Caligus selenecola sp. nov.

Figs 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12

Type host.

Hairfin lookdown Selene brevoortii (Gill) ( Carangidae).

Type locality.

Mexican Pacific, off Mazatlán Port (23 ° 12 ' N, 106 ° 26 ' W), Sinaloa, Mexico.

Prevalence.

15 % (9 / 57).

Type material.

Holotype, adult female preserved in ethanol (ICML-EMUCOP -090621-01), collected on 9 June 2021 . Allotype, adult male preserved in ethanol (ICML-EMUCOP -081121-01), collected on 8 November 2021 . Paratypes, 1 adult female preserved in ethanol (ICML-EMUCOP -090621-02) from the same host individual as the holotype; 1 adult female preserved in ethanol (ICML-EMUCOP -120921-01) from which pair of antennules and P 1 a were dissected and mounted onto two slides, and 1 adult female dissected and mounted onto ten slides (ICML-EMUCOP -120921-02), collected on 12 September 2021; and 1 adult female and 1 adult male preserved in ethanol (ICML-EMUCOP -221121-01) from a single host individual collected on 22 November 2021 .

Site on host.

Gills.

Etymology.

The specific name comes from the host genus name Selene, and the Latin suffix - cola, inhabitor. It is in the nominative singular, gender masculine.

Differential diagnosis.

Caligidae . Female: cephalothoracic shield subcircular, with well-developed distinct paired frontal plates, the latter with large ventral lunules. Genital complex nearly as long as wide, slightly shorter than abdomen. Abdomen indistinctly separated from genital complex. Caudal rami twice as long as wide; armed with six setae. Antennule two-segmented; proximal segment with 27 plumose anterior setae; second segment with 13 naked setae and one aesthetasc. Antenna indistinctly four-segmented; second segment without process; without postantennal process; postantennal area with three setule-bearing papillae. Maxilliped with tiny denticle process in myxal area. Sternal furca absent. P 1 biramous; P 1 ENP vestigial; P 1 EXP two-segmented; P 1 EXP 2 with three plumose setae on posterior margin, distally with lateral spine 1 minute, elements 2 and 4 spiniform without accessory process, element 3 longest with membranous inner flange. P 2 biramous; ENP and EXP tree-segmented; endopodal segments with patch of surface setules anteriorly. P 3 biramous; ENP two-segmented; EXP three-segmented. P 4 uniramous; EXP three-segmented; outer spines of EXP 3 with transverse strip of membrane (modified pecten) close to insertion of spines. Female P 5 vestigial, comprised of small lobe with one seta, and larger elongate lobe with three elements. Male: abdomen with two free somites. Antenna three-segmented; middle segment with two corrugated pads and anterior rows of fine striations; distal segment forming long claw with one accessory process and one tiny seta. Maxillule as in female except for dentiform process with blunt distal process. Maxilliped three-segmented; with two conical projections on myxal area of proximal segment. P 5 with three setae. P 6 represented by two plumose setae.

Description.

Adult female (Figs 1 – 9). Mean total body length measured from anterior margin of frontal plate to posterior margin of caudal rami, 3.1 mm (ranging from 2.8 to 3.4 mm; n = 6). Cephalothoracic shield (Fig. 1 A – C) subcircular, slightly longer than wide; with well-developed distinct paired frontal plates, the latter with large ventral lunules; with posterior sinuses as shown; medial posterior margin of thoracic zone extending beyond posterolateral margins of cephalothoracic shield; with hyaline membrane along distal margin of frontal plates and laterally. Free fourth pedigerous somite (Fig. 1 A – C) slightly wider than long, indistinctly separated from genital complex. Genital complex (Fig. 1 A – C) nearly as long as wide, slightly shorter than abdomen, genital complex: cephalothoracic shield length radio 0.7; with posterolateral processes. Abdomen (Figs 1 A – C, 2 A) indistinctly separated from genital complex; ~ 3.5 × as long as wide; with a slight constriction anteriorly. Caudal rami (Figs 1 A – C, 2 A – D) twice as long as wide; armed with six setae of which two short and four long plumose, ornamented with inner row of setules. Egg sacs (not figured) uniseriate.

Antennule (Fig. 3 A, B) two-segmented. Proximal segment longer than distal, the former with 27 plumose anterior setae. Second segment cylindrical, bearing 13 naked setae and one aesthetasc.

Antenna (Fig. 3 C – E) indistinctly four-segmented. First segment unornamented, second segment without process, third segment unornamented, terminal segment a curved claw with one minute seta. Without postantennal process; postantennal area with three setule-bearing papillae (arrowed in Fig. 3 F).

Mandible (Fig. 4 A, B) of typical stylet-like structure; with twelve marginal teeth.

Maxillule (Fig. 4 C) comprising anterior papilla bearing three unequal naked setae; with posterior moderately long dentiform process.

