Tane 35: 143 - 148 (1995)
FLOATING C O C K L E SHELLS (AUSTROVENUS STUTCHBURYI) THEIR SIGNIFICANCE T O PALEOENVIRONMENTAL
ASSESSMENTS
Bruce W. Hayward' and Jeffrey D. Stilwell
2
2
' Auckland Institute and Museum, Private Bag 92018, Auckland
Department of Geology, University of Otago, P.O.Box 56, Dunedin
SUMMARY
The presence of shells of the estuarine and harbour-dwelling cockle,
Austrovenus stutchburyi (Wood), in fossil assemblages may not always indicate
a sheltered, slightly brackish paleoenvironment. Today, common cockle shells
occur on exposed sandy beaches on the west coast of northern New Zealand, up
to 20 or more kilometres from the mouth of the nearest harbour or estuary.
Our recent observations of thousands of floating cockle shells in Kaipara
Harbour, picked up off the beaches and tidal flats by the incoming tide during
calm conditions, provides a mechanism for transporting numerous cockle shells
out into the tidal channels or harbour entrance where swift tidal currents could
carry them out to the exposed coast.
INTRODUCTION
The New Zealand cockle (Austrovenus stutchburyi (Wood)) is a common and
distinctive fossil in the Pliocene to Holocene rocks of New Zealand, and its
abundance in a fossil assemblage is usually taken to indicate a fossil estuarine
environment, similar to its present-day preferred occurrence.
Cockles live in superabundance in the low tidal and shallow subtidal zones of
most of our present-day estuaries and enclosed bays and harbours (e.g. Morton
& Miller 1968, Hayward et al. 1994). They are often the dominant infaunal
bivalve present, sometimes comprising the entire bivalve fauna in deep shell beds
on sandy mud flats in the upper parts of estuaries and harbours (Beu & Maxwell
1990). They have not been found living off normal salinity open coasts.
Paleoenvironmental assessments
The ecologic distribution of modern organisms is commonly used to assess the
paleoenvironments in which fossil assemblages accumulated but paleontologists
are very much aware that fossil assemblages do not always contain in-situ
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Fig. 1. Map of Northland showing location of observation site, near Matakohe on an arm of the
Kaipara Harbour.
associations of organisms that once lived together. Post-mortem transport and
mixing of plant and animal remains may have to be unravelled to determine the
environments from which the fossils were originally derived and the environment
in which the mixed fossil assemblage accumulated. This unravelling may rely on
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the relative amounts of abrasion on the fossil remains as an indicator of postmortem transport and also on the relative abundance of the different fossils
present. Often the abundant and dominant fossils are inferred to be in-situ or to
have been transported the least distance.
It seems quite reasonable to interpret fossil shell beds that are predominantly
composed of cockles, as being low tidal estuarine or sheltered harbour or bay
deposits (e.g. Morton & Miller 1968, Beu & Maxwell 1990, p. 314). Fossil
assemblages are also encountered that contain numerous cockles together with
open-ocean beach and subtidal bivalves. The interpretation of these assemblages
is more difficult. They have commonly been assessed as bay-bar deposits (Beu
& Maxwell 1990, p. 314), that accumulated near where the two environments
meet. Our observations, reported below, suggest that present-day cockle shells
may be at times transported large distances from their sheltered habitats,
sometimes with very little abrasion of the shell ornament.
PRESENT-DAY OCCURRENCE ON EXPOSED OCEAN BEACHES
Shells of cockles are found on all the exposed west coast beaches of Northland
and Auckland (e.g. Hayward 1990). Although they are not the dominant bivalve
shells present, they often comprise 1-5% of the wash-up. Many of the cockle
shells, and other bivalves, are highly abraded, presumably by the consistent
pounding surf on the beach. Some cockle shells on these beaches are also found
in near pristine condition with crisp radial and comarginal ornament, at least
20km along shore from the nearest sheltered harbour habitat (Hayward 1990).
The dominant bivalve shells on these beaches are Mactra murchisoni
Deshayes, Paphies subtriangulata (Wood) and Spisula aequilateralis (Deshayes).
Their association with cockles in a fossil assemblage may be interpreted as a baybar situation, but on the present-day west coast they are clearly mixed together
and occur in exposed sandy beach settings well away from any estuary, bay or
harbour.
Other common sheltered environment mollusc shells (e.g. mud snail
Amphibola crenata (Gmelin), pipi Paphies australis (Gmelin) and wedge shell
Macomona liliana (Iredale)) are rare on our modern west coast beaches. These
observations have long raised questions about how cockle shells get transported
out of the estuaries and harbours onto and along the exposed west coast beaches,
sometimes with very little abrasion; and how is it that cockle shells are
transported preferentially to other abundant molluscs of sheltered intertidal
habitats.
