Caligus sicarius Kabata, 1984

Boxshall, Geoff, 2018, The sea lice (Copepoda: Caligidae) of Moreton Bay (Queensland, Australia), with descriptions of thirteen new species, Zootaxa 4398 (1), pp. 1-172 : 123-127

Caligus sicarius Kabata, 1984

( Figs. 57 , 58 )

Material examined. 1♀ from Mugil cephalus Linnaeus, 1758 (TC17307) 20 January 2016, QM Reg. No. W53112.

Site on host. Unknown (in body wash).

Description. Adult female ( Fig. 57A ) body length 2.42 mm including caudal rami. Dorsal cephalothoracic shield subcircular (1.66 mm long x 1.56 mm wide); comprising about 70% of total body length. Free margin of thoracic portion of dorsal cephalothoracic shield extending posteriorly beyond rear margins of lateral portions. Lunules present ventrally on frontal plates. Genital complex fused to fourth pedigerous somite; subrectangular, 1.6 times wider than long (0.89 x 0.53 mm); with convex lateral margins and rounded corners ( Fig. 57B ); fifth legs located ventrally close to posterolateral corners ( Fig. 57C ). Genital complex about 4.2 times longer than abdomen. Abdomen 1-segmented; about 1.4 times wider than long (0.30 x 0.21 mm) in ventral view ( Fig. 57B ); carrying paired caudal rami distally; anal slit terminal. Caudal rami with parallel sides, just longer than wide, measured at midpoints of margins. Each ramus armed with short hirsute seta at inner distal angle, slightly longer hirsute seta at outer distal angle, minute hirsute seta located just ventral to outer distal seta, and 3 long plumose setae on distal margin.

Antennule ( Fig. 57D ) 2-segmented; large proximal segment with 25 plumose setae arrayed along anteroventral surface and 2 setae located dorsally; short distal segment bearing 12 elements (10 setae plus 2 aesthetascs) around apex, plus isolated seta on posterior margin. Antenna ( Fig. 57E ) comprising proximal segment bearing spatulate posteriorly-directed spinous process; middle segment short, subrectangular, unarmed; terminal segment forming slender recurved claw bearing accessory spine proximally, and armed with slender distal seta. Post-antennal process ( Fig. 57E ) well-developed, weakly curved; ornamented with 2 unisensillate papillae on basal part and single unisensillate papilla on adjacent ventral cephalothoracic surface. Small process present between base of antenna and post-antennal process.

Mandible of typical stylet-like structure, with 12 marginal teeth. Maxillule ( Fig. 57E ) comprising anterior papilla bearing only 2 unequal, naked setae and posterior, tapering process; sclerite lying medial to papilla enlarged and forming spinous process overlapping base of maxillulary process. Maxilla 2-segmented, comprising elongate unarmed syncoxa and basis: basis bearing membranous subapical flabellum on anterior margin, and terminating in 2 subequal claw-like elements (calamus and canna). Calamus slightly longer than canna, ornamented with strips of serrated membrane arranged obliquely around surface; canna ornamented with strips of serrated membrane. Maxilliped subchelate ( Fig. 57F ); slender proximal segment unarmed and without process on myxal surface; distal subchela with apical claw; short, slender seta present on concave margin.

Sternal furca ( Fig. 58A ) with wedge-shaped tines, gape between tines square-sided. Tapering accessory processes present either side of furca.

First swimming leg pair ( Fig. 58B ) joined by slender intercoxal sclerite; sympod with inner and outer plumose setae derived from basis; endopod represented by unarmed process on posterior margin of basis. Exopod 2- segmented, directed laterally and forming main axis of leg; first segment robust, about 2.9 times longer than wide and armed with small outer (anterior) spine and ornamented with row of setules along posterior margin; second segment short, armed with 3 long plumose setae along posterior margin and 4 distal elements along oblique distal margin. Distal elements ( Fig. 58B , inset) as follows: spine 1 (anterior-most) longest with margin membrane distally; spines 2 and 3 progressively smaller, each with bilateral serrated membrane distally; seta 4 naked, about as long as spine 3, shorter than segment.

Second leg ( Fig. 58C ) biramous, with flattened protopodal segments and 3-segmented rami. Coxae of leg pair joined by intercoxal sclerite bearing marginal membrane posteriorly. Coxa with plumose seta and surface sensilla. Basis armed with outer naked seta; ornamented with surface sensilla, marginal membrane posteriorly, and flap of membrane anteriorly, reflexed back over dorsal surface of segment. Exopodal segment 1 with inner plumose seta and large naked outer spine extending obliquely across surface of ramus but closely aligned to lateral margin, ornamented with flap of membrane anteriorly, reflexed back over dorsal surface of segment; exopodal segment 2 with outer spine directed-distally plus inner plumose seta; segment 3 with 2 outer spines (proximal spine small and naked, distal spine with bilateral membrane); apical spine with marginal membrane laterally and pinnules medially, and 5 inner plumose setae. Endopodal segments 1 and 2 armed with 1 and 2 inner plumose setae respectively; segment 3 with 6 plumose setae; outer margins of all endopodal segments ornamented with fine setules.

