Timing and Synchronization of the Breeding Period in Pilumnus vespertilio (Crustacea: Pilumnidae) in Subtropical Okinawa, Japan

Abstract

Observations on the temporal occurrence of copulating pairs, ovigerous females, monthly brooding periods, and embryo development stages in Pilumnus vespertilio Fabricius were carried out in the wild on Okinawa Island, Japan. The relationship between the female gonad and hepatopancreas during a reproductive period was also studied. These reproductive activities were observed in relation to the lunar cycles. An inverse relationship between mass of the hepatopancreas and development of the gonad was observed. Mating and breeding activities were perfectly synchronized with the lunar periodicity. Five broods from May to September were observed in a single breeding period, and the average brooding period was 21.2 days with an interlude of 8.5 ± 3.1 days between broods/months. The shortest interlude was between May and June (4 days). A sixth brood that started in October was not followed to the end because it started with very few ovigerous females. Embryo development time in days decreased with each stage and averaged 5.3 days per stage. Copulating activity and appearance of ovigerous females during successive broods (months) were clearly synchronized with the lunar cycle. Although copulating frequency was highest after the full moon, nearly 100% of females were ovigerous around the new moon. All females of any one sample carried eggs of the same development stage. All females released their larvae 1-3 days before full moon, coinciding with a high tide. Larvae are probably released during this time as a survival strategy against predators such as planktivorous fish and against adverse intertidal conditions during other times.

In general, brachyuran breeding periodicity can vary considerably; the major controlling factors appear to be latitude, temperature, larval food availability, and intertidal zonation (Sastry 1983). The tendency is toward extended reproductive seasons and continuous reproduction with decreasing latitude in benthic shallow-water crustaceans. With increasing latitudes, breeding seasons become more restricted to periods of higher water temperatures (Giese and Pearse 1974, Sastry 1983, Kyomo 1986). The effect of intertidal zonation on breeding periodicity has been shown in tropical brachyuran species Sesarma messa, S. smithi, and S. erythrodactyla. These species do not breed all year round but for 3, 3, and 5 months of the year, respectively, and their reproductive strategies are related to their positions on the shore (Greenwood and Fielder 1988). Macrophthalmus grandidieri, a shallow-water tropical species, breeds continuously year-round, whereas various species of Uca in the same area but of different zonation do not (Emmerson 1994). For one reason or another, most of the studies on brachyuran reproductive ecology have been reported on different species of only these families: Grapsidae (e.g., Hiatt 1948, Seiple 1979, Saigusa 1981, Kyomo 1986, Omori et al. 1997, Tsuchida and Watanabe 1997), Ocypodidae (e.g., Christy 1978, Zucker 1978, Wada 1981, Henmi and Kaneto 1989, Kosuge et al. 1994), Portunidae (e.g., Archambault et al. 1990, Norman and Jones 1993, Norman 1996), and Majidae (Tsuchida and Watanabe 1991).

Very few reproductive studies, however, have been done on species of the family Xanthidae; for example, Neopanope sayi (Swartz 1978, De Vries and Forward 1989), Panapeus herbstii and Eurypanopeus depressus (McDonald 1982), Leptodius exaratus (Watanabe et al. 1990), and Eriphia smithii (Tomikawa and Watanabe 1992). Knudsen (1960) described reproduction, life history, and larval ecology of four xanthids: Lophopanopeus l. leucomanus, L. bellus diegensis, Paraxanthias taylori, and Cycloxanthops novemdentatus in California. Pilumnus vespertilio Fabricius was a member of this family until very recently (Kyomo 1999, 2001). Notwithstanding, none of these studies on xanthids has reported on the relationship between breeding activities and the lunar cycles.

The only reported studies on Pilumnus vespertilio (Pilumnidae) include its geographical distribution (Sakai 1976), larval development (Lim and Tan 1981), feeding patterns and food habits (Kyomo 1999), and reproductive behavior (Kyomo 2001). The distribution of P. vespertilio ranges widely from Japan, central Pacific to Australia and New Zealand, various areas of the Indian Ocean, and the east coast of Africa (Sakai 1976). In Japan, this species is found in various localities of Sagami Bay and Okinawa Island (Sakai 1976). Preliminary surveys for this study on Okinawa showed that the...