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The littleneck clam flourishes between the mid- and low water, where
it is nourished by microscopic algae filtered from overlying water
for several hours on each tide. As a suspension feeder, the
clam derives its food by ciliary-mucoid capture of the microscopic,
planktonic plant cells contained in a stream of water pumped through
the mantle cavity. While the littleneck clam tolerates moderate
salinity depression, the species is characteristically marine, and
can survive low salinities for short periods, down to 10% seawater
(~4‰ salinity) but cannot feed in less than 50% seawater (~18‰ salinity). The
littleneck clam is characteristic of sheltered harbours and inlets
with extensive sandflats that are exposed at low water and flooded
from the open sea by the rising tide. Its abundance is limited
in estuaries that receive large volumes of fresh water inflow.
Stable fine sand sediments appear to be essential for littleneck
clams to flourish. They are virtually absent from open coast
beaches where mobile, well-sorted sands are inhabited by a variety
of other bivalve species collectively known as surf clams. Littleneck
clams are also absent from excessively muddy sediments.
Growth rates in littleneck clams are extremely variable, not only
between sites, but also between tidal levels at any one site. Populations
frequently contain significant numbers of individuals more than 20
years old, some with shell lengths greater than 60 mm. Sexes
are separate and sexual maturity is thought to be attained at around
18 – 20 mm shell length. Gonadal development peaks in
summer and prolonged spawning takes place over a protracted summer-autumn
season in populations from Auckland to Dunedin.
Several animals prey on littleneck clams, including wading birds,
fish, crabs and sea-stars. Each predator usually targets a
particular size range of clam as preferred prey. Those clams
that avoid being eaten have the potential to be long-lived, surviving
for 25 years and more, reaching a size of >60 mm and a total weight > 90
g.
Littleneck clams have long been the subject of traditional harvest,
their shells being found extensively in middens of varying age throughout
the country. There is an on-going traditional customary and
recreational harvest throughout New Zealand. In the past twenty
years there has been a growing commercial harvest of littleneck clams,
the main sites being located on Snake Bank in the Whangarei Harbour,
at Pakawau Beach in Tasman Bay, and at Papanui and Waitati Inlets
in the vicinity of Otago Peninsula.
Until quite recently the littleneck clam Austrovenus stutchburyi,
was generally known as the common New Zealand cockle, and by its
scientific synonym, Chione (Austrovenus) stutchburyi. The name
littleneck clam is adopted here, partly because the name has growing
use for marketing purposes, but also because the species is not a
true cockle. The genus Austrovenus was erected
by Finlay (1926) to accommodate this species, which had originally
been placed in the genus Venus. The species has other scientific
synomys. Marwick (1927) subsequently recognised Austrovenus
as a subgenus within the genus Chione. Following detailed examination
of both soft and hard part anatomy and morphology in five chionines,
Jones (1979) concluded that “Austrovenus does not belong to
the genus Chione, and may not belong to the [subfamily] Chioninae.
It seems more likely that the similarity of Austrovenus and Chione
in sculpture is the result of convergent evolution of very different
lineages adapting to a shallow infaunal mode of life in shifting
substrata in the intertidal and shallow subtidal zones.”
The current taxonomic classification and nomenclature of the littleneck
clam is as follows:
Class: Bivalvia
Order: Veneroidea
Family:
Veneridae
Subfamily: Chioninae
Genus: Austrovenus
Finlay, 1926 (Type species Venus stutchburyi Gray)
Species: Austrovenus
stutchburyi (Gray, in Wood 1828)
Synonyms: Venus
stutchburyi
Antigona
stutchburyi
Chione
stutchburyi
Common names: littleneck clam, cockle, tuaki / tuangi
Harvest History
The littleneck clam has a long history of exploitation extending
back probably to the very beginning of human settlement. In
the later 14th Century at Purakanui, it was a significant minor food
resource obtained by estuarine gathering (Anderson 1981), ranking
after pipi (Paphies australis), but ahead of the mudsnail (Amphibola
crenata). The species has widespread occurrence in middens
throughout the country and is used as an indicator of estuarine gathering. Comparative
analysis of growth zones in shells from midden and modern material
has been used in attempts to provide information regarding the season
of prehistoric occupation of certain sites, including some in the
vicinity of Otago Peninsula (Coutts 1970, 1971, 1974,).
At several locations littleneck clams have status as "kai wairua",
having particular spiritual significance in the mana and reputation
of adjacent marae. This is true, for example, for dense beds
of large littleneck clams near the entrance of Otago Harbour that
have special significance for Te Runanga Otakau. Similar status
applies to large beds at Port Levy, Banks Peninsula (Owen 1992, p.14).
