Myzostoma seymourcollegiorum, Rouse & Grygier, 2005

Rouse, Greg W. & Grygier, Mark J., 2005, Myzostoma seymourcollegiorum n. sp. (Myzostomida) from southern Australia, with a description of its larval development, Zootaxa 1010 (1), pp. 53-64 : 54-62

Myzostoma seymourcollegiorum n. sp.

Material examined ( in 70% ethanol unless otherwise stated): Holotype: ( SAM E3443 )

found on Cenolia trichoptera at 2 m depth off Encounter Bay (near The Bluff), Fleurieu Peninsula, South Australia (35°35.417’S, 138°36.168’E), coll. G. Rouse, 1­2­2004. Paratypes: ( SAM E3444) 5 specimens, same collection details as holotype; ( SAM E3445 ) 2 specimens found on Cenolia trichoptera at 2 m depth off Encounter Bay (near The Bluff), Fleurieu Peninsula , South Australia (35°35.417’S, 138°36.168’E), coll. I. Eeckhaut, N. Wilson and G. Rouse, 20­9­2003; ( SAM E3446 ) 5 specimens found on Cenolia trichoptera , subtidal Partney Shoal, Sir Joseph Banks Group, Spencer Gulf , South Australia, coll. N. Holmes, 21­1­1986; ( SAM E3447 ) 7 specimens on SEM stub; same collection details as holotype; ( SAM E3448 ) 4 specimens on SEM stub, found on Cenolia trichoptera at 2 m depth off Encounter Bay (near The Bluff), Fleurieu Peninsula , South Australia (35°35.417’S, 138°36.168’E), coll. I. Eeckhaut, N. Wilson and G. Rouse, 20­9­2003; ( SAM E3449 ) approximately 50 larvae on SEM stub; same collection details as holotype.

Other material: ( SAM E3450) 5 specimens, found on crinoid at Port Willunga, Gulf St. Vincent, South Australia, coll. H.M. Laws, 8­12­1971; ( SAM E3451 ) 1 specimen, found on crinoid under rock at low tide, Aldinga Reef, Gulf St. Vincent, South Australia, coll. W. Zeidler & S. Edmonds, 13­11­1980; ( SAM E3452 ) 1 specimen, found on crinoid intertidally under rock at St. Francis Island, Nuyts Archipelago , South Australia, coll. W. Zeidler & A. Gackle, 25­1­1982; ( SAM E3453 ) 1 specimen, found on crinoid intertidally in rockpool at St. Francis Island, Nuyts Archipelago , South Australia, coll. W. Zeidler & P. Aerfeldt, 26­2­1983; ( SAM E34554 ) 12 specimens from crinoid, 6 m depth at Edithburgh Jetty, Yorke Peninsula, Gulf St. Vincent, South Australia, coll. N. Holmes, 23­3­1988; ( SAM E3455 ) 30 specimens, found on crinoids at 2 m depth off Encounter Bay (near The Bluff), Fleurieu Peninsula , South Australia (35°35.417’S, 138°36.168’E), coll. K. Gowlett­Homes & S. Parker, 18­7­1988; ( SAM E3456 ) 3 specimens, found on crinoid under rock at Tinderbox, D’Entrecasteaux Channel , Tasmania, coll. K. Gowlett­Holmes, 2­2­1992; ( AM W22069) 1 specimen on crinoid, AMJ 14790, Cenolia trichoptera at 8 m, Buccewarca Point, Batemans Bay , New South Wales, Australia (150°15’E, 35°44’S), coll. L. Vail & V. Harriot, 14­3­1981; ( AMW 22074) 1 specimen from crinoid, AMJ 14403, Cenolia trichoptera at 24 m . Buccewarca Point, Batemans Bay , New South Wales, Australia (15015’E, 3544’S), coll. L. Vail & V. Harriot, 16­3­1981; ( AM W22096) 1 specimen from crinoid, AM J10933 , Cenolia glebosus , at 12 m, Groper Island, Solitary Islands , New South Wales, Australia (153°14’E, 30°15’S), coll. J. Ogg, 19­8­1977; ( AM W5547 ) 2 specimens from crinoid, AMJ 7890, Cenolia trichoptera , at 30 m, Old Lighthouse, Jervis Bay , New South Wales, Australia (150°48’E, 35°06’S), coll. N. Coleman, 21­4­1973; ( AM W194999 ) 14 specimens, host unknown, at 5 m depth at Western River Cove, Kangaroo Island , South Australia, Australia (138°03’E, 35°50’S), coll. K. Handley, 3­3­1978; ( AM W194995 ) 18 specimens from Cenolia , western edge of Long Reef, Sydney , New South Wales, Australia (33°47’ 50”S 151°26’ 00”E), coll. J.B., 5­2­1966 .