Third leg pair ( Fig. 58D ) forming flattened plate closing posterior part of cephalothoracic sucker as typical for genus. Protopodal part flattened joined by plate-like, intercoxal sclerite (apron) ornamented with marginal membrane posteriorly and along lateral margin anterior to exopod; bearing inner plumose seta at junction with intercoxal plate, and outer plumose seta dorsal to base of exopod; sensillae located adjacent to inner coxal seta and adjacent to origin of endopod. Exopod 2-segmented; first segment armed with straight outer claw directed over ventral surface of ramus and small inner plumose seta; distal segment compound, with 1 proximal and 3 distal spiniform elements on outer margin and 5 plumose setae on distal and inner margins; outer margin of distal segment ornamented with rows of slender setules either side of first outer spine. Endopod 2-segmented; first segment expanded laterally to form flap-like velum closing off space between rami; velum ornamented with row of fine setules along free margin and bearing inner plumose seta; compound distal segment with 6 setal elements increasing in length from outermost to innermost.

Fourth leg 2-segmented ( Fig. 57B ), comprising slender protopodal segment and 1-segmented exopod: protopodal segment armed with plumose seta distally; exopodal segment ( Fig. 58E ) with partial suture marking plane of fusion between former segments; armed with outer spine extending beyond distal margin of segment, long apical element with serrated membrane on along inner margin, plus 2 short naked spines located just lateral to base of apical spine, apical spine with well-developed pecten at base.

Fifth legs located posterolaterally on genital complex ( Fig. 57B ); represented by plumose seta on anterior papilla plus 1 short naked seta and 2 plumose setae on posterior papilla representing exopod ( Fig. 57C ).

Remarks. This small Caligus has not been reported since its original discovery in the Red Sea ( Kabata, 1984) and is redescribed here because it exhibits several unusual character states. It has a secondary (accessory) tine located proximally on the distal segment of the female antenna. This is a common state for males in the genus, as in males of the Caligus diaphanus -group, but is relatively rare among females. The wedge-shaped tines of the sternal furca of C. sicarius are also unusual, especially when combined with the presence of paired accessory processes either side of the furca. Accessory processes are present in several other species, including C. coryphaenae Steenstrup & Lütken, 1861 and some members of the Caligus productus -group such as C. haemulonis Krøyer, 1863 and C. turbidus sp. nov. (Boxshall & El-Rashidy, 2009; present account).

Leg 3 was not fully figured by Kabata (1984). It has a 2-segmented exopod with a compound distal segment representing the second and third ancestral segments. This state is characteristic of other caligid genera such as Anuretes and Mappates , but is less common in Caligus species which most commonly retain a 3-segmented condition. The presence of an inner plumose seta on the first exopodal segment of leg 3 is also uncommon in Caligus .

The exopod of leg 4 is unique: no other Caligus has such a configuration. The exopodal segments are separated only by a trace of the suture marking the plane of the original articulation between the ancestral first and second exopodal segments. There is a long spine on the outer margin derived from the first exopodal segment and the distal margin carries a long apical spine with a conspicuous pecten at its base, plus 2 reduced outer spines. It most closely resembles the leg 4 of Caligus pageti Russell, 1925 , which has a 1-segmented exopod bearing one outer spine about at mid margin, and one long apical spine plus a short outer spine.

The type host was a mugilid, Crenimugil crenilabis (Forsskål, 1775) (as Crenimugil crenilabris ), caught in the Gulf of Aqaba (Red Sea). The single female found in Moreton Bay is only the second record of this copepod and it was collected from another mugilid, Mugil cephalus . This rare but widely distributed species appears to be specific to mugilids. Interestingly, Caligus pageti is also a parasite of mugilids and has been found in the Mediterranean and adjacent Atlantic coastal waters.

Figures

FIGURE 57. Caligus sicarius Kabata, 1984, female. A, habitus, dorsal; B, fourth pedigerous somite, genital complex and abdomen, ventral;C, left corner of genital complex showing leg 5, ventral; D, antennule; E, antenna,post-antennal process and maxillule drawn in situ; F, maxilliped.Scale bars:500 µm on A, B,100 µm on C–D, 200 µm on E–F.

FIGURE 58. Caligus sicarius Kabata, 1984, female. A, sternal furca and accessory processes, ventral view in situ; B, leg 1, with inset showing setationelements of distal margin; C, leg 2; D, leg 3, ventral; E, exopod of leg 4. Scale bars:200 µm on A– D, 100 µm on E.