Dysponetus bulbosus Hartmann-Schroder 1982

Dysponetus bulbosus Hartmann-Schröder, 1982

Figure 1 A–F, 2A–B

Hartmann-Schröder, 1982: p54, Figs 1–4.— Hartmann-Schröder 1986: p32, Figs 1–5.— Pleijel et al. 2012: p5.

Material examined. Cape Naturaliste, Eagle Bay, Western Australia, fine algal bed with sand, holotype (ZMH P- 16752), 07.11.1975; Hallett Cove, Adelaide, south Australia, algae and encrustion in rockpools, 1 specimen (ZMH P- 18720), 13.12.1975; Port MacDonnell, Cape Northumberland, 2 km west of town, lee side of abrasion zone with volcanic rock, algal bed & turf, 1 specimen (ZMH P- 18764), 19.12.1975; Cowbowie Field Station, Yorke Peninsula, Gulf St Vincent, South Australia, Sta. 0 1 (35°05′N, 137°44′E), mixed sand & gravel, 3–5 m, 2 specimens (SMNH 83511), 28.02.2004.

Additional material examined. Dysponetus sp.: inlet by Bonaparte Point, Arthur Harbor, Anvers Island, Antarctica, Sta. 6-63 (64° 46′S, 064° 04′W), from fish trap crushed by small berg, 4 fathoms, 1 specimen (USNM 247268), 24.01.1963.

Morphological re-assessment. The detailed descriptions published by Hartmann-Schröder in each case are still essentially correct, the only amendments necessary being to the details of the first three segments together with some additional observations. The published descriptions for each specimen are slightly different to each other and, in fact, the description by Hartmann-Schröder 1993 is now known to be for a different species (see D. antarcticus n. sp.). However, re-examination, combined with details from larger, more recent specimens, shows that the 1982 and 1986 specimens all exhibit the same characteristics for the initial anterior segments as follows:

Segment 1: The original descriptions of D. bulbosus detail this segment as having either dorsal but no ventral cirri (Holotype—1982; 1993) or both dorsal and ventral cirri (1986). Additionally, either notochaetae but no neurochaetae were detailed (1982, 1993) or both cited as absent (1986). In all cases, even though appendages may have been lost, the cirrophores for the dorsal and ventral cirri are present and both notochaetae and neurochaetae are absent (Figs 1 A, B, D–F; 2A).

Segment 2: The differences in the descriptions for this segment vary. The holotype is described as having a dorsal cirrus with the ventral cirrus described as ‘not definitely absent’, with both notochaetae and neurochaetae present (1982). The specimens from South Australia (1986) are cited with a dorsal but no ventral cirrus and notochaetae but no neurochaetae, and the Antarctic specimen (1993) as having both dorsal and ventral cirri and noto- but no neurochaetae. Examination of material illustrates that the latter case, despite being a different species is, in fact, correct for all specimens (Figs 1 A, B, D–F; 2A); cirrophores indicate where missing appendages were originally present. The holotype and specimen from Adelaide both have obvious emergent acicula on segment 2 (Figs 1 A, B, E, F), not visible on the specimen from Port McDonnell (Fig. 1 C) or the more recent specimens from Yorke Peninsula (Fig. 2 A). It is therefore believed that the emergence is either due to the very small size of the specimens or preservation, but is not a character.

Segment 3: In this case, all descriptions (1982, 1986, 1993) agree that both dorsal and ventral cirri as well as both noto- and neurochaetae are present. The first two, however, are both incorrect in the same respect—the ventral cirrus on this segment is absent (Figs 1 B, D, F). Although clear under SEM (Fig. 2 A), this character is difficult to confirm using normal light microscopy but can be detected at x1000 using oil immersion. At this magnification, it is possible to identify the cirrophores for lost ventral cirri below the neuropodium on any segment. Cirrophores are confirmed absent on the 3rd segment of all 1982 and 1986 material. Ventral cirri are present on segment 3 of the 1993 specimens, however, these specimens are now confirmed as a separate species and are described below as D. antarcticus.

Additional observations: single mouth appendage variable in size: barely visible on holotype (Fig. 1 B), seemingly abnormally large on 1986 specimens (Figs 1 D, F) but average size on more recent 2012 specimens (Fig. 2 A). Notochaetae D-shaped in cross-section (Fig. 2 B), two alternating rows of sharp denticles each side. Accessory chaetae (1–2) present (simple neurochaetae of same form as notochaetae), inserted dorsally and distally on neuropodial lobe.

Final chaetiger (Fig. 1 C) with few notochaetae, small rounded dorsal cirrus, neuropodium with single neurochaeta and accessory chaeta, no ventral cirrus. Pygidium slightly damaged, conical in appearance; single small projection, bluntly rounded, inserted posteroventrally (Fig. 1 C)

Habitat. Algal beds, turf and encrustations in shallow water (0–5 m).

Distribution. South and Western Australia.

Remarks. Prior to the 1993 publication, the only other record of Dysponetus from Antarctica was a damaged specimen identified by Hartman in 1967. The latter specimen is very small (12 segments) with a slightly damaged anterior end. It appears very like D. bulbosus, with possession of a single mouth appendage and a ventral cirrus absent on segment 3. However, it is not deemed in good enough condition to definitively confirm the species identification. The distribution of D. bulbosus is now restricted to Australia.