Caligus lacustris Steenstrup & Lütken, 1861

Boxshall, Geoffrey A. & Bernot, James P., 2023, Resolving taxonomic and nomenclatural problems in the genus Caligus O. F. Müller, 1785 (Copepoda: Caligidae), Zootaxa 5360 (4), pp. 545-567 : 550-552

Caligus lacustris Steenstrup & Lütken, 1861

Material examined by R. F. Cressey: Syntype female from Esox lucius Linnaeus, 1758 caught in “Lake Furr”, Denmark, loaned to R. F. Cressey by the Zoological Museum, Copenhagen. In his unpublished manuscript Cressey stated that the specimen he borrowed from the Zoological Museum was “one of three” syntypes ex Esox lucius from “Lake Furr”, Denmark. However, Margolis et al. (1975) listed the type locality as “Lake Fuur” and “Lake Tiustrup, Denmark ”, while the online catalogue of the Zoological Museum lists only one lot of C. lacustris and gives the locality as “Lake Thystrup”. The Reg. No. of this material is NHMD 84850 (formerly CRU-007006).

Supplementary Description: Abdomen ( Fig. 4A ) about 1.5 times longer than wide. Caudal rami about 1.3 times longer than wide and armed with 6 setae as typical for genus. Antenna ( Fig. 4B ) with slender process on posterior margin of proximal segment and armed with 2 short setae proximally on subchela. Postantennal process ( Fig. 4B ) curved and ornamented with bisensillate papillae. Maxillule ( Fig. 4B ) with tapering posterior process. Sternal furca ( Fig. 4C ) with broad, blunt-tipped, parallel tines. Second exopodal segment of leg 1 ( Fig. 4D ) with 3 plumose setae along posterior margin; spines 1 to 3 on distal margin subequal, all lacking accessory process; seta 4 about equal in length to spines 1 to 3. Leg 2 ( Fig. 4E ) with outer margin of endopodal segment 2 ornamented with setules, slightly spreading onto surface; outer spine on first exopodal segment large and aligned obliquely across surface of ramus, spine on second segment small, curved and aligned along lateral margin of segment; third exopodal segment with II, I, 5 setal formula. Leg 3 exopod ( Fig. 4F ) with first segment lacking inner seta, bearing long, weakly curved, outer spine almost reaching articulation separating second and third segments. Leg 4 ( Fig. 4G ) comprising protopodal segment and 2-segmented exopod: first exopodal segment armed with long, outer spine reaching well beyond middle of second segment; second exopodal segment bearing 3 distal margin spines; inner spine longer than segment, middle and outer spines short.

Remarks: Caligus lacustris is the best known caligid found in fresh water. It has a wide distribution across Northern and Central Europe and western Asia, having been reported from freshwater habitats in Denmark, Germany, The Netherlands, Finland, Sweden, Poland, Ukraine, Russia, Azerbijan, and Uzbekistan as well as from around the Caspian Sea, Aral Sea, Sea of Azov and Baltic Sea (see References in Margolis et al., 1975). Despite the large number of records of C. lacustris , it proved difficult to find reliable descriptive accounts of details of the swimming legs in particular. The supplementary description revealed: the lack of accessory processes on distal spines 2 and 3 on the second exopodal segment of leg 1, the extent of the setular ornamentation on the endopod of leg 2, the small size and unusual configuration of the outer spine on the second exopodal segment of leg 2, the lack of an inner seta on the first exopodal segment of leg 3 and the relative size and ornamentation of the spines on leg 4.

There are interesting and unusual similarities between C. lacustris and the type species C. curtus O.F. Müller, 1785 . Both species share the possession of a large outer spine on the first exopodal segment of leg 2 which is directed somewhat obliquely across the ramus, reaching about to the mid-point of the second segment, combined with an unusually small outer spine on the second exopodal segment. This spine lies parallel with the longitudinal axis of the ramus and curves slightly across the dorsal surface. In addition, spines 1–3 on the second exopodal segment of leg 1 lack an accessory process in both species and seta 4 is only slightly longer than spine 3. Finally, the segmentation and armature of leg 4 is the same in both species. From these morphological similarities, we infer a close phylogenetic relationship between C. curtus and C. lacustris . The geographical distribution of the former species at high latitudes around the North Atlantic basin is contiguous with that of C. lacustris in northwestern Eurasia. We hypothesise that C. curtus is the marine sister taxon of the freshwater C. lacustris and that timing of the colonization of fresh waters by this lineage might be post-glacial.

Figures

FIGURE 4. Caligus lacustris Steenstrup & Lütken, 1861 adult female (body length 6 mm). A, habitus, dorsal; B, antenna, postantennal process and maxillule in situ, ventral; C, sternal furca; D, leg 1; E, leg 2; F, exopod of leg 3; G, exopod of leg 4.