323
Yoldiella ecaudata, Y. sabrina, and Y. valettei all show evidence of lecithotrophic larval
324
development and gonad morphology consistent with what is known for other species of
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deep-sea protobranch bivalves in the Atlantic Ocean. Large egg size, often described in
326
deep-sea benthic invertebrates, enables the uncoupling from primary production, and could
327
drive
328
hermaphroditism will increase the chances of successful fertilisation in areas of low species
329
density or in colonising areas following ice retreat. Given the contrasting morphology and
330
anatomy of these three Yoldiella species and similarities with different protobranch genera,
331
a thorough review of the protobranch taxa of the Southern Ocean should be completed to
332
understand evolutionary links to other deep-sea basins.
333
Acknowledgements
334
Adam J. Reed was supported through a Natural Environment Research Council PhD
335
studentship. We are grateful to the cruise leaders, captains, officers, and crews of PFS
336
Polarstern (ANDEEP III) and of RRS James Clark Ross (JR144 BIOPEARL I, JR179 BIOPEARL II)
337
who enabled us to collect the samples for this study. We thank Richard Pearce for help with
338
SEM analysis, and Paul Tyler and John Grahame for their feedback on an earlier version of
339
this manuscript. This study is part of the British Antarctic Survey Polar Science for Planet
340
Earth Programme funded by The Natural Environment Research Council (NERC), and is
341
ANDEEP publication #192.
continuous
or
quasi-continuous
reproduction.
Additionally,
simultaneous
342
343
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344
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651
486
487
488
489
490
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492
Figure Legends
493
Fig. 1 Photomicrographs of decalcified Yoldiella sabrina before histology sectioning (shell
494
length 7.75mm) from 200m in the Scotia Sea. a) view of the left sowing large digestive
495
gland; b) view of the right showing digestive gland around hind gut loop. m – muscle; ct-
496
ctenidia; dg – digestive gland; hg - hind gut. Scale bars = 1mm.
497
Fig. 2 Transverse histology sections of Yoldiella sabrina. a) 9.1mm shell length from 3500m
498
Scotia Sea showing general anatomy; b) left side digestive gland and oocytes from 5.5mm
499
shell length from 200m Scotia Sea; c) left side digestive gland and testis from 9.7mm shell
500
length from 3500m in the Scotia Sea. oc – oocyte; dg – digestive gland; hg – hind gut; st –
501
stomach; t – testis. Scale bars 100µm unless stated otherwise.
502
Fig. 3 Oocyte diameter measured from histology sections against shell length. a) Yoldiella
503
sabrina collected in February from the South Atlantic (SA) and Scotia Sea (SS); b) Yoldiella
504
ecaudata collected in March 2008 from the 500m in the Amundsen Sea. N.B. two specimens
505
at 2.6mm shell length offset for clarity; c) Yoldiella valettei collected in February from
506
1000m in the Scotia Sea.
507
Fig. 4 Photomicrograph and transverse histology section of Yoldiella ecaudata from 500m in
508
the Amundsen Sea. a) photomicrograph of 2.8mm shell length specimen showing digestive
509
gland and hind gut loop; b) histology of 2.6mm shell length female showing vitellogenic
510
oocytes in the dorsal portion of the gonad above the stomach and surrounding digestive
511
gland. m – muscle; dg – digestive gland; hg – hind gut; st – stomach; oc – oocyte.
512
Fig. 5 Transverse histology sections of Yoldiella valettei from 1000m in the Scotia Sea. a)
513
section of entire 3.1mm hermaphrodite showing oocytes and testis development in relation
22
514
to digestive gland; b) section of a 3.1mm shell length male showing testis enveloping
515
digestive gland and hind gut loop, extending dorsally above the stomach. dg – digestive
516
gland; hg – hind gut; oc – oocyte; t – testis; st – stomach. Scale bars = 100µm.
