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Aethon bicamera sp. nov. (Figs. 1 – 3) Type material: Holotype ♀ and allotype ♂ from Latris lineata (Forster, 1801), Port MacDonnell, South Australia, January 2008; collected by K.S. Hutson; SAMA Reg. No. C 6901. 2♀♀ and 2♂♂ paratypes from L. lineata, Port MacDonnell, South Australia, 06 May 2008; collected by K.S. Hutson; NHMUK Reg. Nos. 2009.276 and 2018.194–195. Etymology: The name of the new species is derived from the Latin bi - meaning two and camera meaning chamber, and refers to the two chambers at the posterior end of the trunk. Description: Female body comprising broad cephalothorax, subquadrate trunk and small urosome completely concealed beneath dorsal trunk plate (Fig. 1A,C). Mean body length 4.71 mm, with a range of 4.65 to 4.76 mm (based on 3 specimens). Cephalothorax comprising cephalosome plus first pedigerous somite; squat, about 2 times wider than long, with weakly convex frontal margin. Lateral margins tapering strongly, widest posteriorly; bearing 2 pairs of lateral lobes, posterior pair more prominent. Both pairs with surface ornamented with cuticular tubercles. Trunk somites completely fused; tergites of second and third pedigerous somites raised, distinct and with surface ornamentation (Fig. 1 A–C). Lateral margin of trunk expanding outwards from prominent anterolateral shoulders backwards to tapering posterolateral processes. Entire dorsal surface of trunk ornamented with cuticular tubercles. Dorsal trunk plate covering fourth pedigerous somite and entire urosome; plate short but extending to full width of body, with weakly convex lateral margins and small median indentation in transverse posterior margin. Urosome comprising fifth pedigerous somite, genital complex and anal somite all fused, wider than long. Anal slit terminal, located between paired caudal rami; caudal rami setation not observed. Antennule (Fig. 2A) indistinctly segmented; segmental setation as follows: 1, 4, 2, 2, 1, 3 + ae, 8 + ae; all setae short and naked. Antenna (Fig. 2B) well developed; basal segment massive, bearing spinous process proximally on medial margin; terminal claw strong with spinous accessory process proximally on concave margin. Mandible stylet-like, armed with 8 marginal teeth distally (Fig. 2C). Maxillule bilobate (Fig. 2D); small inner lobe bearing 2 apical spines; laterally-directed outer lobe elongate, armed with 3 apical spines. Maxilla (Fig. 2E) comprising unarmed syncoxa and long basis bearing curved, bilaterally-serrate, terminal claw plus slender spine apically; basis bearing short spine subapically. Maxilliped (Fig. 2F) comprising robust basal segment and curved distal subchela bearing spinous process proximally and small process distally, at level of suture. Leg 1 biramous (Fig. 2G); unsegmented protopod bearing stout inner spine and slender outer seta: endopod 1- segmented, tapering distally, armed with apical spine about 35% length of segment: exopod indistinctly segmented bearing 5 stout spines around distal margin. Leg 2 (Fig. 2H) unsegmented and modified into large, heavily sclerotized, inwardly-curved process bearing accessory median process proximally; armed with small lateral papilla bearing outer protopodal seta; unarmed distal process representing exopod. Leg 3 (Fig. 2I) with large foliaceous rami; exopod oriented mainly vertically but curving ventrally to form lateral plates enclosing posterior part of trunk both laterally and ventrally (Fig. 1B,C): exopod rigid and strongly sclerotized, ornamented externally with tubercles: endopod unornamented, oriented mainly horizontally, separating upper trunk chamber containing loosely coiled egg sacs from lower chamber containing adult male. Leg 3 pair meeting in midline at extreme anterior end and posteriorly but with distinct heart-shaped space separating legs in anterior third (Fig. 1B). Leg 4 (Fig. 2J) biramous with outer protopodal seta present proximally on dorsal margin; rami foliaceous, rounded distally, entirely enclosed by modified rami of leg 3. Leg 5 not observed. Male. Body divided into cephalothorax incorporating first pedigerous somite, and unsegmented trunk representing fused second to fifth somites, genital somite and abdomen (Fig. 3A). Total body length 1.87 and 2.09 mm (2 specimens). Cephalothorax about 1.3 times longer than wide, with weakly convex lateral margins; comprising 57% of body length. Trunk about equal in width to cephalothorax, tapering posteriorly; comprising second to fifth pedigerous somites, genital complex and abdomen, all fused. Anal somite tapering posteriorly, bearing