Caligus sarosensis, Özak & Kurt & Kamanli & Akbulut & Yanar & Boxshall, 2024

Özak, Argun Akif, Kurt, Tuba Terbiyik, Kamanli, Seyit Ali, Akbulut, Gürkan, Yanar, Alper & Boxshall, Geoffrey A., 2024, Two new species of planktonic Caligus O. F. Müller 1785 (Copepoda: Caligidae) from Türkiye with an updated review and checklist of planktonic caligids, Zootaxa 5437 (3), pp. 301-335 : 315-325

Caligus sarosensis sp. nov.

LSIDurn:lsid:zoobank.org:act: C1548B11-522F-4D50-A2C0-0E6A65FAE685

Type material

Holotype adult female stored in collections of the Natural History Museum, London (Reg, No, NHMUK 2022 . 200). Type Locality: Stn. SABSW1 , Gulf of Saros, Türkiye (40°37ʹ52.52ʺN 26°43ʹ39.70ʺE): depth 60 m; date 20.02.2019.

Etymology: The species name refers to the Gulf of Saros (Çanakkale, Ţrkiye) where the material was collected.

Description

Adult female [ Figs. 8–14 ]. Body ( Figs. 8A , 9A ) comprising cephalothorax, free fourth pedigerous somite, genital complex, and 1-segmented abdomen.Total body length 2.06 mm including caudal rami. Dorsal cephalothoracic shield longer than wide, 1.06 × 0.77, narrowing anteriorly towards narrow frontal plate bearing pair of large lunules; lateral margins convex, with broad hyaline membrane around lateral margins. Cephalothorax comprising approximately half of total body length. Thoracic zone of shield about as long as wide, 0.56 × 0.63, forming large posterior sinuses on either side; posterior margin extending beyond posterior end of lateral zones. Fourth pedigerous somite about 2.6 times wider than long (0.10 × 0.26). Genital complex ( Figs. 8A, B , 9A ) 0.54 × 0.38, subrectangular, with parallel lateral margins and weakly lobate posterolateral corners, anterior part indented, forming folded narrow transition between fourth pedigerous somite and genital complex; lateral indentation and partial surface suture present on anteroventral surface of genital complex at about one third of complex length ( Figs. 8B black arrows, 9B white arrows). Two subrectangular flap-like processes on medioventral surface either side of median line ( Figs. 8B , 9B, 9C arrowheads) (SV-SAR5), left and right processes subequal (41 × 74 µm, 45 × 74 µm, respectively). Posteroventral surface of genital complex swollen and produced into lobate posterolateral corners overlying oviduct openings (egg sac attachment area) ( Figs. 8C , 9D ). Free abdomen 1-segmented, slender, 2.3 times longer than wide (0.32 × 0.14), genital complex and abdomen fused laterally, abdomen with numerous overlapping cuticular folds anterolaterally ( Figs. 8C black arrows, 9D white arrows) (SV-SAR6): abdomen with paired swellings on anteroventral surface either side of mid-line, inner margins of each swelling connecting to another digitiform cuticular medial swelling carrying two tiny, triangular papilliform processes ( Figs. 8C arrowheads, 9D arrowheads) (SV-SAR6), mid-anteroventral surface ornamented with 2 sensilla ( Figs. 8C long arrows, 9D long arrows) (SV-SAR6). Combined length of genital complex and abdomen about 84% of length of cephalothorax. Caudal ramus ( Figs. 8C , 9D ) subrectangular, about 1.6 times longer than wide (0.08 × 0.05), bearing 6 plumose setae plus tuft of spinules along inner margin.