Etymology. This species is named in honour of the students and staff of Seymour College, Adelaide, South Australia.

Diagnosis. Moderate­sized myzostomes with flattened, almost perfectly circular body. Dorsal body surface smooth, ventral surface with ciliated patches. Colour in life varies from uniform dark brown to light brown to yellow, but generally slightly translucent, especially on body margin. Ten pairs of short, conical marginal cirri; these contractile, all of similar length when extended. Introvert with between 4 and 7 (usually 6) pairs of pointed buccal papillae; introvert pouch opening in front of first parapodia, close to body margin. Parapodia arranged in circle slightly ahead of mid­body region. Parapodia cirrate, elongate, in sockets in cylindrical bases. Penes prominent when extended, emerging from outer bases of 3 rd parapodia. Fours pairs of large, conspicuous lateral organs located between parapodia and body margin. Emergent hooks of parapodia with moderately thick shaft, tip curving to 90° with respect to shaft. Replacement hooks, one or two per parapodium, with shorter shafts than emergent hooks. Acicula shorter and thinner than emergent hooks, manubrium on one side only. Ectocommensals on Cenolia spp.

Description. Holotype with thin, flattened, almost perfectly circular body 3.4 mm in diameter ( Figs 1 A­C), free­living on arm of crinoid Cenolia trichoptera . Ventral body surface flat with extensive ciliated patches ( Figs 1E, F, I). Colour in life dark brown to purple dorsally, ventrally mottled purple to translucent, especially around body margin ( Fig. 1C). Colour of preserved holotype white, translucent. Ten pairs of short, conical marginal cirri, all of similar length when extended (150 µm), equidistantly distributed around margin ( Figs 1 A­C). Cirri contractile (to 50 µm), with marked wrinkles and sitting in indentation on outer body ( Fig. 1G). Opening of introvert pouch small and ventral, closer to anterior margin than to parapodia 1 ( Fig. 1E) and at level with ‘ring’ of lateral organs. Introvert capable of protruding 0.9 mm ( Figs 1C, D). Thirteen pointed buccal papillae, thickness and length varying, around distal introvert margin ( Figs 1C, D). Gut branches stop at level of lateral organs, leaving moderately wide, translucent edge to body ( Fig. 1C). Cloacal opening for digestive tract and female genital duct located two­thirds of way from last

parapodia to rear body margin ( Figs 1C, E). Parapodia, five pairs, arranged elliptically, centred slightly ahead of mid­body ( Figs 1C, E). Parapodia each lying about one­third in from body margin to centre ( Figs 1C, E), with anterior pair closer to each other than posterior pair. Parapodia cirrate; in socket in cylindrical base elongate, finger­like distal part, extensible to 300 µm ( Figs 1C, F, H). Parapodial cirri low, hemispherical lobes on distal part of each parapodial base ( Figs 1E, F). One pair of penes at lateral bases of parapodia 3, capable of extending at least 200 µm ( Figs 1 C, E, J). Lateral organs in four pairs, large, with openings 100 µm across, visible as round holes between and outside of parapodia ( Fig 1D), forming ring adjacent to translucent body margin ( Figs 1C, E, I). Each parapodium with emergent hook, replacement hooks, and acicula. Emergent hooks up to 650 µm long, shafts moderately thick, with distal fifth bowed slightly outward; tip short, sharp, curving 90° ( Figs 1H, 2A). Replacement hooks 500­600 µm long, one or two per parapodium, with shorter shafts than emergent hooks, otherwise similar ( Fig. 2B). Acicula (support rod) of each parapodium longer than emergent hook, 720 µm long, and two­thirds as thick ( Fig. 2C), coming to sharp point ( Fig. 2D); manubrium developed on one side of shaft ( Fig. 2D), with small ridges along outer margin.