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OBSERVATIONS ON FLOATING C O C K L E S
On 11 July 1993, the authors were searching for fossils near Matakohe, on
the shore of the upper Kaipara Harbour (Fig. 1). The day was unusually calm
with absolutely no wind or ripples on the sea surface and the tide was rising quite
rapidly. We noticed that numerous single valves of cockles were floating past
(Fig. 2). Realising the significance of this as a possible post-mortem transport
mechanism, we made the following observations:
The intertidal beaches and mud flats of the huge Kaipara Harbour are covered
in abundant shells of cockles with less abundant pipi, wedge shells and mud
snails. The more stable orientation for cockle shells on the beaches is convex up,
with only approximately 1% of their shells concave up. As the ripple-less tide
rises it picks up and will float off many of the concave up shells. Only concave
upwards cockle shells float. They were floating in a zone within about 10m of
the shore and were carried up the harbour with the tidal current. The current
speed at our location was 8cm per second and the density of floating cockles was
1-10 per linear metre of shoreline.
Also seen floating were rare specimens (concave up) of wedge shells, pipi,
Mactra ovata (Gray) and Protothaca crassicosta (Deshayes). Field
experimentation indicated that of these, only P. crassicosta could float convex up;
that the mud snail can sometimes float with its aperture upwards; and that joined
valves of dead cockles and wedge shells sometimes floated with air trapped inside
but none stayed afloat for more than one minute.
Assuming that the perfectly calm conditions we experienced on the Matakohe
Arm were present throughout the harbour, we roughly calculate that there were
in the order of 1 million cockle shells floating in the Kaipara Harbour at the time
of our observations.
POSTULATED POST-MORTEM TRANSPORT MECHANISM
Cockle shells will only be picked up and floated away on an incoming tide
during very calm weather. Additionally, only concave up shells that are lying on
the beach at or very close to horizontal and contain no water in them will float
off. Shells containing no water must either have been turned over during the last
tidal cycle, perhaps by birds, or have been sitting in the sun for some time to
evaporate off the water left in the shell when the tide went out (Cadee 1994).
Thus few shells are likely to be floated off during overnight low tide cycles or
during rainy or cloudy days, and only a few incoming tides each year will satisfy
the requirements for floating off shells.
When the tide turns, the floating cockles will be carried out towards the
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Fig. 2. Photograph of cockles floating on the incoming tide and being carried away by the tidal
current, Kaipara Harbour.
harbour entrance so long as the sea surface remains calm. The harbour entrance
is never calm enough for shells to be floated all the way out to the exposed coast.
Flotation, however, is a probable mechanism for moving cockle shells out from
the relatively calm, sheltered intertidal and subtidal muddy flats to swift-flowing
tidal channels, particularly near the harbour entrance where some shells are likely
to be carried either in suspension or along the bottom and out to the exposed
shore. Once out of the harbour confines, longshore drift is presumably capable
of transporting the shells tens of kilometres along the coast before they are
washed up on the beach.
CONCLUSIONS
Cockle shells are commonly found washed up, together with surf clam
species, on the exposed west coast beaches of Northland and Auckland. We infer
that they have been transported there by longshore drift from the mouths of one
of the several major harbours that empty onto this coast (Fig. 1).
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We suggest that many of the cockle shells are transported out of these
harbours by strong tidal currents, having initially floated from their sheltered
intertidal mudflat habitats into the tidal channels and towards the harbour
entrances during extremely calm conditions.
ACKNOWLEDGEMENTS
We thank Hugh Grenfell and Jack Grant-Mackie for reading the manuscript and suggesting
improvements.
REFERENCES
Beu, A . G . & Maxwell, P. 1990: Cenozoic mollusca of New Zealand. New Zealand Geological
Survey Paleontological Bulletin 58.
Cadee, G.C. 1994: Floating shells, Dutch Wadden Sea. Journal of Paleontology 68(4): 903-904.
Hayward, B.W. 1990: Kawerua molluscs. Tane 32: 1-9.
Hayward, B.W., Blom, W., Morley, M . , Stephenson, A.B. & Hollis, C.J. 1994: Benthic ecology
of Whangape Harbour, Northland. Records of the Auckland Institute and Museum 31: 219-230
Morton, J. & Miller, M . 1968: "The New Zealand Sea Shore." Collins, Auckland.
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