Antennule ( Fig. 8D ) 2-segmented; proximal segment about 1.7 times longer than slender distal segment; proximal segment armed with 25 plumose setae anteroventrally near anterior margin plus 2 naked dorsal setae ( Fig. 8D arrowheads); slender distal segment carrying 1 subterminal seta on posterior margin and 11 terminal setae plus 2 aesthetascs ( Fig. 8D arrows). Antenna ( Figs. 10A , 11A ) 3-segmented; proximal segment without posterior process; middle segment subrectangular; distal segment forming acutely curved claw, with minute distal seta on outer margin at mid-length ( Figs. 10A arrowhead, 11A arrowhead) plus proximal seta. Postantennal process ( Figs. 10B , 11B ) weakly curved, carrying accessory tine proximally ( Fig. 11B arrowhead) plus two papillae on basal part, each with 6 sensilla; similar papilla with 6 sensilla present on adjacent ventral surface. Maxillule ( Figs. 10C , 11C ) comprising slightly curved dentiform posterior process, anterior papilla bearing 1 long and 2 small setae; posterior part of dentiform process ornamented with narrow marginal flange bilaterally. Mouth tube ( Fig. 10D ) approximately 1.1 times longer than wide. Distal margins of labium and labrum fringed with hyaline membrane. Labrum with submarginal row of minute denticles. Mandible ( Fig. 10E ) curved distally and bearing 12 inner teeth near apex. Maxilla ( Fig. 10F , 11D ) 2-segmented, brachiform; proximal segment (lacertus) unarmed; distal segment (brachium) armed with small subdistal outer hyaline membrane (flabellum) plus 2 elements at apex (short canna and long calamus) ( Fig 11E ), canna with bilaterally serrated hyaline membrane, calamus longer than canna and ornamented with spirally arranged strips of serrated membrane ( Figs. 10F , 11E ). Maxilliped ( Figs. 10G , 11F ) comprising large proximal segment (corpus) and distal subchela representing fused endopodal segments plus curved terminal claw; subchela armed with small seta at base of claw, tip of claw extending almost to middle of corpus; myxal area smooth. Sternal furca ( Fig. 10H ) with short, straight, weakly divergent tines rounded at tip, tines with marginal flanges.

Leg 1 ( Fig. 12A ) biramous, with 2-segmented exopod and lobate vestigial endopod carrying minute denticle at apex ( Fig. 12B ). Sympod armed with lateral plumose seta and inner seta. First exopodal segment ( Fig. 12A ) bearing small spine at outer distal corner but lacking row of setules along free posterior margin ( Figs. 12A , 14A ). Distal exopodal segment ( Figs. 12C , 14A ) with 4 terminal elements (spines 1–3 and inner seta 4) on distal margin; spine 1 (outermost) shortest, middle 2 spines almost equal in length and each bearing single accessory process ( Fig. 12C ), seta 4 (innermost) about 4 times longer than middle two spines 2 and 3 and slightly longer than segment; posterior margin with three plumose setae ( Fig. 14A ), each distinctly longer than segment.

Leg 2 ( Fig. 12D ) biramous with 3-segmented rami. First exopodal segment with pinnate seta on inner margin and long spine at outer distal corner reflexed obliquely back across surface of second exopodal segment; second segment lacking outer spine, armed only with pinnate seta on inner margin ( Figs. 12E , 14B ): third exopodal segment with 5 inner plumose setae plus 3 spines; proximal outer spine (smallest) naked and slender ( Figs. 12E,F , 14B arrow), middle spine with hyaline membrane bilaterally; terminal spine ornamented with hyaline membrane along outer margin and row of setules along inner margin ( Figs. 12E, F , 14B ). First endopodal segment armed with inner plumose seta; second endopodal segment armed with 2 inner plumose setae and ornamented with rows of setules along outer and inner margins; third segment with 6 plumose setae and bearing tuft of setules proximal to outermost seta ( Fig. 12G ).

Leg 3 ( Fig. 13A ) with narrow intercoxal sclerite, coxa and basis fused into flattened apron-like protopod ornamented with extended strips of hyaline membrane along lateral and posterior margins. Inner coxal seta and outer basal seta both pinnate. Exopod 3-segmented, first segment with outer spine extending beyond middle of second segment, orientated parallel with longitudinal axis of ramus, base of spine with pecten-like strip of with hyaline membrane; second segment with outer spine and inner plumose seta, plus setules along outer margin; third segment with outer row of setules and 3 outer spines ( Fig. 13B ) (first and second spines almost equal in length, third spine slightly shorter than first 2) plus 4 pinnate setae ( Fig. 13A ). Endopod ( Fig. 13A ) 2-segmented; first segment armed with long, inner pinnate seta and forming flap-like velum extending to base of exopod, ornamented with row of fine setules along free posterior margin; second segment bearing 6 pinnate setae and bearing row of long setules along outer margin.

Leg 4 ( Fig. 13C ) uniramous. Protopodal segment with outer seta derived from basis. Exopod 2-segmented; first segment armed with long slender, bilaterally flanged, outer distal spine; compound distal segment with 1 lateral spine and 3 apical spines along oblique distal margin; inner apical spine longest. Inner and middle apical spines each with pecten at base ( Figs. 13D , 14C ). Armature of rami of legs 1–4 as follows (Roman numerals indicating spines and Arabic numerals indicating setae).

