A revision of Micropodarke (Psamathini, Hesionidae, Polychaeta)

Micropodarke Okuda, 1938 (Psamathini, Hesionidae, Polychaeta) is revised based on examination of all available types, other museum specimens, and a large number of newly collected specimens from Japan, Hong Kong, Papua New Guinea, Australia, New Caledonia, California, the west coast of Canada, and the Red Sea. The previous synonymy of Micropodarke amemiyai Okuda, 1938 with M. dubia (Hessle, 1925) is substantiated, and M. trilobata Hartmann‐Schröder, 1983 is newly synonymized with M. dubia, leaving Micropodarke monotypic. A lectotype is selected for M. dubia. Micropodarke dubia is identified by the apomorphies of a proboscis diaphragm, segmental ventral adhesive papillae, distally nobbed and slightly curved neuroaciculae, and median neurochaetae with few, basally situated prolonged teeth (“spurs”), with abrupt transition to the following, much shorter teeth. Micropodarke dubia has a main distribution in the tropical and warm temperate Indo‐Pacific region, and appears to be absent from the Atlantic Ocean.


Introduction
We present a taxonomic revision of Micropodarke, based on examination of newly collected specimens of Micropodarke from southern and eastern Japan, Hong Kong, the Great Barrier Reef, New South Wales, South Australia, New Caledonia, western Canada, California, and the Red Sea, together with all available types and other deposited museum specimens. Micropodarke was erected by Okuda (1938) for a new hesionid from Japan, M. amemiyai Okuda, 1938. At present it includes three nominal species: M. dubia (Hessle, 1925) and M. amemiyai from Japan, and M. trilobata Hartmann-Schrö der, 1983 from Australia. Okuda, in his original description of M. amemiyai, had clearly overlooked Hessle's earlier study on hesionids from Japan, including the description of Kefersteinia dubia from the same locality where he found M. amemiyai, and his species M. amemiyai was synonymized with M. dubia by Imajima and Hartman (1964). The generic name Micropodarke, in contrast, has generally been considered valid by later authors. Micropodarke trilobata was described from an anterior fragment only, although Hartmann-Schrö der in two later studies (1986,1989) recorded further specimens from southern and south-western Australia. Several attempts to recollect specimens from the type locality in Dunsborough, Western Australia, unfortunately proved unsuccessful, although other specimens from elsewhere in Australia have been collected for comparison.

Material and methods
Specimens were extracted from scuba or intertidally collected sand samples by decantation through a 250 mm sieve, relaxed in a mixture of 7% magnesium chloride (in distilled water) and filtered sea-water (see Rouse and Pleijel 2001 for details). Relaxed specimens were studied alive, processed for long-term storage, light microscopy, and scanning electron microscopy (SEM). For long-term storage specimens were preserved in 10% formaldehyde (i.e. 25% formalin) in filtered seawater for one or a few days, rinsed in distilled water and transferred to 70-80% alcohol (all newly collected specimens were fixed in this way unless otherwise noted in the ''Material examined'' sections). Live, relaxed specimens were also mounted for light microscopy (LM) in BDH Gurr AquamountH on slides for chaetal studies. Specimens for SEM were similarly relaxed, fixed in 1% osmium tetroxide (diluted in the MgCl 2 solution or in filtered sea-water) for 1 h, rinsed in distilled water, dehydrated in a graded ethanol series, critical point dried, and sputter-coated. Specimens preserved for DNA sequencing were fixed and stored in 70 or 95% ethanol.

Apomorphies
Presence of proboscis diaphragm, segmental ventral adhesive papillae, distally nobbed and slightly curved neuroaciculae, and three to six median neurochaetae with few, basally situated prolonged teeth (''spurs''), with abrupt transition to following, much shorter teeth.

Reproduction
Banse and Hobson (1968) reported a mature female in May from Washington, with egg size up to 60 mm. We have collected specimens approaching maturity in July in Hong Kong and in November in New Caledonia. The eggs are transparent to faint rose, and the egg size agrees with Banse and Hobson's observation. Males from Hong Kong had spermatids in large clusters, though no mature sperm were present.

Colour
Live specimens transparent, without pigmentation. Posteriormost part of gut with white pigmentation. Eyes light red to orange. Preserved specimens more opaque, eyes darker red.

Distribution and habitat
Micropodarke dubia is mainly distributed in warm-temperate and tropical regions ( Figure 6). Notably there are no current records from the whole Atlantic, the Pacific coast of South America, the Arctic, and the Antarctic. As presently known it is vertically distributed from the intertidal down to 450 m, and occurs in well-sorted sand and muddy sand; it may occur in high densities on sandy beaches.

