Zootaxa, Annelida, Myzostomida

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Zootaxa 1010: 53–64 (2005) ISSN 1175-5326 (print edition) www.mapress.com/zootaxa/ ZOOTAXA 1010 Copyright © 2005 Magnolia Press ISSN 1175-5334 (online edition) Myzostoma seymourcollegiorum n.sp. (Myzostomida) from southern Australia, with a description of its larval development GREG W. ROUSE1 & MARK J. GRYGIER2 1 Marine Invertebrates, South Australian Museum, North Terrace, Adelaide, SA 5000 Australia & School of Environmental & Earth Sciences, The University of Adelaide, SA 5005 Australia. E-mail: [email protected] 2 Lake Biwa Museum. Oroshimo 1091, Kusatsu, Shiga, 525-0001 Japan. E-mail: [email protected] Abstract Few Myzostomida have been recorded from southern Australian waters. Most myzostome taxa to date have been described from the warmer waters of the Indo-Pacific, where their crinoid echino- derm hosts are most diverse. In this paper a new myzostome, Myzostoma seymourcollegiorum n. sp., is described from the crinoid Cenolia trichoptera (Comasteridae) taken from Encounter Bay near Adelaide, South Australia; further records suggest its range extends also to Tasmania and around eastern Australia to central New South Wales. Myzostoma seymourcollegiorum n. sp. is an ectocommensal on C. trichoptera and C. glebosus and there has previously been no myzostome recorded from any species of Cenolia. The new species resembles a number of other Myzostoma species that are quite flat and have a nearly circular shape with 20 short marginal cirri. The early stages of its larval development are also outlined and compared with those of other Myzostomida. Key words: Crinoidea, Myzostomida, larval development, symbiont Introduction Myzostomida comprises a small taxon of around 150 nominal species. There is an ongoing debate on the overall position of the group, with some authors favoring a position among polychaetes (e.g. Rouse & Pleijel 2001), while others favour a placement outside annelids and closer to platyhelminths (Eeckhaut et al. 2000) or rotifers (Zrzavy et al. 2001). Myzo- stomida are all obligate symbionts, mainly with crinoids but some with other echinoderms. Many are mobile and roam over the host, stealing food from it, but others remain sessile near a convenient ‘feeding’ site. Some induce the host to form galls or cysts around them, while others are endoparasitic, living in the gut lumen, coelom, or gonads of their host Accepted by B. Sket: 14 Jun. 2005; published: 21 Jun. 2005 53.

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