Megalomma cf. miyukiae Nishi, 1998

Capa, María & Murray, Anna, 2009, Review of the Genus Megalomma (Polychaeta: Sabellidae) in Australia with Description of Three New Species, New Records and Notes on Certain Features with Phylogenetic Implications, Records of the Australian Museum 61 (2), pp. 201-224 : 212-216

Megalomma cf. miyukiae Nishi, 1998

Figs 5C , 10

Megalomma miyukiae Nishi, 1998: 53–59 , figs 1–4.

Material examined. Northern Territory. MAGNT W331 (1 spec.), Coral Bay, Port Essington , 11°11'50"S 132°02'55"E, collected by D. Staples, 17 May 1983 , broken reef, 3–5 m; MAGNT W17354 (1 anterior end on SEM stub, crown in vial), Cullen Bay Marina, Darwin Harbour , 12°27'09"S 130°49'18"E, coll . CSIRO CRIMP survey team, 30 Mar. 1999 ; MAGNT W17355 (1 detached crown only), same site and collectors ; MAGNT W17356 (1 spec.), Inner Stokes Hill Wharf , Darwin Harbour, S 12°28'13"S E 130°50'59"E, coll . CSIRO CRIMP survey team, 16 Aug. 1999 . Queensland. AM W35502 (1 crown + spec. on SEM stub), Lorim Point Wharf, Weipa , 12°40'S 141°57'E, P990 WI, coll. CRC Reef Research Centre Ltd , Oct. 1999 , from pile scraping, 7 m.

Description. Only one specimen (MAGNT W331) complete, the rest are missing posteriors or in poor condition (branchial crown is detached from body). Complete specimen with 10 thoracic and 120 abdominal chaetigers. Crown with 18 radioles on each semicircular lobe. External margin of radioles rounded, without lateral flanges. Tip of radioles digitiform and longer than distal pinnules, except radioles bearing compound eyes, where tip is shorter. Radiolar skeleton with 8–10 rows of cells ( Fig. 5C ). Three pairs of dorsalmost radioles with subdistal compound eye, with distinct ommatidia, diminishing in size ventrally and with dorsalmost pair of eyes almost surrounding the radiole, leaving a small gap on the outer margin. Dorsal lips with radiolar appendages as long as two thoracic chaetigers. Ventral lips rounded and well developed. Parallel lamellae and ventral sacs present ( Fig. 10A ). Caruncle absent. Smooth keel present between dorsal lips ( Fig. 10B ). Posterior peristomial collar with rounded dorsal margins and with wide gap between them but with low fleshy ridges that continue obliquely from middorsal faecal groove to vestigial pockets, pockets level with notochaetae of first chaetiger ( Fig. 10B ); lateral margins of collar smooth, perpendicular to body axis and covering the junction of thorax and crown; ventral lappets prominent, rounded and overlapping ( Fig. 10A ). Ventral shields quadrangular, all similar in width down the body, almost in contact with the neuropodial tori ( Fig. 10A ). First ventral shield longer than the rest, with m-shaped anterior margin although not conspicuous. First chaetiger with about 20 superior and inferior elongate narrowly hooded notochaetae, superior longer than inferior. Chaetigers 2–8 with about 20 elongate narrowly hooded superior chaetae and 20–25 broadly hooded inferior chaetae ( Fig. 10C–E ) (type B). Neuropodial tori slightly diminishing in width posteriorly. Thoracic uncini with several rows of small teeth, similar in size, above main fang ( Fig. 10F ); uncinus with well developed breast, handle similar in length to the distance from breast to main fang. Companion chaetae with asymmetrical membrane and small teeth covering most of its surface ( Fig. 10H ). Neuropodia of anterior abdominal chaetigers with broadly hooded chaetae ( Fig. 10I ). Abdominal notopodia with uncini similar to thoracic but with shorter handles ( Fig. 10G ). Pygidium bilobed, eyespots absent. Chitinous hard tube (specimen MAGNT W17356), covered with fine sand on anterior third of length.

Variation. The other specimens have eight thoracic chaetigers and 15–25 radioles on each branchial lobe. Specimens possess one to three pairs of subdistal radiolar eyes, and some have eyes missing on the third dorsalmost radioles, in which case they are present on the fourth pair; relative size of eyes varies from one specimen to another—in some the two dorsalmost pairs are large and the third pair significantly smaller, while in others the difference in size is more gradual, diminishing ventrally. The specimens also vary in the size of the gap between ventral shields and tori. In some specimens the ventral lappets do not overlap.

Colour pattern. Body slightly pigmented anteriorly, particularly the collar, ventral shields, and ventral margins of parapodial tori, although tori unpigmented. Base of crown and anterior margin of peristomium dorsally coloured; radioles with some transverse irregular bands and with some pinnules pigmented.

Remarks. The identification of this species in Australia should be confirmed with more material in better condition. Some features, such as separation of the dorsal margins of the collar from the faecal groove, the presence of vestigial pockets of the collar and the presence of subdistal eyes in only a few dorsal radioles agree with the original description of M. miyukiae , described from Thailand ( Nishi, 1998). But there are some differences between these specimens and the types of M. miyukiae . Vestigial pockets of the collar are present on the specimens described herein, and although we consider this a feature that might be overlooked and interpreted as “pockets absent” when there is no fusion of collar with the faecal groove, Nishi (1998) is aware of the possibility of vestigial pockets being present as he states in Remarks when comparing his new species to others, that M. heterops Perkins, 1994 , “has vestigial pockets joining ridges and dorsolateral margins”, and mentions that only a “small bulge” is present on M. miyukiae specimens. In the absence of examination of type material of M. miyukiae , we can only assume that our specimens differ from M. miyukiae in this character. The first ventral shield is not as long as twice the length of the following shields, and also radiolar tips are longer than pinnules except for the ones bearing compound subdistal eyes, but these differences could be due to intraspecific variation. These specimens should also be compared with M. kaikourense , described from New Zealand, as they share some features such as dorsal collar margins not fused to faecal groove, presence of vestigial pockets (according to original description by Knight-Jones, 1997), and eyes on dorsalmost radioles. The differences between the two species are that the specimens from Australia bear eyes on other radioles, and also, using a perhaps more significant species-character, that the inferior thoracic chaetae have progressively tapering distal tips (type B) in the specimens described herein, whereas they narrow abruptly (type A) in M. kaikourense .

Figures

Figure 5. Drawings to scale of radioles sections, at the base: (A) Megalomma

Figure 10. SEM of Megalomma cf. miyukiae, AM W35502: (A) anterior thoracic chaetiger and posterior peristomial ring collar, ventral view; (B) same, dorsal view; (C) notochaetae, first chaetiger, ventral view; (D) inferior thoracic chaetae, anterior chaetiger, ventral view; (E) whole fascicle; (F) thoracic uncini; (G) abdominal uncini; (H) companion chaetae; (I) anterior abdominal neurochaetal fascicle.