Remarks
Hessle's syntypes were originally separated in four tubes, UUZM 674a-d. Of these, 674a contains two specimens in good condition, tube 674b a specimen without head or posterior end, tube 674c an anterior end, but also two nereidid pieces, and tube 674d is empty. One of the specimens from UUZM 674a is here selected as lectotype, and obtains the new museum number UUZM 674a1.
There appears to be no type material of M. amemiyai. Nevertheless, a neotype designation would serve little purpose, since at present there are no doubts regarding the identity of M. amemiyai and its synonymy with M. dubia. The type localities are virtually the same, both being located in Sagami Bay on east Honshu in Japan, and examination of a large number of specimens from this area provided no evidence that several Micropodarke species may be involved. Banse and Hobson (1968) included a description of M. dubia, which clearly agrees with the description above, as well as with specimens collected from the same area. Their remarks on M. amemiyai, however, appear confused. They interpreted the statement ''uniramous parapodia'' in Okuda's (1938) original description to mean lack of notoaciculae, but this is likely to be incorrect, since Okuda made no explicit mention or drawing of either noto-or neuroaciculae. Furthermore, Banse and Hobson argued that lack of notoaciculae and presence of additional parapodial lobes (5ventral adhesive papillae) would indicate a close relationship to Nereimyra, but none of these features are present in this taxon. In fact, notoaciculae are present in all hesionids described to date.
Hartmann-Schrö der (1983) described M. trilobata from Dunsborough, south-west Australia, based on an anterior fragment in poor condition. The new species was justified by differences in the shapes of the neuropodia, the prostomium, and the adhesive papilla. Although it is unclear on what descriptions or specimens she based her comparisons, none of these differences could be corroborated from examinations of the type and her additional specimens (see Hartmann-Schrö der 1986, 1989, other Australian specimens, and topotypes of M. dubia. Contrary to her description, the parapodia have one pre-and one postchaetal lobe, the prostomium lacks a distinct posterior incision, and the ventral part of the neuropodium has the same form as other M. dubia. Since no other consistent differences were observed, M. trilobata is here treated as a junior synonym of M. dubia. Populations from the west coast of North America differ from other M. dubia in having much longer adhesive papillae ( Figure 5D, E) and a slightly larger number of chaetae (often 20-28).
Live specimens of M. dubia can often be distinguished from other hesionids in samples by their habit of coiling up when disturbed. This may be related to the fact that they often live in unstable sediments that are exposed to waves and currents, and coiling up around sand grains or larger particles and attaching to them by means of the adhesive glands prevents them from being swept away.

Discussion
Following Pleijel (1998), Micropodarke belongs to Psamathini, which includes also Hesiospina Imajima and Hartman, 1965, Psamathe Johnston, 1836, Bonuania Pillai, 1965, Nereimyra Blainville, 1828, Sirsoe Pleijel, 1998, and Syllidia Quatrefages, 1866 where it is sister to the latter four. The relationships within Psamathini are currently not well supported and require further studies. Although it was based on erroneous observations (see above), the suggestion by Banse and Hobson (1968) that Micropodarke and Nereimyra may be closely related can therefore not be excluded at present.
The monophyly of Micropodarke, as noted above, is supported by four apomorphies: the proboscis diaphragm, segmental ventral adhesive papillae, distally nobbed and slightly curved neuroaciculae, and median neurochaetae with few, basally situated prolonged teeth (''spurs''), which have an abrupt transition to the following and much shorter teeth, even though several of these features may be homoplastic. For instance, a similar proboscis diaphragm may also be present within Podarkeopsis Laubier, 1961 (although this warrants further investigation), segmental ventral adhesive papillae are present also in Sinohesione genitaliphora Westheide, Purschke and Mangerich, 1994, and distally truncated aciculae are present also in Heteropodarke (Pleijel, 1999). In spite of these homoplastic occurrences, we consider M. dubia well supported, since none of the other taxa where these features occur are closely related (see Pleijel 1998). The fourth apomorphy, the prolonged teeth (''spurs''), is highly characteristic and members of M. dubia can readily be identified from this feature. Nevertheless, prolonged teeth do occur also within other Psamathini, such as Psamathe (Pleijel 1998, Figure 17F) and Hesiospina (Pleijel 2004), and, although absent in many other hesionids, also in the more distant groups Chrysopetalidae (personal observation) and Nereididae (Glasby 1999). In all these taxa, however, the blades differ in that the length of the teeth is less extreme, and in that they lack the abrupt transition to shorter teeth.