Exopod Endopod Leg 1 I-0; III,1,3 vestigial Leg 2 I-1; 0-1; II, I, 5 0–1; 0–2; 6 Leg 3 I-0; I-1; III, 4 0–1; 6 Leg 4 I-0; I, III absent

Leg 5 ( Fig. 13E ) located on posterolateral ventral surface of genital complex and represented by 2 small papillae; outer (protopodal) papilla with single long pinnate seta; inner (exopodal) papilla carrying 2 shorter unequal pinnate setae; sensilla located on surface close to inner papilla.

Remarks

The new species, C. sarosensis sp. nov. exhibits numerous detailed similarities in the structure and setation of legs 1 to 4 with C. izmiriensis sp. nov. (described above) and Metacaligus yucatanensis is very similar to both new species, having an unusual leg 1 with a rounded distal margin on the second exopodal segment and spines 1 to 3 are small and clustered close to the base of seta 4. In addition, spines 2 and 3 each bear an accessory process and seta 4 is longer than the segment. This configuration differs from that of M. yucatanensis only in the lack of accessory processes on spines 2 and 3, which are absent according to Suárez-Morales et al. (2012b).

In C. izmiriensis sp. nov. the second exopodal segment of leg 2 carries a tiny vestige of an outer margin spine but both C.sarosensis sp. nov. and M. yucatanensis lack any vestige of this spine. The loss of this spine is an extremely rare character state in Caligus . In leg 3 of C. izmiriensis sp. nov. the gap between the rami is smaller than in the other two species, so the velum is not quite so broad, but all three share an elongate second exopodal segment and the outer spine on the first exopodal segment is straight and reaches only about two thirds of the distance along the second segment. There is also a pecten-like, rounded strip of hyaline membrane positioned across the base of the outer spine on the first exopodal segment in all these species. The segmentation and armature of leg 4 is similar in these three species and all have the same arrangement of pectens. Despite these detailed similarities, these species can be readily distinguished by the sternal furca: in C. izmiriensis sp. nov. the sternal furca has long, closely adpressed tines, compared with short, widely separated tines in C. sarosensis sp. nov., and by the complete absence of the sternal furca in M. yucatanensis .

The paired cephalothoracic appendages of the two new species are rather generic and lack any distinctive characteristics. However, the sensory papillae associated with the postantennal process are multisensillate in both species and the presence of 6 or 7 sensilla per papilla is unusually high as there are rarely more than 4 in the great majority of Caligus species where the number is known. The presence of an accessory tine on the postantennal process of C sarosensis sp. nov., serves to distinguish it from C. izmiriensis sp. nov. which lacks such an accessory tine.

The new species from the Gulf of Saros exhibits a slender dorsal cephalothoracic shield which narrows anteriorly towards the frontal plate which bears the paired lunules, and the lateral margins of the shield are provided with unusually broad marginal membrane. This general morphotype is typical of so-called “planktonic” caligids, such as Caligus undulatus Shen & Li, 1959 . Ohtsuka et al. (2020) listed the narrow frontal plate as a diagnostic characteristic of their newly proposed Caligus undulatus -group of species which comprised five, possibly six, species, each of which had originally been described from plankton samples rather than from material obtained from a host fish. As defined by Ohtsuka et al. (2020), the C. undulatus -group is characterized by: 1), leg 4 is 3-segmented with IV spines on compound distal exopodal segment; 2), three plumose setae are present on posterior margin of second exopodal segment of leg 1; 3), outer spines on distal exopodal segment of leg 2 are small or reduced; 4), antenna with weak or well-developed process on proximal segment; 5), body with relatively narrow frontal plate; 6), female genital complex longer than wide, sometimes with outer margin undulated; and 7), male urosome slender, with 2-segmented abdomen. In addition to C. undulatus, Ohtsuka et al. (2020) included in this species group C. evelynae Suárez-Morales, Camisotti & Martin, 2012 , C. longiramus Venmathi Maran, Ohtsuka & Jitchum, 2012 , C. ogawai Venmathi Maran, Ohtsuka & Shang, 2012 , C. tripedalis Heegaard, 1972 , and possibly C. hyalinae Heegaard, 1966 . Subsequently, Boxshall & Bernot (2023) considered that C. hyalinae should be treated as a junior subjective synonym of C. chelifer Wilson, 1905 , and they confirmed that this species does also belong in the C. undulatus -group. Both new species share C. undulatus -group characteristics 1, 2, 4, 5, and 6. Caligus izmiriensis sp. nov. also shares character 7 but the male (and, therefore, the state of characteristic 7) is unknown in C. sarosensis sp. nov. Characteristic 4 is not robust since it incorporates the two extremes (and presumably all states in between) of a well-developed process on the proximal segment of the antenna versus proximal process lacking. Characteristic 3 is also variably expressed within the species group, ranging from minute in C. ogawai ( Venmathi Maran et al. 2012a) , via slender and setiform in C. evelynae ( Suárez-Morales et al. 2012a) , to about the same size as the distal outer spine in C. longiramus ( Venmathi Maran et al. 2012b) . In C. sarosensis sp. nov. the proximal outer spine on the distal exopodal segment of leg 2 is minute and in C. izmiriensis sp. nov. it is small.