Variation. Paratypes range from 1 to 3 mm in diameter. Colours in life ranged from yellow to dark purple brown, with some specimens showing an intermediate ring of lighter colour ( Fig. 1B). A darker spot of pigment is also often seen over the penis regions. Specimens left in dishes overnight were found to have irregularly shaped, large (up to 200 µm), flat spermatophores attached to them, usually on the dorsal surface ( Fig. 1A). The spermatophores lacked the obvious ‘arms’ as described by Jägersten (1939a) and Eeckhaut & Jangoux (1991) for Myzostoma cirriferum Leuckart , but have lobes at the distal end. One of the paratypes ( SAM E33446 ) had only four pairs of parapodia, and fewer marginal cirri than normal. Most specimens had the introvert withdrawn so papillae counts were not possible. Four specimens from Kangaroo Island ( AM W194999) showed 8, 11, 13, or 14 buccal papillae respectively. One of these specimens was 3.8 mm in diameter.

Remarks. Myzostoma is a large, heterogenous genus containing over 120 described species ( Grygier 2000; Rouse & Pleijel 2001). There has not been a detailed revision of the group. However, Grygier (1990) identified a series of informal groupings within Myzostoma based on body shape, lifestyle, cirri and parapodial shape and distribution. He called these the crosslandi ­group, the platypus ­group, the costatum ­group, the ambiguum ­ group, and the wyvillethomsoni ­group. These groupings represented around 25% of the described Myzostoma species and Myzostoma seymourcollegiorum n.sp. clearly does not fall within any of them. Rather, M. seymourcollegiorum shows most similarities to a range of Myzostoma that have a circular body shape, smooth dorsum, and ten pairs of short to moderate­length cirri. Grygier (1992) also identified what he referred to as the M. gigas species group containing M. gigas von Graff , M. fimbriatum von Graff , M. giganteum Nansen , M. pseudogigas (Jägersten) and M. coriaceum von Graff. Grygier (1992) discussed these species in relation to M. pallidum von Graff and M. triste von Graff. These species are more or less circular with 20 very short cirri but have opaque, semi­lenticular bodies unlike M. seymourcollegiorum and need not be considered in detail here. However, of the gigas ­species group, Myzostoma pallidum from the Philippines would appear, from von Graff’s (1877) description, to be the most similar to M. seymourcollegiorum , except that the introvert is longer in the latter and the cirri are longer (200 µm) in the former for similar­sized specimens. Myzostoma triste , also from the Philippines, was described from a single specimen, 4.5 mm across, that has a granular dorsal surface and no marginal translucent zone ( Grygier 1992). Material from Hong Kong, Philippines, and Saipan referred to as Myzostoma sp. by Grygier (1992), and discussed by him with reference to M. pallidum and M. triste , is similar to seymourcollegiorum but seems closer to M. pallidum . Other Myzostoma that could be considered with the M. gigas ­group in having a circular body, smooth dorsum and 20 short to moderate length cirri, but with partially transparent bodies as in M. seymourcollegiorum , include M. brevicirrum von Graff , M. brevipes von Graff , M. cerriferoidum McClendon , M. pictum von Graff , and M. stochoeides Atkins.

Myzostoma brevicirrum , described from Tonga, differs from M. seymourcollegiorum in that its cirri are very short and wart­like and in that the largest specimen studied by Graff (1884) was only 0.5 mm across. The introvert opening in M. brevicirrum also lies outside the lateral organ ring, unlike the situation in M. seymourcollegiorum . Myzostoma brevipes , from the Caribbean, has very short parapodia and the dorsal surface has fine peripheral folds that match the branching of the intestine ( Graff 1883; Graff 1884), both features not seen in M. seymourcollegiorum . Myzostoma cerriferoidum , also from the Caribbean, has very small lateral organs ( McClendon 1907) and otherwise was so briefly described and not illustrated as to preclude any further comparison. Grygier (pers. obs.) has studied the types of M. cerriferoidum (USNM 5780) and this species may well be synonymous with M. brevipes , although the types of the latter are lost (Grygier pers. obs.). Myzostoma pictum from Montserrat in the Caribbean was described briefly by von Graff (1883) and in further detail by von Graff (1884). Only gross morphology and coloration were recorded, and the ventral side was not examined in detail, but M. pictum has a clear dorsal longitudinal crest and longer marginal cirri than M. seymourcollegiorum .

Myzostoma stochoeides from the Torres Strait (northern Australia) differs markedly from M. seymourcollegiorum in that its parapodia lie halfway between the body centre and outer body margin ( Atkins 1927). Material identified as M. stochoeides by Eeckhaut et al. (1998), which has extended this species’ range from Singapore to around the northern coast of Australia, also shows a wide marginal zone and is not referable to M. seymourcollegiorum . The presence or absence of parapodial cirri is not considered important in the present discussion since the structures were very small in M. seymourcollegiorum and only visible under SEM. Similar small parapodial cirri may be present in some of the Myzostoma species discussed above but have never been noted in any of them.

