Decapod Crustaceans of the Headwater Streams of Pohnpei, Eastern Caroline Islands, Federated States of Micronesia

Abstract

Two species of Macrobrachium (Palaemonidae) and three atyid shrimp (Atyidae) species were identified among 986 decapod crustaceans collected in headwater streams on Pohnpei, Micronesia, during 1999 and among incidental collections in 2000. None is endemic to the island; all are indigenous or at least not of recent human introduction, and all occur widely in the Indo-West Pacific region and have a diadromous life history pattern. Both Macrobrachium lar (Fabricius, 1798) and M. latimanus (Von Martens, 1868) are common in Pohnpei rivers, but M. latimanus outnumbers M. lar especially in the uppermost reaches. Atyoida pilipes (Newport, 1847) accounted for 72% of the atyid sample, and it was the only decapod recorded at elevations as high as 604 m; Caridina weberi (De Man, 1892) composed 21% of the sample and C. typus (H. Milne Edwards, 1837) 6.4%. Ovigerous females were collected throughout the year for three of the five species; the absence of ovigerous Macrobrachium lar and Caridina typus during August-November was possibly due to small sample sizes. Freshwater shrimps do not currently form an important part of the diet of Pohnpei islanders, but historical accounts suggest that shrimps were utilized more in the past when imported foods and advanced technology for harvesting marine resources were not readily available.

Information on the distribution and abundance of the freshwater biota of Micronesian islands is scanty and widely scattered in the literature. Maciolek and Ford (1987) furnished the first comprehensive inventory of stream macrofauna on Pohnpei. They reported on 44 species, including 10 decapod crustaceans (seven atyids, two palaemonids, and one grapsid crab); their samples were largely from the Nanpil-Kiepw River, and at elevations below about 200 m. No other crustaceans were added to this list during subsequent surveys of the Nanpil and Lehn Mesi Rivers (also at sites below about 200 m) in 1996 (Nelson et al. 1996); there have been no other surveys. This study is the first report on the distribution of freshwater decapods in headwaters (mainly above 200 m) on Pohnpei, and it is based largely on surveys of six rivers during February-November 1999, with ancillary material from March and April 2000. New information is presented on the distribution, relative abundance, morphology, and reproductive condition among two species of Macrobrachium (Palaemonidae), M. lar (Fabricius, 1798), and M. latimanus (Von Martens, 1868), and three atyid shrimps (Atyidae), including Atyoida pilipes (Newport, 1847), Caridina typus (H. Milne Edwards, 1837), and C. weberi (De Man, 1892).

Study Area

Pohnpei is a moderately sized (355 km² [MacLean et al. 1986]), ruggedly mountainous, tropical Pacific island located 766 km north of the equator in the eastern Caroline Islands, Federated States of Micronesia (Figure 1). Maximum elevations in the central highlands are just under 800 m (U.S. Army Corps of Engineers 1986). Densely forested steep ridges and deep valleys give rise to several major watersheds and radiate outward and downward to the coastal lowlands and surrounding mangroves. A discontinuous barrier reef encloses a lagoon up to about 6 km wide. The average annual air temperature is 27°C, and the average monthly temperature does not vary from the annual average by more than 1° (Laird 1982). Mean annual rainfall in Kolonia, the main settlement (on the northern coast), is approximately 485 cm, with estimates for the central highlands ranging to 1015 cm (Merlin et al. 1992).

The vegetation is largely rain forest dominated by dohng (Campnosperma brevipetiolata) and sadak (Elaeocarpus carolinensis) with patches or stands of the endemic kotop palm (Clinostigma ponapensis). Tree ferns (Cyathea spp.) are common in the understory, and epiphytic mosses and ferns festoon tree trunks, limbs, and branches. Trustrum (1996) reported a decline in native upland forest from about 42% of vegetation cover in 1975 to about 15% in 1995, attributing it largely to clear-cutting of numerous, small plots for cultivating sakau (kava), Piper methysticum. The crushed roots of this plant yield a beverage consumed for its psychoactive properties, and which has long been a part of Pohnpeian tradition and culture. In recent...