Caligus izmiriensis sp. nov. and C. sarosensis sp. nov. both exhibit the same general morphotype as members of the C. undulatus -group, especially the narrow frontal plate and the broad marginal membrane around the dorsal cephalothoracic shield. We propose to place them in the C. undulatus - group together with C. undulatus , C. evelynae , C. longiramus , C. ogawai , C. tripedalis and C. chelifer . Interestingly Metacaligus yucatanensis also shares C. undulatus -group characteristics 1, 2, 4, 5, 6 and 7. Regarding characteristic 3, the proximal outer spine on exopodal segment 3 of leg 2 in M. yucatanensis is long although extremely slender and setiform in appearance ( Suárez-Morales et al. 2012b), somewhat like that of C. evelynae . Despite this impressive array of shared character states, the two new species can be readily distinguished from M. yucatanensis by the presence of a sternal furca in both new species. All species currently placed in Metacaligus lack a sternal furca ( Dojiri & Ho 2013; Suárez-Morales et al. 2012b) and the presence of a sternal furca prevents the placement of the new species in Metacaligus . However, in view of the close similarity between the new species and M. yucatanensis , it is necessary to review the validity of Metacaligus as a generic level taxon.

Figures

FIGURE 8. Caligus sarosensis sp. nov. Adult female. A. Habitus dorsal; B. Genitoabdomen (ventral) with partial surface suture on anteroventral surface of genital complex (arrows) and two subrectangular flap like processes (arrowheads); C. Abdomen (ventral) with overlapping cuticular folds (arrows) on anterolateral parts and digitiform cuticular medial swelling carrying two tiny triangular papilliform processes (arrowheads) plus 2 sensillae (long arrows) on mid-anteroventral surface; D. Antennule with 2 naked dorsal setae (arrowheads) on proximal segment and 2 aesthetascs (arrows) on distal segment.

FIGURE 9. Caligus sarosensis sp. nov. Adult female. Drishti applied confocal laser scanning microscope images (CLSM-D). A. Habitus dorsal; B. Habitus ventral; C. Two subrectangular flap like processes (arrowheads) on either side of the medioventral surface of genital complex (arrowheads); D. Abdomen (ventral) with overlapping cuticular folds (white arrows) on anterolateral parts, digitiform cuticular medial swelling carrying two tiny triangular papilliform processes (arrowheads) and 2 sensillae (long black arrows) on mid-anteroventral surface.

FIGURE 10. Caligus sarosensis sp. nov. Adult female. A. Antenna, minute distal seta (arrowhead) on outer margin at midlength of terminal claw; B. Postantennal process; C. Maxillule; D. Mouth cone; E. Mandible; F. Maxilla; G. Maxilliped; H. Sternal furca and intercoxal sclerite of leg 1.

FIGURE 11. Caligus sarosensis sp. nov. Adult female. Drishti applied confocal laser scanning microscope images (CLSM-D). A. Antenna, minute distal seta (black arrowhead) on outer margin at mid-length of terminal claw; B. Postantennal process with accessory tine at base (white arrowhead); C. Maxillule; D. Maxilla; E. Distal segment of maxilla with spirally arranged strips of serrated membrane on calamus and bilaterally serrated membrane on canna; F. Maxilliped.

FIGURE 12. Caligus sarosensis sp. nov. Adult female. A. Leg 1; B. Endopod; C. Terminal elements on distal segment of leg 1; D. Leg 2; E. Exopod of leg 2; F. Proximal outer spine on third exopodal segment of leg 2; G. Endopod of leg 2.

FIGURE 13. Caligus sarosensis sp. nov. Adult female. A. Leg 3; B. Outer spines on third exopodal segment of leg 3; C. Leg 4; D. Exopodal spines on leg 4; E. Leg 5.

FIGURE 14. Caligus sarosensis sp. nov. Adult female. Drishti applied confocal laser scanning microscope images (CLSM-D). A. Distal segment of leg 1; B. Exopod of leg 2, proximal outer spine on third exopodal segment arrowed; C. Leg 4 exopodal spines surrounded with serrated hyaline membrane.