Leitoscoloplos mawsoni Benham 1921

Leitoscoloplos mawsoni (Benham, 1921)

Figure 11

Scoloplos kerguelensis: Willey 1902: 275; Monro 1939: 124 (in part). Not McIntosh 1885. Scoloplos mawsoni Benham, 1921: 78 –81, pl. 9, figs. 91–94.

Leitoscoloplos mawsoni: Mackie 1987: 5 –7, fig. 4.

Material examined. Antarctica: Adelie Land, Commonwealth Bay, Boat Harbor, 67°00′S, 142°36′E, 8 m, coll. J. Haswell, 15 Oct 1912, 55 syntypes (AM W769).— Wilkes Land, Vincennes Bay, Casey Station, coll. Australian Antarctic Division, Brown Bay, middle, Sta. S1P1R1, 0 4 Dec 2005, 66.272°S, 110.567°E, diver cores, 15–25 m, (50, including 15 juveniles, AM); O’Brien Bay-1, T2P1R2, 14 Dec 2006, 66.312°S; 110.515°E, diver cores, 12–25 m, (24, AM); Wilkes Bay, Sta. S 2P1R3, 15 Dec 2005, 66.526°S; 110.526°E, diver cores, 10–20 m, (26, including 10 juveniles, AM); Sta. S1P2R3, 15 Dec 2005, 66.526°S; 110.526°E, diver cores, 10–20 m, (44, AM).

Description. A moderate-sized species, syntypes up to 32 mm long, 1.5 mm wide for about 100 setigers; other specimens with similar maximal measurements. Color in alcohol: light tan; large specimens from Casey Station S1P1R1 with dusky dark pigment on prostomium, peristomium, and first 3–4 setigers, most intense on prostomium and anterior peristomium. Thoracic region inflated, not depressed, with 10 setigers, wider than abdominal segments.

Prostomium short, conical, rounded or blunted on tip, not acute (Fig. 11A); two nuchal slits sometimes apparent in dorsolateral locations; without eyespots. Peristomium a single achaetous ring, tapering anteriorly, about one-fourth longer than first setiger (Fig. 11A).

Thoracic parapodia similar, inconspicuous, with short postsetal lobes with rounded tips increasing in size along thorax (Fig. 11A); notopodial lobes elongated, oval-shaped; neuropodial lobes more triangular, but with rounded, not pointed tips (Fig. 11B). Abdominal notopodial postsetal lobes short, blunt slightly asymmetrical in anterior segments (Fig. 11C), becoming longer, more symmetrical, somewhat triangular posteriorly (Fig. 11D); neuropodia short, rounded, expanded without obvious notch; subpodial flange weakly developed in anterior abdominal setigers (Fig. 11C), becoming larger, blister-like in far posterior setigers.

Branchiae from setiger 11 (rarely 12), short, strap-like at first; becoming wide at base, smoothly tapering to rounded tip (Fig. 11C–D); far posterior branchiae about twice the length of notopodia.

Thoracic notosetae arranged in single spreading fascicle of two rows, with setae of anterior row shorter and thinner; neurosetae arranged in two fascicles dorsal and ventral to postsetal lobe. Abdominal notosetae including crenulated capillaries and 0–3 furcate setae; furcate setae with unequal tynes connected by row of fine needles (Fig. 11E). Abdominal neurosetae thin, weakly crenulated.

Pygidium a simple ring, lacking cirri.

Remarks. Leitoscoloplos mawsoni was redescribed by Mackie (1987) and is similar to L. geminus in having up to 10 thoracic setigers and branchiae from setiger 11–12. The two species are difficult to separate and differentiated mostly on soft parts that may be affected by preservation. The branchiae of L. mawsoni are initially short and straplike, narrow basally, becoming wider basally further posterior and not appearing asymmetrical. In contrast, the branchiae of L. geminus are short and wide at the base from the first and triangular in shape, becoming larger and distinctly asymmetrical in posterior setigers, bending medially. The posterior neuropodial lobes of L. geminus are more distinctly notched than in L. mawsoni; the former has a more conspicuous subpodial flange with a notch separating it from the neuropodium; the flange of L. mawsoni is poorly developed, but both species develop enlarged blister-like flanges in posterior setigers.

Biology. As is typical for Southern Ocean orbiniids, little is known concerning the biology of L. mawsoni. However, collections from the Australian Antarctic program from December 2005 had two obvious size classes: (1) large mature adults with ripe gametes and (2) small juveniles. Many specimens from Dec. 2005 were packed with eggs with average diameters of 215 µm.

Distribution. Southern Ocean: Adelie Land, Wilkes Land and Ross Sea; 3– 25 m.