Maxilla (Fig. 5 A – C) two-segmented, brachiform, comprising elongate unarmed lacertus and slender brachium, the latter with flabellum (Fig. 5 C) slightly above halfway inner margin and with long calamus and shorter canna; calamus with strips of serrated membrane arranged obliquely around surface; canna with bilateral strips of serrated membrane.

Maxilliped (Fig. 5 D – G) subchelate; corpus concave proximally, with tiny denticle process in myxal area; claw with naked seta on posterior surface (arrowed in Fig. 5 G).

Sternal furca absent.

P 1 (Fig. 6 A – E) biramous, with slender, naked intercoxal sclerite. Sympod with one inner and one outer plumose seta, and one proximolateral seta. Endopod vestigial, represented by unarmed process bearing one tiny apical element. Exopod two-segmented. P 1 EXP 1 with inner row of setules and one small spine at outer distal corner; P 1 EXP 2 with three plumose setae on posterior margin, and four distal elements as follows: spine 1 minute, arising on outer lateral margin of segment (arrowed in Fig. 6 B, C, E); elements 2 and 4 spiniform, seemingly without accessory process, the former slightly longer than the latter, both visibly shorter than element 3; the latter longest, with membranous inner flange.

P 2 (Fig. 7 A – C) biramous, with subquadrate intercoxal sclerite bearing distal hyaline membrane. Coxa with one inner plumose seta and one anterior sensillum. Basis with one outer small seta, one inner sensillum, and with inner hyaline membrane. Exopod three-segmented; first segment as long as second and third segments combined, with one plumose inner seta, one outer stout spine, and with dorsally flexed membrane along outer margin; second segment smallest, with one inner long plumose seta, one small outer spine, and ornamented with inner row of setules; third segment with five plumose inner setae, and two outer spines of which proximal shortest and one outer subdistal seta, the latter stout and with inner hyaline membrane. Endopod three-segmented; first segment with one inner plumose seta, with patch of surface setules anteriorly; second segment with two inner plumose setae and ornamented with anterior patch of setules; third segment with six plumose setae and ornamented with anterior patch of setules.

P 3 (Fig. 7 D – G) with coxa and basis fused into flattened apron-like sympod, with one small outer plumose seta near insertion of exopod, one inner long plumose seta near intercoxal sclerite, and two widely separated sensilla along posterior margin. Sympod and intercoxal sclerite with extended strips of hyaline membrane along lateral and free posterior margins. Exopod three-segmented; first segment with one apical spine longer than segment and reaching slightly beyond articulation between second and third exopodal segments; second segment with one outer spine and one inner plumose seta, ornamented with outer row of setules; third exopodal segment with three outer spines (proximal outer smallest), and four plumose setae, ornamented with outer setules. Endopod two-segmented; first segment extended laterally for form velum, and armed with long inner plumose seta; second segment with six plumose setae.

P 4 (Fig. 8 A – F) uniramous. Protopodal segment with 1 distal seta. Exopod three-segmented; first and second segments with one outer spine each; third segment with three outer spines, with transverse strip of membrane (modified pecten) close to insertion of spines. All spines subequal in length.

P 5 (Fig. 9 A, B) vestigial, situated on ventral surface, near outer margin of posterolateral lobe of genital complex; comprised of small lobe with one plumose seta, and larger elongate lobe with three plumose setae.

Armature formula of P 1 – P 4 as follows (Roman numerals for spines; Arabic numerals for setae):

P 6 possibly represented by pair of protuberances located posteromedial to P 5 (Figs 1 A – C, 2 A).

Adult male (Figs 10 – 12). Total body length measured from anterior margin of frontal plate to posterior margin of caudal rami 1.9 mm long (n = 1). Cephalothoracic shield (Fig. 10 A, B) as in female but with narrower anterior region, slightly wider than long. Free fourth pedigerous somite (Fig. 10 A, B) slightly wider than long, indistinctly separated from genital complex. Genital complex (Fig. 10 A, B) as long as wide. Abdomen (Fig. 10 A, B) with two free somites; first somite slightly wider than long, second somite slightly longer than wide. Caudal rami (Fig. 10 A, B) ~ 2 × as long as wide, with two short and four long plumose setae. All appendages as in female, except for antenna, maxillule, and maxilliped.

Antenna (Fig. 11 A, B) three-segmented; proximal segment small and unarmed; middle segment with two corrugated pads and anterior rows of fine striations; distal segment forming long curved pointed claw with one accessory process and one tiny seta near its base. Postantennal area (not shown) as in female.

Maxillule (Fig. 11 C, D) as in female except for dentiform process with blunt distal process.

Maxilliped (Fig. 12 A, B) three-segmented; myxal area of proximal segment with two conical projections bearing subterminal tooth; subchela as in female.

P 5 (Fig. 12 C, D) located at approximately midway along lateral margin of genital complex; composed of small lobe bearing three plumose setae.

P 6 (Fig. 12 C) represented by two plumose setae at tip of a posteroventral protuberance on genital complex.