Alebion maculatus Wilson, 1932

(Fig. 2)

Material examined. 1♂ from Carcharhinus cf. limbatus (Müller & Henle, 1839) (TC17372), 21 January 2016, QM Reg. No. W53041.

Site on host. Body surface.

Differential diagnosis. Cephalothorax dorsoventrally flattened with well-developed marginal membranes; frontal plates well developed, lacking lunules. Fourth pedigerous somite with bilobed dorsal plate in female; with small dorsal expansions in male (Fig. 2A). Female genital complex with lateral swellings and elongate fifth legs extending beyond tip of abdomen. Male genital complex about 1.5 times longer than wide; lateral margins linear, tapering slightly posteriorly (Fig. 2A). Abdomen 2-segmented in both sexes. Caudal rami incompletely fused to anal somite. Male antenna with blunt spinous process on proximal segment; subchela with accessory claw (Fig. 2B). Post-antennal process without tine, sclerite forming corrugated adhesion pad; associated papillae unisensillate (Fig. 2B). Posterior process of maxillule broad and flat, forming adhesion pad ornamented with fine surface striations (Fig. 2B). Post-oral process present; elongate with corrugated surface. Maxilliped with smooth myxal margin in female; with prominent myxal process in male. Sternal furca lacking. Leg 1 with 2-segmented endopod armed with 0-0; 3 setae: distal exopodal segment with 3 plumose setae on posterior margin: distal margin (Fig. 2C) armed as follows: spine 1 simple, spine 2 modified plate-like, spine 3 short and stout, with accessory process, seta 4 naked, longer than spine 3. Leg 2 sexually dimorphic: with modified plate-like spines on second and third exopodal segments of female; spines forming highly ornamented rods in male (Fig. 2D), spine on second segment about as thick as, and extending nearly to tip of, spine on third segment. Leg 3 with 3-segmented rami: spine on first exopodal segment linear and directed away from ramus (Fig. 2E); spine on second segment and 2 proximal spines on third segment modified, plate-like: endopod (Fig. 2F) with unarmed first segment expanded to form velum; second segment elongate, with outer velum-like expansion and 2 inner plumose setae; small third segment with 4 plumose setae. Leg 4 reduced to single segment bearing 4 short setal elements. Fifth leg of male located near mid-length of genital complex, comprising isolated outer seta plus exopodal lobe bearing 3 setae (Fig. 2G). Body length of adult male 4.50 mm.

Remarks. Cressey (1972) revised the genus Alebion, recognizing eight species as valid and providing a key to the adults of both sexes. Following Cressey’s key, the single male from Moreton Bay is identified as A. maculatus, based on the size and form of the modified spines on the second exopodal segment of leg 2 (Fig. 2D).

Alebion maculatus was first figured by Bassett-Smith (1898a) who reported two mature females (under the name Alebion carchariae Krøyer, 1863) taken from an unidentified “small shark” caught off Aden. Wilson (1932) pointed to significant differences in the shape of the genital complex and abdomen between Bassett-Smith’s description and the original description of A. carchariae by Krøyer (1863) and proposed A. maculatus as a new species to accommodate Bassett-Smith’s specimens. This species has previously been recorded from Aden, India (Gnanamuthu, 1951; Rangnekar, 1959), Sri Lanka (Kirtisinghe, 1956) and Madagascar (Cressey, 1967). This is the first report of A. maculatus from Australian waters, and this represents a significant range extension.

The only identified host species known for A. maculatus is Carcharhinus limbatus caught at Nosy Bé, Madagascar (Cressey, 1967, 1972); most other reports have named hosts only as “carcharhinid shark” (e.g. Gnanamuthu, 1951 - as Alebion alatus Gnanamuthu, 1951, and Kirtisinghe, 1956 - as Alebion megacephalus Kirtisinghe, 1956), or “unidentified shark” (e.g. Rangnekar 1959 - as Alebion spinosus Rangnekar, 1959).