Caligus bonito Wilson, 1905

(Figs. 21, 22)

Syn: Caligus kuroshio Shiino, 1959

Material examined. 20♀, 9♂♂ from Euthynnus affinis (Cantor, 1849) (TC18121), 21 November 2016; 10♀♀, 4♂♂ QM Reg. No. W53058; 10♀♀, 5♂♂ NHMUK Reg. Nos 2017.237–246.

Site on host. Gill cavity.

Differential diagnosis. Cephalothorax dorsoventrally flattened with well-developed marginal membranes; frontal plates with lunules. Genital complex 1.04 times longer than wide (measured along dorsal midline) with small posterolateral lobes (Fig. 21A); abdomen elongate, about 3.8 times longer than wide and longer than genital complex. Antenna with small, blunt posterior process (Fig. 21B). Post-antennal process with weakly-curved tine; associated papillae each bisensillate. Posterior process of maxillule slender (Fig. 21B). Maxilla with smooth distal margin on brachium (Fig. 21C). Maxilliped of female with slight expansion on myxal margin (Fig. 21D). Sternal furca with slender, diverging tines without lateral flanges (Fig. 21E). Distal exopodal segment of leg 1 (Fig. 21F) with 3 plumose setae on posterior margin, basal setules on margin of plumose setae nearest apex unusually stout; distal spine 1 simple, longer than spines 2 and 3, each armed with accessory process; seta 4 about twice as long as spines and about as long as segment, ornamented with setule row on one side and marginal membrane on other. Leg 2 with strong denticles along outer margins of endopodal segments 1 and 2 (Fig. 21G); endopodal segment 3 ornamented with 2 shallow membranous ridges on ventral surface; outer spines on exopodal segments 1 and 2 lying obliquely across axis of ramus (Fig. 21H); proximal outer spine on segment 3 tiny, much shorter than distal spine. Leg 3 with 3-segmented exopod; first exopodal segment (Fig. 22A) bearing small slightly curved outer spine, lacking inner seta; second segment with outer spine and inner seta; third segment with 3 naked spines, plus 4 plumose setae; distal endopodal segment with swollen lateral margin. Leg 4 uniramous, 3-segmented (Fig. 22B); first and second exopodal segments with I and IV spines, respectively. Mean body length of female 4.80 mm, range 4.46 to 5.31 mm (based on 10 specimens).

Male (Fig. 22C) with cephalothorax as in female. Genital complex slender with linear margins, about 1.7 times longer than wide. Abdomen 2-segmented; first segment just shorter than second; including caudal rami, about 3.4 times longer than wide and longer than genital complex. Fifth legs located ventrally near posterior corners of genital complex, not visible in dorsal view. Post-antennal process (Fig. 22D) more strongly recurved than in female; associated papillae bisensillate. Maxilliped (Fig. 22E) with 3 small processes along myxal margin, middle process largest with bifid apex. Mean body length of male 4.91 mm, range 4.31 to 5.27 mm (based on 5 specimens).

Remarks. The distinctive features of C. bonito, within the C. bonito -group, are its long genital complex and long abdomen combined with the presence of stout denticles along the outer margins of endopodal segments 1 and 2 of leg 2, plus the unusually stout basal setules on the margin nearest the limb apex of the 3 plumose setae on the posterior margin of the distal exopodal segment of leg 1. Interestingly, Ho & Lin (2004) noted the existence of two forms of C. bonito in Taiwanese waters. Lin & Ho (2001) initially reported C. bonito from Katsuwonus pelamis (Linnaeus, 1758) and Euthynnus affinis, and their redescription of the female showed an abdomen about 3.7 times longer than wide and shorter than the cephalothorax, a sternal furca with tapering, slightly curved tines that lacked marginal flanges, bisensillate papillae on the post-antennal process, and seta 4 on the distal exopodal segment of leg 1 is much longer than the segment. In their subsequent study of C. bonito from Sarda orientalis (Temminck & Schlegel, 1844) Ho & Lin (2004) illustrated another form characterized by an abdomen that is about 4.0 times longer than wide and markedly longer than the cephalothorax, a sternal furca with divergent linear tines that have marginal flanges, multi-sensillate papillae on the post-antennal process, and seta 4 on the distal exopodal segment of leg 1 is only just longer than the segment. There are other minor differences, for example, the posterior process on the female antenna is much larger in the latter form (from S. orientalis).

The Moreton Bay material can be attributed to typical C. bonito as described by Wilson (1905), Kabata (1979) and Pillai (1985), all of which show females with the abdomen shorter than the cephalothorax. The detail provided by Pillai (1985) confirms the presence of other relevant character states, such as the tapering tines without marginal flanges on the sternal furca, the bisensillate papillae on the post-antennal process, and a seta 4 on the distal exopodal segment of leg 1 that is much longer than the segment. The Moreton Bay sample shares these character states with the material described by Lin & Ho (2001). The status of the second, atypical, form (from S. orientalis) described by Ho & Lin (2004) needs to be re-assessed as it may represent a distinct species.

Although it has been reported from at least 24 different hosts (Ho & Lin, 2004), C. bonito is typically a parasite of scombrids of the tribes Sardini and Thunnini (Cressey & Cressey, 1980) and of dolphinfish ( Coryphaenidae) (Burnett-Herkes, 1974; Carbonell et al., 1999; Hutson et al., 2011), and it has a broad distribution in low to middle latitudes in all oceans (Cressey et al., 1983: Fig. 10). Caligus bonito has previously been reported from Queensland on Euthynnus alletteratus (Rafinesque, 1810) as C. kuroshio (Kabata, 1965b), from New South Wales on Sarda australis (Cressey & Cressey, 1980) and from southeastern Australia on Arripis truttacea (Cuvier, 1829), A. trutta (Forster, 1801) and Coryphaena hippurus Linnaeus, 1758 (Catalano & Hutson, 2010; Hutson et al., 2011). The material collected in Moreton Bay was found on Euthynnus affinis, a previously known host of C. bonito (Cressey & Cressey, 1980) .