Polydora and Related Genera Associated with Hermit Crabs from the Indo-West Pacific (Polychaeta: Spionidae), with Descriptions of Two New Species and a Second Polydorid Egg Predator of Hermit Crabs

Abstract

Polydora and related genera associated with hermit crabs from shallow subtidal coral reef areas of the Indo-West Pacific are described. Over 2000 hermit crabs were collected from localities in the Philippines and Indonesia between July 1997 and April 1999. In total, 10 species of polychaetes among five genera (Boccardia, Carazziella, Dipolydora, Polydora, and Tripolydora) were identified and described. Adult morphology of these species was investigated with light microscopy and scanning electron microscopy. The study includes the description of two new Polydora species, including the second known polydorid egg predator of hermit crabs. Six of the species burrow into calcareous substrata, living in burrows within live or dead gastropod shells or coralline algae attached to shells. Two species were found in mud tubes within crevices of gastropod shells inhabited by hermit crabs. The zoogeography and biodiversity of polydorids from the West Pacific are discussed. The diversity of polydorids from the Philippines is comparable with that of other central Pacific and Indo-West Pacific islands, but it is lower than that in areas of the North and Southwest Pacific; lower diversity probably reflects disparity in sampling efforts between these regions. A key to the Philippine polydorids is provided.

Polychaetes are common associates of hermit crabs, either living on, within, or burrowing into occupied gastropod shells (e.g., Jensen and Bender 1973, Stachowitsch 1980, Hoberg et al. 1982, Karlson and Shenk 1983, Walker 1995). In particular, Polydora and related genera (termed polydorids, nine genera of the family Spionidae that contain a modified fifth segment) are known to burrow into calcareous substrata including gastropod shells occupied by hermit crabs (Blake and Evans 1973, Read 1975, Radashevsky 1993, Blake 1996, Martin and Britayev 1998, Williams 2000). The burrowing behavior of these species has been investigated in substantial detail because of their effects on commercially important mollusk species (see Radashevsky and Williams 1998). Polydorids have been considered to be facultative or obligate commensals of hermit crabs, but recent research indicates that the worms can have negative effects on their hosts (Buckley and Ebersole 1994, Williams 2000). The purpose of this investigation was to examine the systematics and ecology of polydorids associated with hermit crabs from coral reef areas of the Indo-West Pacific.

The Spionidae is one of the most conspicuous polychaete families of Indo-West Pacific islands, in many areas second only to the family Syllidae in number of recorded genera (Knox 1957, Bailey-Brock 1995, Paxton and Chou 2000). Polydorids are common inhabitants of coral reef areas, and some species are bioeroders found to burrow in live and dead corals and mollusk shells (Hartman 1954, Bailey-Brock 1995). In spite of the amount of work completed in the North and Southwest Pacific (see Blake and Kudenov 1978, Radashevsky 1993, Sato-Okoshi 1999, Radashevsky and Hsieh 2000a,b), polydorids from central Indo-West Pacific areas such as the Philippines remain largely unknown. Before this investigation only two polydorid species had been identified from the Philippines, although a number of faunistic surveys of the polychaetes from the Philippines and surrounding areas have been completed (Pillai 1965, Williams 2000). Part of the success of polydorids is due to the possession of long prehensile palps allowing the worms to suspension and deposit feed or a combination of both (Williams and McDermott 1997). Other taxonomically important features of these worms include major spines of the fifth segment, branchial distribution, neuropodial hooded hook morphology, and pygidium morphology.

This paper represents part of a series of studies on the systematics, ecology, and feeding biology of polydorids associated with hermit crabs from the Indo-West Pacific. The first paper provided the description of Polydora robi, an obligate commensal of hermit crabs from the Philippines and Indonesia (Williams 2000). Polydora robi was documented to ingest the embryos of host hermit crabs, and the reproduction and larval development of the species have been investigated (Williams 2001). The study reported here provides records of 10 species of polydorids from the Philippines and Indonesia, nine of...