Tane 35: 143 - 148 (1995)

FLOATING COCKLE SHELLS (AUSTROVENUS STUTCHBURYI) - THEIR SIGNIFICANCE TO PALEOENVIRONMENTAL ASSESSMENTS

Bruce W. Hayward' and Jeffrey D. Stilwell2 ' Auckland Institute and Museum, Private Bag 92018, Auckland 2 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

143 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

144 the relative amounts of abrasion on the fossil remains as an indicator of post• mortem 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 bay- bar 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.

145 OBSERVATIONS ON FLOATING COCKLES

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

146 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).

147 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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