Desmonema scoresbyanna Gershwin & Zeidler, 2008, sp. nov.

Desmonema scoresbyanna, sp. nov.

(Plate 6)

Desmonema gaudichaudi. - Southcott & Glover 1987: 131 –132.

Material examined. Holotype: SAM H349, in The Coorong opposite Point Campbell, SA, coll. W. Potts & D. Phillips, 19.ii.1986; female (BD 129.2 mm, tentacles per group: 15, 13, 13, 15, 15, 15, 16, 15).

Paratypes: SAM H348 (RVS A1156), Encounter Bay, West Island, SA, coll. S.A. Shepherd, 29.iv.1969; one damaged specimen (BD 139.4 mm, tentacles per group: 15, 16, 10 (damaged), 15, 14, 6 (damaged), 15, 14). AM G15562, Budgewoi Lake, NSW, 33° 14’S 151° 34’E, winter 1952; one specimen (BD 90 mm, tentacles per group: 11, 11, 13, 12, 10, 13, 11, 12).

Diagnosis. Desmonema with unbranched radial canals in tentacular lappets; with 12–16, typically 15, tentacles per cluster.

Description. Bell flat, slightly thicker in center; with finely granular exumbrellar warts. Subumbrellar musculature with fine circular muscles covering radial stomach pouch region, continuous over radial septa in web-like pattern, with muscle striae meeting septum at 45° angle toward the margin; radial muscles lacking.

Tentacles in eight adradial clusters of 12–16, typically 15; issuing from subumbrella in single straight row peripheral to circular muscles at base of tentacular lappets; hollow, flattened laterally, with longitudinal muscle furrows; those near center of cluster more robust, to 6.2 mm width.

Rhopaliar lappets 16, spatulate, with triangular extension of corresponding radial stomach pouch, slightly diverticulated; lacking radial canals. Tentacular lappets eight, broadly rounded, much larger than rhopaliar lappets; with unbranched radial canals alternating in position with tentacles (Plate 6A). Rhopalia eight, four interradial, four perradial; located within niches situated proximal to margin, open to subumbrella and between lappets (Plate 6B, C); exumbrellar pit shallow, blind ending; rhopaliar base possessing two gelatinous papillae on subumbrellar side.

Gonads four, interradial, external, in pleated bag-like bundles. Radial stomach pouches 16, tentacular pouches wider than rhopaliar pouches peripherally. Radial septa 16, puckering but not interrupting circular muscles; with simple proximal terminus; tilting toward rhopaliar lappets distally.

Oral arms four, perradial, curtain-like proximally, tapered distally with crenulated margin. Manubrium inconspicuous. Mouth approximately quadrate, with gastric cirri showing through.

Color in life unknown.

Nematocysts. Tentacular nematocysts smoothly scattered on all sides of tentacles, not in distinct warts. Tentacular cnidome comprising small euryteles (7.69–9.04µm long x 5.22–7.14µm wide, n=16). Oral arms densely dotted with small clusters of about a dozen large oval euryteles (9.47–11.84µm long x 7.64–8.83µm, n=18) and with warts containing small, medium, and large euryteles (S: 2.77–4.26µm diameter, n=18; M: 4.62–5.36µm diameter, n=6; L: 7.79µm diameter, n=1).

Etymology. Named to honour Dr. Scoresby A. Shepherd, who has inspired many marine biologists and natural historians, and has caught many specimens of interesting Australian medusae, including one of this species, and his wife Anna, who has opened her home and pantry to wayward biologists on many occasions. Noun in apposition.

Associations. Two types of juvenile fishes were found living with the specimens, identified as trevally and hardyheads (Southcott, 1987).

Distribution. Presently known only from The Coorong and Encounter Bay, South Australia, and Budgewoi Lake, New South Wales. Both The Coorong and Budgewoi Lake are semi-enclosed systems with narrow openings to the sea. Thus, the salinity can vary considerably in both localities. Many species of plants and animals exhibit sharp discontinuities at Bass Strait (Dawson 2005, and references therein); thus, it is possible that unrealized differences exist between the South Australian and New South Wales forms of Desmonema.

Remarks. The genus Desmonema was thoroughly revised by Larson (1986). Desmonema scoresbyanna differs from other species of the genus in that canals on the lappets are unbranched and present only on the tentacular lappets, and in its tentacle number being in a narrow range of only 12–16 (most commonly 15) per group.

Southcott and Glover (1987) considered their specimens to be D. gaudichaudi based on Larson’s (1986) key. However, while the tentacle number falls within the range of 10–60 per group as for D. gaudichaudi, D. scoresbyanna differs from D. gaudichaudi (as well as the other Desmonema spp.) in several important respects (see table 1). In D. scoresbyanna, the tentacle number is of a narrow range of 12–16 per group; it is the only known species of Desmonema with distinctive unbranched radial canals exclusive to the tentacular lappets, whereas the canals of all other species of the genus are dendritically branched and are on all lappets; and the subumbrellar muscles are not broken into fields as in the other species. Larson (1986) stated that tentacle number was valid only for adult medusae greater than 25 cm BD, implying that maturity is reached by 25 cm. However, the three specimens of D. scoresbyanna are all less than 25 cm, and clearly reproductively mature as revealed by gonadal biopsy. Thus, D. scoresbyanna appears to be a smaller species than others in the genus.

Tentacle number per octant Radial canals Lappets w/ canals Musculature D. gaudichaudi (Lesson, 1829) 10–60 branched all 16 fields

D. comatum Larson, 1986 100–200 branched all 16 fields

D. glaciale Larson, 1986 <10 branched all 16 fields

D. scoresbyanna, n. sp. 12–16 simple tentacular only continuous