paired caudal rami (Fig. 3B) on posterolateral margins; rami about 2.3 times longer than maximum width; tapering irregularly towards tip; bearing 4 setae, with long dorsal seta located proximally, lateral seta situated about at mid-length, plus apical and subapical spiniform setae. Antennule 7-segmented and showing traces of geniculation between segments 5 and 6 (Fig. 3C); armed with setae as follows: 1, 4, 2, 5, 1, 2 + ae, 9 + ae. Antenna massive (Fig. 3D) subchelate; comprising unarmed, robust basal segment and strong terminal claw bearing curved spinous process proximally on concave margin. Mandible and maxillule as in female. Maxilla (Fig. 3E) basis with bifid subapical process; terminal claw densely ornamented with fine spinules. Maxilliped (Fig. 3F) comprising muscular basal segment and curved terminal subchela; basal segment ornamented with extensive patches of tiny tubercles proximally on myxal surface, and with patches of spinules distally; subchela with small process near concave margin. Leg 1 biramous (Fig. 3G); unsegmented protopod bearing stout inner spine and slender outer seta: endopod 1-segmented, tapering distally, armed with apical spine about 65% length of segment: exopod 1-segmented armed with 5 spines along distal margin; protopod and both rami ornamented with patches of spinules, as figured. Leg 2 (Fig. 3H) biramous; protopod bearing outer seta on papilla; endopod slender tapering towards tip, bearing apical spine about 47% length of segment; exopod unsegmented, lobate, ornamented with rounded tubercles in zone of wrinkled integument on distal surface. Leg 3 (Fig. 3I) reduced, lamellate, bearing outer seta on elongate papilla; posteriorly directed lamella weakly bilobed and armed with 2 small spines on apex of outer lobe. Leg 4 (Fig. 3J) bi-lobed; bearing outer seta on papilla; larger lamelliform outer lobe ornamented with isolated sensillae and armed with 2 reduced setae distally; smaller inner lobe unarmed. Leg 5 (Fig. 3A) represented by small outer lobe bearing single seta. Unarmed, plate-like operculum closing off each genital aperture probably representing leg 6. Remarks: The genus Aethon currently comprises only four species (Walter & Boxshall, 2018): the type species Aethon quadratus Krøyer, 1837, A. percis (Thomson, 1890), A. garricki Hewitt, 1968 and A. morelandi Hewitt, 1968. The type species is known only from the Atlantic, whereas the other three species were all originally described from New Zealand waters (Thomson, 1890; Hewitt, 1968). The new species can be readily distinguished from A. garricki by the shape of the posterior part of the trunk, the dorsal trunk plate, which is about 1.6 times longer than wide in A. garricki, whereas in the new species the dorsal trunk plate is 1.6 times wider than long. In addition, the lateral margins of the cephalothorax carry distinct lobes at the posterior extremity in the new species but these margins are evenly convex in A. garricki. In both A. morelandi and A. percis the dorsal trunk plate is about as long as wide, compared to the short, squat plate of the new species. The free posterior margin of the plate forms two evenly rounded lobes separated by a median indentation in A. percis, whereas in A. morelandi this margin is more sinuous as it has a median indentation plus paired lateral indentations which produce the appearance of angular posterolateral corners. The new species differs from both of these species in having an almost linear, truncated rear margin with just a tiny median indentation. These differences are sufficient to justify the establishment of a new species. The male was lodged inside a chamber enclosed laterally and ventrally by the sclerotized exopods of the third legs and dorsally by the unsclerotized third leg endopods plus the lamellate fourth legs which together form a horizontal internal wall separating the male from the dorsally-located, upper chamber containing the loosely coiled egg strings. The partitioning of the brood chamber into a dorsal space enclosing the egg strings and a ventral space housing a single adult male has not been reported before for any Aethon species. The male was attached directly to the gill filament of the host using its paired antennae which extend out through the anteriorly-located, heart-shaped gap between the third legs (Fig. 1B). It seems likely that the male attaches to the gill of the host and the adult female then moves into position over the male and envelopes it within its ventral chamber by closing the third legs around it. In this position both male and female are still attached to the host. |
