Myxicola nana, Capa, María & Murray, Anna, 2015

Capa, María & Murray, Anna, 2015, A taxonomic guide to the fanworms (Sabellidae, Annelida) of Lizard Island, Great Barrier Reef, Australia, including new species and new records, Zootaxa 4019 (1), pp. 98-167 : 132-136

Myxicola nana n. sp.

( Figs 14–15 )

Material examined. Holotype: AM W.45485, MI QLD 2445 (on SEM).

Description. Holotype 3.5 mm long, 0.4 mm wide. Six thoracic and 30 abdominal chaetigers. Specimen with white crown and light orange body when alive. Red interramal eyespots on abdominal segments. Pigmentation faded on preserved specimen. Radiolar crown with semicircular radiolar lobes fused dorsally bearing six pairs of radioles ( Fig. 14 A). Dorsal and ventral basal flanges absent. Basal membrane between radioles along 1/3–1/4 of their length; radioles with narrow radiolar flanges along their length, and long (1/3 of their length), digitiform bare tips ( Fig. 14 A). Two rows of vacuolated cells supporting radioles basally. A pair of subdistal radiolar eyes may be present in at least most radioles. Dorsal lips with short radiolar appendages, pinnular appendages absent; ventral radiolar appendages absent, ventral lips developed, extending dorsoventrally along inner surface of base of radiolar lobes (generic feature); parallel lamellae and ventral sacs absent. Posterior peristomial ring collar absent ( Fig. 14 A, C–D). Anterior peristomial ring with a rounded ventral lobe ( Fig. 14 C). Narrow glandular ridge on second chaetiger ( Fig. 14 C–D). Ventral shields inconspicuous. Interramal eyespots absent in thoracic region. First notopodia with two thin elongate-narrowly hooded chaetae. Following thoracic chaetigers with inconspicuous notopodia with 4–5 narrowly-hooded chaetae arranged in a transverse row anterior to the uncini ( Fig. 15 A–B). Thoracic neuropodia with acicular uncini with four rows of teeth decreasing in size distally over main fang, covering half its length ( Fig. 15 C). Companion chaetae absent. Interramal eyespots present in abdomen. Abdominal neurochaetae narrowly-hooded, on inconspicuous neuropodia ( Fig. 15 D–E). Abdominal notopodial tori forming almost complete cinctures around body (generic feature, Figs 14 E–G, 15D–F), with avicular uncini with two rows of 2–3 teeth each over main fang, covering most of main fangs length ( Fig. 15 D–F), breast and handle absent. Pygidium rounded, eyespots not seen ( Fig. 14 F–G). Pygidial cirrus absent ( Fig. 14 F–G). Tube not studied.

Remarks. Myxicola is an homogenous genus characterised by radioles that are united for most of their length by a basal membrane, abdominal uncinial tori forming almost complete cinctures, a transparent, gelatinous tube, and an atypical arrangement of dorsal and ventral lips ( Fitzhugh 1989, 2003; Capa et al. 2011). About 20 species have been described in the genus, the most recent in 1928 ( Read 2015), but the number of currently accepted species is approximately six, due to various subsequent synonymisations. Some of these revisions have led to the consideration of Myxicola infundibulum ( Renier, 1804) as a cosmopolitan species (e.g., Fauvel 1927; Day 1961; Imajima 1968; Hutchings & Glasby 2004; Edgar 2008), and that its presence also in Australia may be due to anthropogenic translocation out of its natural distribution range ( Boyd et al. 2002; Hutchings & Glasby 2004; Smith & Carlton 2007; Dane 2008).

The single specimen found in Lizard Island is very small compared to other reported Myxicola specimens (generally 15–400 mm long), so some of its morphological features could be size-dependant. It exhibits, for example, a short radiolar basal membrane joining the radioles for less than one third of their length instead of for most of the length of the radioles, as in other congeners. This species is also characterised by the presence of radiolar eyes, with a similar arrangement as those described in M. ommatophora Grube, 1878 , from the Philippines. Myxicola nana n. sp. is unique, if compared with other congeners, in the morphology of the abdominal uncini, which have up to six teeth, with an irregular arrangement and which also vary in size even between proximal uncini ( Fig. 14 F), instead of a single tooth on top of the main fang as reported for most other species (e.g., Grube 1878; Fauvel 1927; Dane 2008; Giangrande et al. 2012). However, detailed studies have shown that M. infundibulum , at least, has some additional (one to four) smaller flanking teeth in addition to the main tooth ( Dane 2008), and it was suggested that the small variation observed between the geographically distant populations studied was size-related (the larger the specimens, the more ′secondary ′ teeth they possess). This is obviously not the case for the specimens described herein, which are very small compared to those studied by Dane (2008) and which possess large and more numerous uncinial teeth.

The only Myxicola species reported in Australia so far is the apparently introduced M. infundibulum and, as mentioned above, this specimen differs from it in several features (e.g., number of thoracic chaetigers, presence of radiolar eyes, uncinial morphology; see also Table 4 for species comparison). In Australia, the genus has only been recorded from South Australia and Victoria, so its presence is a new record for Queensland.

* In populations from Europe, North America and Australia ( Dane 2008) Etymology. The species epithet refers to the very small size of the specimen described, in comparison to other reported members of this genus, and is derived from the Latin word “nanus” meaning dwarf.

Habitat. Muddy sediment with Halimeda , 25 m depth.

Type locality. Lizard Island.

Distribution. Australia (Queensland: Lizard Island).

Figures

FIGURE 14. Myxicola nana n. sp., scanning electron micrographs. A. Anterior end showing radiolar crown and mediumlength basal membrane, radiolar flanges and glandular ridge on second chaetiger (arrow); B. Detail of radioles and radiolar flanges; C. Base of radiolar crown and anterior thoracic chaetigers, lateroventral view; glandular ridge (white arrow) and anterior ventral peristomial lobe (black arrow) shown; D. Anterior thoracic chaetigers, dorsal view; notopodial chaetae (black circles) and neuropodial uncini (white circles) are minute; E. Midabdominal chaetigers with notopodial tori encircling the animal (arrows); neuropodial chaetae minute (circles); F. Posterior abdominal chaetigers and pygidium, lateroventral view, G. Same, laterodorsal view.

FIGURE 15. Myxicola nana n. sp., scanning electron micrographs. A. Anterior thoracic parapodium, with notopodial chaetae (black arrows) and neuropodial uncini (white arrows); B. Thoracic chaetae, narrowly-hooded; C. Thoracic uncini with four rows of teeth dismissing in size distally over main fang, covering half its length; D. Abdominal parapodia with cinctures of glandular tori and uncini and narrowly-hooded neurochaetae; E. Same from posterior view; F. Abdominal uncini each with two rows of 2 – 3 teeth over main fang (variable within chaetigers, see arrows).

Tables

TABLE 4. Comparison of main diagnostic features of some of the better documented species of Myxicola

M. aesthetica	M. infundibulum	M. ommatophora	M. nana n. sp.
Size up to 40 mm	up to 400 mm	43 mm	3.5 mm
N. thoracic <6 chaetigers	8	8	6
Basal membrane>¾ length of crown	>¾ length of crown	>¾ length of crown	1/ length of crown 3
Radiolar eyes absent	absent	paired, subdistal	paired, subdistal
Anterior ventral lobe well developed, acute	well developed, acute	low, rounded	low, rounded
Abdominal uncini? one tooth over main fang	one main tooth and up to four additional small flanking teeth*	? one tooth over main fang	3–6 large teeth over main fang