510 
Nocturna! occurrence of the swimming crab 
Ovalipes punctatus in the swash zone 
of a sandy beach in northeastern Japan 
Kazutaka Takahashi 
Kouichi Kawaguchi 
Ocean Research Institute 
University of Tokyo 
1-15-1 Minamidai 
Nakano, Tokyo 164-8639, Japan 
Present address (for K. Takahashi): Tohoku National Fisheries Research Institute 
3-27-5 Shinhama 
Shiogama, Miyagi 985-0001 Japan 
E-mail address (for K. Takahashi): issey@affrc.go jp 
Swimming crabs belonging to the genus 
Ovalipes (Crustacea: Brachyura: Por- 
tunidae) are distributed worldwide 
along sandy coastlines of subtropical 
and temperate waters (Stephenson and 
Rees, 1968). They are extremely well 
adapted to life on sand, with their 
strong swimming and burrowing abil- 
ities (Brown and McLachlan, 1990). 
Crabs of this genus are voracious carni- 
vores and significant predators of com- 
mercially important mollusks on sandy 
beaches (Du Preez, 1984; Brown and 
McLachlan, 1990). Recently, interest in 
their importance as a possible fisher- 
ies resource has increased (Sasaki and 
Kawasaki, 1980; Wear and Haddon, 
1987). 
Ovalipes punctatus (De Haan) is 
common in the coastal waters of Ja- 
pan and China. It is found on sandy 
bottoms below the intertidal zone, but 
peak abundance is at 5-60 m. This 
species has an ontogenetic migration 
into deeper water (Kamei, 1976; Sasa- 
ki and Kawasaki, 1980). Sasaki and 
Kawasaki (1980) investigated the life 
history of O. punctatus in Sendai Bay 
(100 km south of the present study 
site), on the Pacific coast of northeast- 
ern Japan. Female crabs spawn from 
mid-September until mid-November in 
offshore water 40-60 m deep. The in- 
cubation period of the eggs is about 20 
days. After a planktonic phase, the lar- 
vae settle on sandy bottoms in March 
and April. Settled crabs of both sexes 
grow to carapace widths (CW ) of 50-65 
mm in the first year. In the second 
year, females and males grow to 70-90 
and 80-85 mm CW, respectively. The 
life span of this crab is 2 to 2.5 years. 
The minimum size at maturity is es- 
timated to be 45-50 mm CW (Sasaki 
and Kawasaki, 1980), but the spawning 
population consists mainly of 2-year- 
old crabs of 75-80 mm CW. However, 
knowledge of the diet and feeding hab- 
its of this species has been extremely 
limited until now. 
Ovalipes species, including O. punc- 
tatus, generally live below the intertid- 
al zone. However, they are often found 
in the intertidal zone, especially at 
night (McLachlan et al., 1979; Brown 
and McLachlan, 1990). McLachlan et 
al. (1979) suggested that this occur- 
rence is related to feeding, but details 
are still unknown. During the course of 
an ecological study of the sandy beach 
in Otsuchi Bay, we frequently found O. 
punctatus in the swash zone at night. 
This note examines the significance of 
the swash zone in the life history of the 
swimming crab O. punctatus in rela- 
tion to feeding and molting. 
Materials and methods 
The study was conducted at Koshira- 
hama Beach, in Otsuchi Bay, on the 
Pacific coast of northern Honshu, main- 
land Japan, in September of 1994 and 
August of 1995 (Table 1). The beach 
is about 120 m long, bounded on both 
ends by rocky shores, and is catego- 
rized as a “sheltered beach” according 
to McLachlan’s (1980) rating scheme. 
Mean depth at 20, 50, and 100 m from 
the shoreline is 1.6, 3.5, and 5.5 m, 
respectively. The beach has a 1:13 (rise: 
run) slope and median particle diam- 
eter of the sediment is 260 pm. Other 
details of the study site are given in 
Takahashi and Kawaguchi (1995). 
Crabs were sampled with a beach 
seine 1.1 m high by 8.3 m wide with 
1.0-mm mesh. The net was hauled par- 
allel to the shoreline at a depth of 
ca. 0-1 m. Samples were taken along 
the entire shoreline of Koshirahama 
Beach. All crabs were preserved in 
10% formalin, and the carapace width 
(CW), sex, maturity and molt stage 
of each individual were recorded. The 
foregut was removed and stored in 
70% ethanol for dietary analysis. Sex- 
ual maturity of O. punctatus was as- 
signed by size classes as defined by 
Sasaki and Kawasaki ( 1980): juvenile 
(<30 mm CW), immature (31-50 mm 
CW), or adult (>51 mm CW). Molt 
stages were based on the criteria of 
Drach and Tchernigovtzeff (1967) and 
Norman and Jones (1992): 1) soft, no 
calcification of the new exoskeleton; 2) 
early papershell, thin, flexible exoskel- 
eton, easily depressed when touched; 
3) late papershell, hard exoskeleton 
except for the branchiostegite region 
which is compressible; 4) intermolt, 
completely hard exoskeleton; and 5) 
premolt, teeth on chelae well worn 
and having a less rounded appearance 
compared with those in earlier stages, 
complete exocuticle developed beneath 
the exoskeleton. 
The diet of O. punctatus was an- 
alyzed by using the points method 
and the percentage occurrence meth- 
od (Williams, 1981; Wear and Haddon, 
1987). The points method assesses di- 
et composition in terms of both foregut 
fullness and estimated volume of food 
in the foregut. First, the relative degree 
of foregut fullness of each crab was es- 
timated visually by using six ordered 
classes (empty=class 0; trace=class 1; 
25%=class 2; 50%=class 3; 75%=class 
4; full=class 5). A visual assessment 
of fullness was possible because, ex- 
cept for the gastric mill, the foregut of 
O. punctatus is like a thin-walled trans- 
lucent bag. 
Manuscript accepted 18 December 2000. 
Fish. Bull. 99:510-515 (2001) 