517
Fig. 6 Prodissoconch I (PI) images taken using scanning electron microscopy. Arrow indicates
518
artefact on shell. a) Yoldiella valettei; b) Yoldiella ecaudata. Scale bars =100µm.
519
23
520
521
Table 1 Species collection and reproductive data for Yoldiella ecaudata, Yoldiella sabrina,
522
and Yoldiella valettei from the Southern Ocean. ‘n’ indicates total number of animals used
523
to obtain egg sizes and sex ratios, including those of unknown sex.
Species
Location
& Date
Cruise&
Gear
Yoldiella
ecaudata
Yoldiella
sabrina
Yoldiella
sabrina
Yoldiella
sabrina
Yoldiella
sabrina
Yoldiella
valettei
Amundsen Sea
March 2008
Amundsen Sea
March 2008
Scotia Sea
February 2005
Scotia Sea
February 2005
South Atlantic
February 2005
Scotia Sea
February 2006
BIOPEARL II
EBS
BIOPEARL II
EBS
ANDEEP III
AGT
ANDEEP III
AGT
ANDEEP III
EBS
BIOPEARL I
EBS
Depth
(m)
male/female
Max. oocyte
size (µm)
n
500
3/6
130.4
15
500
3/1
/
10
200500
6/5
55.68
17
3500
2/1
187.9
5
4730
3/1
132.0
5
1000
9/12
120.6
34
524
525
24
526
Fig. 1 Photomicrographs of decalcified Yoldiella sabrina before histology sectioning (shell
527
length 7.75mm) from 200m in the Scotia Sea. a) view of the left sowing large digestive
528
gland; b) view of the right showing digestive gland around hind gut loop. m – muscle; ct-
529
ctenidia; dg – digestive gland; hg - hind gut. Scale bars = 1mm.
530
531
25
532
Fig. 2 Transverse histology sections of Yoldiella sabrina. a) 9.1mm shell length from 3500m
533
Scotia Sea showing general anatomy; b) left side digestive gland and oocytes from 5.5mm
534
shell length from 200m Scotia Sea; c) left side digestive gland and testis from 9.7mm shell
535
length from 3500m in the Scotia Sea. oc – oocyte; dg – digestive gland; hg – hind gut; st –
536
stomach; t – testis. Scale bars 100µm unless stated otherwise.
537
26
538
Fig. 3 Oocyte diameter measured from histology sections against shell length. a) Yoldiella
539
sabrina collected in February from the South Atlantic (SA) and Scotia Sea (SS); b) Yoldiella
540
ecaudata collected in March 2008 from the 500m in the Amundsen Sea. N.B. two specimens
541
at 2.6mm shell length offset for clarity; c) Yoldiella valettei collected in February from
542
1000m in the Scotia Sea.
543
27
544
Fig. 4 Photomicrograph and transverse histology section of Yoldiella ecaudata from 500m in
545
the Amundsen Sea. a) photomicrograph of 2.8mm shell length specimen showing digestive
546
gland and hind gut loop; b) histology of 2.6mm shell length female showing vitellogenic
547
oocytes in the dorsal portion of the gonad above the stomach and surrounding digestive
548
gland. m – muscle; dg – digestive gland; hg – hind gut; st – stomach; oc – oocyte.
549
28
550
Fig. 5 Transverse histology sections of Yoldiella valettei from 1000m in the Scotia Sea. a)
551
section of entire 3.1mm hermaphrodite showing oocytes and testis development in relation
552
to digestive gland; b) section of a 3.1mm shell length male showing testis enveloping
553
digestive gland and hind gut loop, extending dorsally above the stomach. dg – digestive
554
gland; hg – hind gut; oc – oocyte; t – testis; st – stomach. Scale bars = 100µm.
555
29
556
Fig. 6 Prodissoconch I (PI) images taken using scanning electron microscopy. Arrow indicates
557
artefact on shell. a) Yoldiella valettei; b) Yoldiella ecaudata. Scale bars =100µm.
558
30