Eisothistos poseidon, Knight-Jones & Knight-Jones, 2002

Knight-Jones, E. W. & Knight-Jones, Phyllis, 2002, Four new species of Eisothistos (Anthuridea: Isopoda) from tubes of Spirorbidae (Serpuloidea: Polychaeta), Journal of Natural History 36 (12), pp. 1397-1419 : 1408-1413

Eisothistos poseidon sp. nov.

(gures 5–7)

Material. Post-partum female holotype and ve paratypes, one brooding two embryos, all from tubes of Pileolaria militaris collected in big intertidal pool at Cape Schanck, 38ss30¾S, 144ss00¾E, 7 May 1983 . Juvenile in spirorbid tube from West Point, Flinders , 38ss30¾S, 145ss00¾E, 8 May 1983 . Two mature females and two males in and among spirorbid tubes from depths 3–4 m in Two Fold Bay , 37ss07¾S, 144ss57¾E near Eden, NSW, 19 May 1983 . Three females in spirorbid tubes from Wynyard , 40ss59¾S, 145ss41¾E, collected 3 May 1983 . One in tube of Protolaeospira tri abellis Knight- Jones from 5–10 m depths oV Bruny Island , 43ss05¾S, 147ss25¾E, 29 April 1983 . One brooding seven eggs in tube of Protolaeospira tricostalis Lamarck from 1 m depth on north side of Port Arthur harbour, ca 43ss08¾S, 147ss50¾E, 25 April 1983 .

Description. HOLOTYPE. 2.2 mm long, tail-fan 0.5 mm wide but rest of body very slender (gure 5A). Each eye with ve or six ocelli. Telson with longitudinal row of six keel spines, hind margin serrated to form pointed teeth separated by angular notches, from some of which setae originate. Largest pair of these marginal setae are plumose. Notches from which these plumose setae arise delimit transverse adcentral row of four to six marginal teeth (four in holotype), which may vary in size irregularly and may include small setae (one asymmetrical seta in holotype) arising in gaps between. Laterally marginal teeth become smaller, extending round telson sides but still pointing posteriorly. Each plumose seta is accompanied by small companion seta, arising in same notch. In next notch laterally, on each side, arises a fairly large but not plumose seta, and in the next there is a third seta of no more than average size on each rounded ‘corner’ of the telson.

Each antennule basal article with leaf-like process which normally points anteriorly (gure 5A), but may protrude prominently if antennule is re exed strongly (gure 6A); article 4 shorter than neighbours, articles 5–9 tapering, without aesthetascs, article 9 with two or three small terminal setae. Antennae with three basal articles (article 3 the longest) and six agellar articles, several with small setae. Pereopods seven pairs, without ‘wheel-joints’ between propodus and dactyl. Ischia in pereopods 2 and 3 (shown enlarged in gure 5B, C) each with a long seta and seven or eight pointed studs distally. Pereopod 3 ischium also has three wart-like studs proximally (gure 5C). Each uropodal exopod and endopod has three large fangs pointing distally, with four to seven small teeth in each embayment separating the fangs. In gure 5A (ventral view of tail-fan) left fang of right exopod is obscured behind pleon and pereonite 7.

PARATYPES. Four post-partum females, like holotype but smaller (lengths 1.7–2 mm), yet telson keel tends to have more spines (from six to 12) and hind margin usually more teeth in adcentral row (three to eight). Small setae tend to arise with bilateral symmetry in gaps between those teeth, e.g. two each have single seta in central gap (as in gure 5F), whilst others each have two setae, one on each side of a central or nearly central tooth (gure 5G). Hatching embryo ca 0.7 mm long (gure 5E), one of two with a female 1.9 mm long, has tips of most limbs covered by membranous sleeves and does not show the characters of the species. One large female 2.3 mm long, like holotype but with tail-fan 0.9 mm wide, hind telson damaged, body plump and limbs of the premating type (i.e. pereopods with wheel-joint between each propodus and dactyl—see gure 6A, C), has an aesthetasc on each antepenultimate and penultimate article of antennules, and two or three terminal setae on article 9. Most antenna articles bear two or three setae. This specimen had antennules and antennae re exed posteriorly, with processes from antennule bases protruding dorsally like ears of a mouse (gure 6A).

Juvenile from West Point, Flinders, 1 mm long and still a manca, lacking pereonite 7 (gure 6D), with only seven studs on ischium of pereopods 3 (gure 6F), but with antennules 1 and 2 (gure 6E) already typical of mature females (gure 6A) and a tail-fan typical of the species.

The two males from Two Fold Bay, 1.5 and 1.4 mm long, had large eyes each with at least six protuberant ocelli, numerous aesthetascs mostly arising from basal article 3 of each antennule, many scales on propodus of each pereopod 1, and pleonites 1–3 each about as long as pereonite 7, which is about half length of other pereonites (gure 7A). Each uropod has three endopod teeth larger than their neighbours (gure 7B), as in tridents of females. Hind telson (gure 7C) bears two small dorsal setae in adlateral positions. Smaller male, hind body viewed obliquely from side (gure 7D), shows uro-exopods with at least two fangs and many pleopods, including inner rami of both pleopods 2 diverging from nearly mid-ventral origin on pleonite 2. A copulatory stylet (indicated by an arrow) lies alongside each pleopod 2 inner ramus and extends a short distance distally (as in E. petrensis Kensley, 1984 , gure 6d).

Five Tasmanian females from Wynyard to near Hobart , noted above, were identi ed to species by their uropods, which each bore two tridents. Eggs in brood from Port Arthur were oval, diameters 0.2Ö 0.26 to 0.22 Ö 0.3 mm, and full of fat globules (gure 5D) .

Etymology. Named after Greek sea-god, who bore a trident, like each uropodal ramus of this species.

Remarks. Numbers of mid-line keel spines on female telson (not seen in males) varied from six to 12. Numbers of teeth in adcentral row between plumose setae on hind telson varied from three to eight (cf. gure 5G, F). One or two small setae in gaps between adcentral teeth were usually (but not always) symmetrically placed, e.g. one in a central gap, or one each side of a central tooth. Absence of aesthetascs and wheel joints in post-partum females suggests that they have had an ecdysis associated with copulation. Wägele (1979, 1984) noted de ciences in mouthparts after spawning, and used the term ‘atrophic’ for these and for the loss of aesthetascs and setae from antennules 1 and 2.

Eisothistos teri (from Barbados) is like E. poseidon in having forward projections from antennule bases in females, not males, but its uropodal exopod is not tridentate and bears a recurved tooth on the main fang. Eisothistos poseidon has no such tooth and its tridents are unlike any described previously.