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DEPTH AND SIPHON CROPPING IN SCROBICULARIA
BURYING DEPTH OF THE BENTHIC BIVALVE
SCROBICUIARJA PLANA (DA COSTA) IN RELATION TO
SIPHON CROPPING
A field experiment tested the hypothesis lhat siphon cropping influences the burying depth of the benthic
tellinid Scrobicitlaria plana (da Costa). Cropping was simulated by removing part of the inhalant siphons
and then burying the animals at the original depth. The depth was measured in situ each day afterwards
with the aid of a nylon thread attached to the shell. The resulting increase of depth with siphon weight in
the experimental animals resembles the relation between depth and siphon size in the natural population.
Heavy cropping of the siphon reduced the body weight, whereas the loss of < 10 mg of the siphon had no
discernible effect on the condition of the animals. When a Scrobicularia with a short siphon reduces its
depth, it enlarges its feeding opportunity (radius around the burrows for deposit-feeding), but also increases
the risk of being eaten by a predator. A short siphon alone does not force Scrobicularia to abandon
the refuge provided by depth: only animals also in a poor body condition expose themselves to a
larger predation risk. Siphon cropping reduces the burying depth and makes the bivalves more accessible
for wading birds like Oystercatchers (Haemaiopus ostralegus L.). Burying depth is hence the outcome of
conflicting demands; this implies that under each set of conditions an optimum can be defined.
Introduction
Benthic bivalves at first sight appear safe from predators,
hidden as they are in the substruium. Many estuarine
species, however, regularly suffer the loss of ihe
siphon tips by 'cropping', "nipping' or 'browsing'
predators, mostly juvenile flatfish (Edwards & Steele
1968. Trevallion etal. 1970. Trevall ion 1971, Kuipers
1977, de Vlas 1979a. 1985, Peterson & Quammen
1982).
Deposit-feeding bivalves are more vulnerable to
these croppers than suspension feeders which can filter
their food from the overlying water while the siphon is
below the surface (Peterson & Quammen 1982). Even
a typical deposit feeder, like the tellinid Scrobicularia
plana, is. however, a suspension feeder at high water or
limits its siphonal activities to ihe sides of mouth of the
inhalant burrow, apparently to minimize the risk of losing
a (big) part of the siphon to predatory fish (Hughes
1969). During deposit feeding at low waler. Scrobicularia
extend their siphon 5 to 8 cm out of the burrow,
but they show an anti-predator reaction to feeding
wading birds: there is a quick withdrawal of the in­
85
halant siphon when there is any vibration through the
mud (Linke 1939, Hughes 1969). Nevertheless.
Curlews (Numenius aiyuaia) manage to graze siphons
oi Scrobicularia at a rate of 1 to 10 per min (Ens &
Zwarts 1980a. unpubl. data).
Bivalves are able to regenerate the siphon (Trevallion
I971.de Vlas 1979a. 1985. Hodgson 1982a). bin
siphon predation has several negative consequences
for diem. Regeneration of the siphon may take place at
the expense of the development of the gonads (Trevallion
et al. 1970. Trevallion 1971). the body condition
(Trevallion et al. 1970, Trevallion 1971. Hodgson
1982a). or somatic growth (Peterson & Quammen
1982). The same sublethal effects have been shown for
estttarine worms suffering tissue loss (de Vlas 1979b.
Wocxlin 1984.Zajac 1985).
Zwarts & Wanink (1989) suggest another negative
effect regular amputation of the siphon would force
ihe bivalves to live nearer to the surface and so expose
themselves mote to the risk of being eaten by birds.
These authors show lhat Scrobicularia occur in a large
vertical range within the substratum: animals of > 25
mm live in summer at a depth between 0 and 10 cm