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Table 6. Size al which there is a maximal investment in siphon tissue
(Fig. 12) and si/e al which egg produclion starts. The latter according
to Broiisseau I 1978). Commilo 11982) for Mya arenaria:
kiisk-nscn 11957), Seed .v Brown 11977) for Cerastoderma edule;
Caddy (1967). Commito (1982) for Macoma balihica: Hughes
11972) for Scrobicularia plana.
Species Maximal investment
in siphon tissue
SIPHON SIZE AND DEPTH IN BENTHIC BIVALVES
Start,
production
Mya illinium 20-45 mm > 20-45 mm
Cerastoderma edule 7-17 >I8
Macoma hallhu a 5-8 >X-10
Scrobicularia plana < 15-25 >20
even the largest Mya and Macoma (Blundon &
Kennedy 1982a). They dig up bivalves to a depth of c.
10 cm (Blundon & Kennedy 1982b), which is thus less
than the probing depth of Curlews feeding on Mya. but
more than the depth from which Oystercatchers take
Macoma. This may explain why in Chesapeake Bay
the latter species live at greater depths than their European
conspeeifics. h is worthwhile to do more comparative
research to understand the importance of
burying depth as a defensive adaptation.
Siphon investment and predation risk
The increase of burying depth wilh size (Figs. 2-5) corresponds
wilh a larger investment in siphon mass (Fig.
6). The depth/size relationship in Cerastoderma. Mya.
110
Scrobicularia and Macoma can be described wilh an
S-curve (Figs. 2-5). so it is reasonable to suppose that
siphon weight, known from Fig. 6. but recalculated
here as a proportion of total body weight, is relatively
low lor juveniles, maximal for the size classes which
burrow deeper, and gradually decreasing when burying
depths level off. This is indeed the case (Fig. 12, see
also Pekkarinen (1984) for a similar graph for Macoma
in the Baltic Sea).
There is a very high mortality among juvenile benthic
bivalves. A significant percentage of this mortality
is caused by predation of Oat fishes, shrimps and crabs
(Hancock & Urquhart 1965. Reise 1977. de Vlas
1979a. Pihl 1982. Pihl & Rosenberg 1984. Jensen &
Jensen 1985. Le Mao 1986, Sanchez-Salazar el al.
1987b). It is not yet clear to what degree si/e and depth
offer protection againsi these shallow-feeding predators.
In any case, quick growth is apparently a first priority,
to be followed by increase in siphon mass to
in,ike possible a greatei burying depth
It has been suggested that reproduction in these
benthic bivalves is delayed in order to divert its re
sources into rapid early growth (Lammers 1967 for
Macoma: Seed & Brown 1978 for Cerastoderma.
Brousseau 1979. Commito 1982 for Mya). Table 6
shows lhat maximal siphon growth also takes place before
sexual maturity is reached. The three priorities
during the course of the life of a benthic bivalve therefore
seem to be: (1) to grow fast: (2) to increase burying
depth: and (3) to produce offspring.