Hatschekia aigoi Izawa, 2016

Hatschekia aigoi Izawa, 2016 View in CoL

Syn: Hatschekia sp. of Kabata (1991)

Material examined: 6 ♀♀ from gills of S. fuscescens ( TC17928 ) collected at Wynnum North, Moreton Bay on 05 July 2016, QM Reg. Nos. W55128 View Materials ; 4 ♀♀ from gills of Siganus fuscescens (Houttuyn, 1782) ( TC17799 ) collected off Wellington Point, Moreton Bay on 01 July 2016 ; 1♀ gills of S. fuscescens ( TC19646 ) collected in Moreton Bay ; NHMUK Reg. Nos. 2022.214-216 .

Supplementary description of female

Total body length excluding caudal rami ranging from 1.24 to 1.59 mm, with a mean of 1.46 mm (n = 7). Body ( Fig. 2A View FIGURE 2 ) comprising anterior cephalothorax and long cylindrical trunk bearing minute conical genitoabdomen posteriorly. Cephalothorax about 1.14 times wider than long (172 x 196 μm), widest in posterior third. Dorsal cephalothoracic shield supported by m-shaped subsurface chitinous frame ( Fig. 2A View FIGURE 2 ) with long median and paired lateral longitudinal bars; median bar with 2 pairs of short side branches posteriorly. Cephalothorax length typically comprising 12 to 14% of trunk length. Trunk about 5.2 times longer than wide (range 3.7 to 6.2 times); with more-or-less parallel lateral margins; posterior margin truncate, without expanded lobes at corners. Genitoabdomen wider than long ( Fig. 2B View FIGURE 2 ) comprising fused genital and abdominal somites; bearing paired genital apertures dorsally. Caudal rami about 2.1 times longer than wide (30 x 14 μm); armed with 6 naked setae of different lengths; lateral seta located about at 45% of lateral margin. Mean number of eggs per egg sac = 9.5 (range 9 to 10, n =2).

Rostrum lacking lateral processes. Antennule ( Fig. 2C View FIGURE 2 ) short, indistinctly 5-segmented: segmental setation pattern 10, 5, 4, 1, 12 + ae; 2 unequal setae located on antero-dorsal surface of first segment. Antenna ( Fig. 2D View FIGURE 2 ) 3- segmented, comprising short unarmed coxa, slender tapering basis, and distal subchela: surface of basis ornamented with minute pits; subchela with swollen base bearing inner element, plus curved distal claw. Parabasal papilla ( Fig. 2D View FIGURE 2 ) small and irregularly lobate, located lateral to insertion of antenna. Mandible stylet-like, bearing apical and subapical teeth ( Fig. 2E View FIGURE 2 ). Maxillule bilobed ( Fig. 2F View FIGURE 2 ): both lobes armed with 2 unequal setae; inner seta on inner lobe broad and curved. Maxilla ( Fig. 2G View FIGURE 2 ) armed with stout inner seta proximally on first segment; subchela comprising long segment armed with slender seta at inner extremity and distal claw with bifid tip.

Swimming legs 1 and 2 biramous; members of each leg pair joined by slender interpodal bars ( Fig. 2H View FIGURE 2 ). Leg 1 ( Fig. 2I View FIGURE 2 ) with fused sympod armed with outer and inner setae: exopod distinctly 2-segmented; proximal segment with outer distal spine; distal segment bearing 4 blunt-tipped setal elements around distal margin: endopod 2- segmented; proximal segment unarmed; distal segment armed with 4 blunt-tipped setae around apical margin plus 1 blunt seta midway along inner margin. Leg ornamented with 2 curved rows of minute spinules on sympod and on both exopodal segments. Leg 2 ( Fig. 2J View FIGURE 2 ) with fused sympod bearing outer seta; exopod 2-segmented; proximal segment longer than distal, armed with outer distal spine; distal segment bearing 1 short and 2 longer setae around apex: endopod 2-segmented; proximal segment with inner seta; terminal segment armed with 3 setae on distal margin and 1 seta distally on inner margin. Leg ornamented with curved rows of minute spinules on sympod and both rami: 2 each on sympod and exopodal segment 1, 3 on exopodal segment 2, and 1 each on endopodal segments. Leg 3 located laterally on trunk at 31% of length (arrowed in Fig. 2A View FIGURE 2 ), represented by small posteriorly-directed lobe drawn out into spinous process armed with 2 setae ( Fig. 2K View FIGURE 2 ). Leg 4 located laterally on trunk at 83% of length ( Fig. 2A View FIGURE 2 ), represented by single seta originating directly on trunk surface.

