The Distribution and Occurrence ofAcholoë squamosa(Polychaeta: Polynoidae) a Commensal with the Burrowing StarfishAstropecten irregularis(Echinodermata: Asteroidea)

The distribution and abundance ofAcholoë squamosa, a commensal with the burrowing starfishAstropecten irregularis, at 17 sites off the western and south-western coasts of the British Isles are described. A total of 4329Astropectenand 3362Acholoëwere examined. Population density ofAstropectenranged from 5 to 592 individuals ha−1with higher densities typically associated with finer-grained sandy or muddy sediments. There was considerable inter-site variability in colonization rates with maximum colonization approaching 90%. The occurrence ofAcholoëwas negatively correlated with the modal arm size of the host population and with water depth, factors which were themselves significantly correlated. Most adult worms occurred in the ambulacral grooves, and worms were sometimes observed with their head and anterior body region inside the mouth of the host, presumably feeding on stomach contents. Marked seasonal trends in abundance of theAstropectenpopulation were not accompanied by any comparable changes in the levels of colonization byAcholoë. Never more than one adult worm (i.e. >15 mm in length) occurred on the same starfish, indicating that intra-specific factors may control the distribution ofAcholoë; multiple colonization comprised a single adult worm with up to three juveniles (i.e. worms <15 mm). In late autumn and late spring to early summer, newly recruitedAcholoëwere found between the inferomarginal plates of the host where they were afforded protection by the over-arching ossicle spines. The sex ratio appeared to be strongly biased (20:1) towards females.