Lifestyle
The littleneck clam is well adapted to life in the stable beach
sediments of sheltered shores where they flourish from mid-tide level
down to below low water of spring tides. The adult shell is
heavy and plumply rounded, heart-shaped in section, and greyish white
to rust brown, with prominent external sculpture of radial ribs intersecting
with growth lines that are concentric with the outer shell margin. The
interior of the shell is strongly marked with purple. The heavy
sculpture stabilises the clam in sand, where it burrows to a depth
of only two or three centimetres. A pair of short conical siphons,
fringed with small papillae, gives the animal the name of littleneck
clam. One siphon draws an intake from the overlying water,
and the other expels an exhaust stream. In the process of
suspension feeding, particles that are entrapped from the water stream
as it passes over the gills, are then transported to the mouth in
a mucus sheet. The short siphons must remain within reach of
the sediment surface, though they are usually withdrawn during the
low-tide period. The foot is strong and compressed, and smaller
littleneck clams occasionally crawl along the surface, leaving a
furrow usually less than a metre long. Clams normally remain
immobile with the end of the shell often protruding.
Protruding clams become eroded and often develop a distinctive epibiotic
community. Epibionts include the asexually reproducing anemone
Anthopleura aureoradiata, sometimes with several attached to the
shell of a single clam. The estuarine barnacle Elminius modestus,
is common wherever salinities are depressed. Green algal staining
provides a food source for a small limpet Notoacmea helmsi that lives
by grazing the shell surfaces. Tubicolous amphipods often provide
a tuft of parchment-like tubes on each shell. Spionid worms
etch their U-shaped tubes into the shell surface of living clams,
providing a refuge from which their feeding palps are extended into
the overlying water.
Harvesting Effects
In his study of the effects of harvesting on littleneck clams in
Waitati Inlet, Irwin (1999) showed a return in the space of one years
to over 80 % of original biomass, in areas recovering from a 57%
reduction through harvesting effort. This recovery was attributed
in part to migration, in addition to growth and recruitment. In
a follow-up elective study on this observation, McDonald (1999) concluded
that migration occurs from high density areas surrounding areas affected
by harvest.
Parasitic infestation has also been implicated in impairing the
ability of littleneck clams to burrow, leaving them lying on the
sediment surface and prone to predation by the parasite’s definitive
oystercatcher host (Poulin et al.2000).
Experimental Studies
Experimental transplantation at different densities between low,
mid and high shore levels were undertaken and maintained for 12 months
in Otago Harbour (Dobbinson 1985, Dobbinson et al. 1989). Mid
and high shore clams showed enhanced growth and condition, and lower
mortality, when transplanted to lower shore levels. Lower shore clams
transplanted to higher shore levels did not grow measurably and had
lower condition and higher mortality. Mortality increased with
height on the shore. On the basis of these results, though
they were tempered by density effects, it was suggested that transplantation
of smaller older clams from higher shore levels, to replace the low
shore harvest, was a possible management strategy to counter problems
of natural recruitment.
The effects of harvesting as a form of manipulation on biomass,
length frequency and recruitment in remnant populations was undertaken
in Waitati Inlet by Irwin (1999). Irwin found evidence for
enhanced growth, recruitment and immigration following harvest removal
of 55 – 60% of original biomass, though there was considerable
spatial variation between his sites.
A series of discretely separate littleneck clam populations are
to be found in the stretch of coast from Blackhead in the south,
to Shag Point in the north. South of Blackhead is a long stretch
of coast extending to the Catlins Estuary, beyond Nugget Point, on
which there are only minor populations to be found. The largest
estuary in this area, that of the Taieri River, is unfavourable for
littleneck clams by virtue of its intermittently high silt load and
low salinity values. North of Shag Point the coast is comprised
of sandy beaches, rocky shores and gravel, with only minor estuaries
suitable for littleneck clams, extending as far as Banks Peninsula.
Planktonic larval life of the littleneck clam is to be measured
in weeks (~3) rather than days or months which suggests that all
the populations in the vicinity of Otago Peninsula are likely to
a have significant exchange of larvae. This is particularly
true from north of Cape Saunders to Shag Point because the counter-current
eddy that forms in the northern downstream lee of Otago Peninsula
under most wind and tidal regimes will strongly favour the retention
of larvae in this area (Murdoch 1989, Murdoch et al. 1990).
Recognition of the from area Shag Point to Blackhead as a single
management area would enable a sensible and potentially sustainable
plan to be developed for the future harvest of littleneck clams.
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