There has been no previous record of myzostomes on any species of Cenolia Clark. This genus of crinoids is restricted to Australian and New Zealand waters ( Rowe & Gates 1995) and currently contains five species. Cenolia trichoptera (Müller) has a range along the southern coast of Australia from Western Australia to the northern coast of New South Wales and around Tasmania ( Rowe & Gates 1995). Our examination of museum collections has revealed that M. seymourcollegiorum can be said to largely match its host distribution and to occur in South Australia, Tasmania, and New South Wales. For the northernmost NSW record (AM W22096) from the Solitary Islands, the host is not Cenolia trichoptera , but Cenolia glebosus Rowe et al. , which has a range starting in southern New South Wales and extending north into tropical waters ( Rowe & Gates 1995). However, examination of the specimens (Grygier pers. obs.) was superficial and in­depth examination of fresh specimens from C. glebosus is desirable to confirm conspecificity with those from C. trichoptera . Myzostoma seymourcollegiorum is not present in the myzostome holdings at the Museum and Art Gallery of the Northern Territory (Darwin), Queensland Museum (Brisbane), Tasmanian Museum & Art Gallery (Hobart) or Western Australian Museum (Perth), with one possible record each in Museum Victoria and Natural History Museum, London holdings (Grygier pers. obs.; specimens not removed from crinoid bottles for close inspection).

Larval development. Specimens maintained in petri dishes filled with filtered seawater spawned fertilized eggs spontaneously over several days. The eggs were spherical to slightly ellipsoidal, measuring 50 by 60 µm (n =5). After several hours, 60 x 70 µm (n = 5) embryos were present ( Fig. 2F). Timing of the development of embryos was not followed since temperature was not controlled, although the entire process outlined below took place over only 3 days. The embryos developed into protrochophores with an apical tuft and an equatorial band of scattered tufts of cilia that may represent a prototroch ( Fig. 2G). No mouth opening was visible at this stage. The larvae began to elongate at this stage until they reached 125 µm and developed two sets of long capillary chaetae that emerged behind the anterior ciliary band (prototroch?) ( Figs 2H, J). A posterior band of cilia, inter­ preted here as a telotroch, was visible on these later­stage larvae. The chaetae, reaching up to 150 µm in length, numbered 6­8 in each bundle and were ‘barbed’ along most of the length. The latest larval stages observed, seen in Figures 2H and K, showed the development of a second pair of chaetal bundles behind the first pair. These had much shorter and thinner chaetae. A mouth was visible ventrally at this stage with cilia clearly in front of the opening ( Fig. 2K). The larvae died shortly after reaching this stage.

The development outlined above is similar to that reported for the few other myzostomes studied to date ( Beard 1884; Eeckhaut & Jangoux 1992; Eeckhaut & Jangoux 1993; Jägersten 1939b; Kato 1952; Eeckhaut et al. 2003). The differences among the previous studies have largely had to do with larval ciliary bands. In Myzostoma cirriferum and M. alatum von Graff , an anterior ciliary ring develops behind the oral opening ( Beard 1884; Eeckhaut & Jangoux 1992; Eeckhaut et al. 2003). In M. glabrum Leuckart ( Jägersten 1939b: as M. parasiticum nomen nudum) and a species erroneously referred to as Myzostoma ambiguum von Graff by Kato (1952), there are two anterior ciliary bands, one preoral and one postoral. Myzostoma seymourcollegiorum also appears to have two ciliary bands. The term prototroch can be defined as a ring of (usually compound) cilia that is derived from a group of specific cells, called trochoblasts ( Damen & Dictus 1994), and positionally it lies anterior to the mouth opening. No detailed developmental studies have been done to date to determine whether the cells giving rise to this anterior ciliary band are in fact trochoblasts. Eeckhaut et al. (2003) referred to the single postoral band in M. cirriferum as a metatroch and showed that a prototroch is absent in M. cirriferum . Given that Rouse (1999) defined trochophore larvae by the presence of a prototroch, this implies that M. cirriferum and M. alatum lack trochophores, yet other myzostomes may have them. Clearly further study is required. The larval development outlined here for M. seymourcollegiorum is unique in one respect; no other myzostome has been described with two pairs of chaetigers.

Further studies on the development of M. seymourcollegiorum are planned to understand the nature of the development of its larvae, especially with regards to the ciliary bands. This may have significant implications with regards to our understanding of the overall phylogenetic position of Myzostomida .