NOTE Takahashi and Kawaguchi: Nocturnal occurrence of Ovalipes punctatus 
513 
Rank of foregut fullness index 
□ class 0 □ class 1 □ class 2 EB class 3 □ class 4 nclass 5 
Figure 2 
Difference in the foregut fullness index of the swimming crab Ovalipes punc- 
tatus in the swash zone of Koshirahama Beach, Otsuchi Bay between day 
and night. Class 0 = empty foregut, class 1 = trace items in foregut, class 2 = 
foregut 25% full, class 3 = 50%> full , class 4 = 75% full, class 5 = full. 
Discussion 
Ovalipes punctatus occurred nocturnally in the swash 
zone of Koshirahama Beach, Otsuchi Bay. The nocturnal 
occurrence in the swash zone for congeners such as O. tri- 
maculatus from South Africa (reported as O. punctatus, 
see Schoeman and Cockcroft, 1993) is often observed else- 
where (McLachlan et ah, 1979; Brown and McLachlan, 
1990). McLachlan et al. (1979) suggested that occurrence 
in the swash zone is related to feeding. Generally, Ovali- 
pes species are active nocturnally (Caine, 1974; Du Preez, 
1983). Our observations of foregut fullness and abundance 
support these findings. Furthermore, because peracarid 
crustaceans that live close to the sandy shoreline domi- 
nated the foregut contents (Kamihira, 1979; Takahashi 
and Kawaguchi, 1995; authors’ pers. obs.), we concluded 
that part of the crab population migrates from deeper 
waters into the swash zone at night to exploit the crusta- 
cean fauna of sandy beaches. 
Feeding in the swash zone is advantageous for the crabs 
because important prey, such as the amphipod H. japoni- 
cus, the mysid A. kokuboi, and the isopod E. chiltoni, are 
abundant in the swash zone of Koshirahama Beach. These 
species are considerably more abundant near the shore- 
line than other macrobenthic organisms that live in the 
offshore area (authors’ pers. obs.). The peracarids, especial- 
ly mysids and isopods, burrow into the sand during day- 
time, then emerge into water column in the swash zone 
at night (Takahashi and Kawaguchi, 1997; authors’ pers. 
obs.) which makes them more vulnerable to predation by O. 
punctatus and fishes (Takahashi et al., 1999). The predomi- 
nance of peracarid crustaceans in the foreguts of the crabs 
suggests that O. punctatus capture the peracarids effective- 
ly in the swash zone of Koshirahama Beach at night. 
Species of Ovalipes are known as opportunistic, broad- 
spectrum predators. They are extremely versatile, feed- 
ing on both less active prey, such as mollusks, and on 
mobile animals, such as amphipods, mysids, isopods, and 
fish (Caine, 1974; Haefner, 1985; Wear and Haddon, 1987; 
Ropes, 1989; Stehlik, 1993). 
The swash zone also serves as a refuge from predation. 
Species of Ovalipes (including congeneric species) are oc- 
casionally found in the guts of fish and decapod crusta- 
ceans (McDermott, 1983; Du Preez and McLachlan, 1984; 
Mitchell, 1984; Wear and Haddon, 1987; Stehlik, 1993). 
Cannibalism is a major cause of mortality in many por- 
tunid crabs during their early life stages; the importance 
of refuges has been emphasized as a factor regulating 
crab recruitment in natural populations (Hines and Ruiz, 
1995). In estuarine ecosystems, habitats with structural 
complexity, i.e. seagrass beds, mussel beds, filamentous 
algae, are effective as refuges for juvenile crabs, whereas 
risk of predation is higher in open sand, the main habitat 
of Ovalipes (Wilson et al., 1987; Ryer et al., 1997, Moksnes 
et al., 1998). In sand habitat, small Ovalipes burrow deep 
by using reverse gill current to hide from predators during 
the day (Barshaw and Able, 1990). 
Nocturnal emergence of Ovalipes makes them more vul- 
nerable to cannibalism. In the swash zone of Koshiraha- 
ma Beach at night, almost all the O. punctatus were ju- 
venile and immature crabs. This finding suggests that 
small individuals avoid the deeper zone, where the risk of 
cannibalism is high. For blue crabs, Callinectes sapidus, 
shallow water areas function as refuges from predation 
