NOTE Takahashi and Kawaguchi: Nocturnal occurrence of Ovalipes punctcitus 
511 
Table 1 
Details of the sampling dates and times (JST) and results 
in the swash zone of Koshirahama Beach, Otsuchi Bay. 
Date 
Day or 
night 
Sampling 
time (h) 
No. of crabs 
collected 
22 Aug 1995 
Day 
8:02 
3 
25 Aug 1995 
Day 
13:12 
2 
30 Aug 1995 
Day 
13:15 
3 
12 Sept 1994 
Night 
20:54 
30 
25 Aug 1995 
Night 
21:06 
23 
30 Aug 1995 
Night 
20:48 
35 
Food items were identified to the lowest possible taxon 
under a binocular dissecting microscope. The relative con- 
tribution of each prey category to the total volume of the 
foregut contents was assessed subjectively in the follow- 
ing way: a category representing 95-100% of the total con- 
tents was awarded 100 points; 65-95%, 75 points; 35-65%, 
50 points; 5-35%, 25 points; 5% or less, 2.5 points; empty, 
0 points. The points that each prey category received were 
weighted by multiplying by a factor that depended on the 
degree of foregut fullness, i.e. full = 1, 75% = 0.75, 50% = 0.5, 
25% = 0.25 and trace = 0.02 (Wear and Haddon, 1987). The 
maximum and minimum weighted points possible for a 
single category in a single foregut were 100 ( 100 x 1.0) and 
0.05 (2.5 x 0.02), respectively. The following percentages 
were calculated for each prey category (Williams, 1981): 
Percentage points for zth prey = 
V i 
100; and 
Percentage occurrence for zth prey = (6/AD100, 
where a tJ - the number of points for prey item i in the 
foregut of the yth crab; 
A = the total points for all the crabs and all the 
prey items in all the foreguts examined; 
N - the number of crabs examined with food in 
the foregut; and 
b t = the number of crabs with foreguts containing 
prey category i. 
Results 
Die! change in the swash zone occurrence pattern 
A total of 96 crabs (41 males, 55 females), ranging from 
12 to 80 mm CW, were collected from the swash zone of 
Koshirahama Beach (Fig. 1). No ovigerous females were 
collected. 
Almost all specimens were collected at night (88 indi- 
viduals, 92% of the total catch) and only 8 crabs were col- 
lected during the day (Fig. 1). Juvenile (35) and immature 
m 
ro 
u 
"D 
> 
c 
10 r- 
Day 
J I I L 
10 20 30 40 50 60 70 80 90 
Carapace width (mm) 
Figure 1 
Histograms of carapace width for the swimming crab 
Ovalipes punctatus found in the swash zone of Koshira- 
hama Beach, Otsuchi Bay, during the day and at night. 
All the specimens were collected in September 1994 or 
August 1995. 
(39) crabs dominated the nocturnal catches, accounting 
for 40% and 44% of the total nocturnal catch, respectively 
(Table 2). Fourteen adult crabs were also caught at night, 
constituting 16% of the nocturnal samples (Table 2). Day- 
time catches consisted of 1 immature and 7 juvenile crabs 
(Table 2). On average, crabs caught at night (36 mm CW) 
were significantly larger than crabs caught during the day 
(25 mm CW) (P<0.01; f-test). 
All crabs were in the intermolt or the papershell stages 
(early and late papershell); no soft-shelled and premolt 
crabs were collected. During the daytime, 7 out of 8 crabs 
were in the papershell stages and only one individual was 
classified as intermolt (Table 2). At night, 5 juvenile and 5 
immature papershell crabs were collected, accounting for 
14% and 13% of the total catch for each category, respec- 
tively (Table 2). Of the adults collected at night, 9 were 
in the papershell stages, accounting for 64% of the total 
adult crabs taken. 
Feeding activity and diet composition 
Because feeding habits of early and late papershell crabs 
are not different from those of intermolt stages in por- 
tunid crabs (Norman and Jones, 1992), analysis for fore- 
gut fullness and diet was conducted by pooling all molt 
stages. The percent frequency of each class of foregut full- 
ness is shown separately for day and night in Figure 2. 
Although the daytime sample was small, 50% of the crabs 
had empty foreguts (Fig. 2), but at night over 70% of the 