Remarks

Four nominal species of Hatschekia have been reported from siganid hosts: H. aigoi Izawa, 2016 from Siganus fuscescens caught in Japanese waters, H. sigani Uma Devi & Shyamasundari, 1980 from S. javus (Linnaeus, 1766) in Indian waters, H. siganicola El-Rashidy & Boxshall, 2011 from the invasive host S. luridus (Rüppell, 1829) in the eastern Mediterranean, and H. teuthidis Yamaguti, 1954 from Indonesia (off Sulawesi). The host name given by Yamaguti (1954) for H. teuthidis was Teuthis sp. but Izawa (2016c) attributed Yamaguti’s “ Teuthis sp. ” to Siganus , presumably reflecting usage in Japan, so we follow Izawa in considering H. teuthidis as a parasite of a siganid host. [The generic name Teuthis Linnaeus, 1766 is recognized as a synonym of the acanthurid genus Acanthurus Forsskål, 1775 (see Eschmeyer, 1998; Froese & Pauly, 2024), but at the species level, most species formerly placed in Teuthis are currently attributed to Siganus .] In addition, Kabata (1991) provided a partial description of an unnamed Hatschekia sp. from Siganus fuscescens (as S. nebulosus ) caught in Moreton Bay.

The type host of H. aigoi is Siganus fuscescens caught in the Sea of Kumano, Mie Prefecture, Japan ( Izawa, 2016c) and comparison between the description of H. aigoi and the new Australian material from the same host reveals no significant differences.The original material from Japanese waters had a female body length of 1.60 to 1.69 mm, whereas the Australian females ranged from 1.24 to 1.59 mm ( Kabata, 1991; present account). The Australian material shares basically the same pattern of subsurface chitinous frame supporting the dorsal cephalothoracic shield (despite the somewhat different representations in the different sets of line drawings). Differences in antennulary setation are minor and probably reflect the different angle of observation: since Izawa’s figure shows the antennule in situ attached to the rostrum whereas in the Australian material the antennule was dissected off. Izawa (2016c) doesn’t mention any teeth on the mandible, but two are present. The maxillules are very similar and both share the possession of a curved inner element on the inner lobe. Legs 1 and 2 share identical setation patterns, although the short setae on the rami of both legs are depicted with acute tips by Izawa (2016c) but have rounded tips in the Australian material. Leg 3 bears 2 setae but Izawa (2016c: Fig. 1J View FIGURE 1 ) shows them originating on a simple rounded lobe whereas in the Australian material the lobe is drawn out into a posteriorly-directed process (cf. Fig. 2K View FIGURE 2 ). Overall, these differences are minor, with many explicable by the different viewing angles illustrated by Izawa (2016c). The geographical range of H. aigoi is therefore expanded to encompass the waters off southern Queensland.

Detailed comparison with Kabata’s (1991) description of an unnamed Hatschekia sp. from S. fuscescens in Moreton Bay again reveals no major differences. The differences in antennulary setation simply reflect Kabata’s illustration of the major setae but omission of the smaller setae. A single difference is apparent in the leg setation: the second exopodal segment of leg 1 is shown with 5 distal setal elements by Kabata but only 4 were present in our material (and Izawa’s). Given the apparent variability observed in leg setation, leading to the consideration of many aspects of setation patterns as unreliable for taxonomy ( Kabata, 1991), it appears probable that Kabata’s unidentified material should also be attributed to H. aigoi , especially as it was found on the same host species and in the same locality as the material described here.