Cornigerius, Mordukhai-Boltovskoi, 1967

Dumont, Henri J. & Vierstraete, Andy, 2019, Cornigerius lacustris of Lake Hazar, Turkey, a synonym of Cornigerius maeoticus (Pengo) of the Ponto-Caspian (Cladocera: Onychopoda), Zootaxa 4619 (1), pp. 177-183 : 178-181

Taxonomy of Cornigerius .

For morphological illustrations, the reader is referred to Mordukhai-Boltovskoy (1967) and Rivier (1998). Four species have been classified in Cornigerius . The first, C. maeoticus ( Pengo, 1879) , used to be a common and widespread animal (before the Mnemiopsis invasion), found across the entire Caspian Lake, the north Black Sea, and the mouth of all major rivers debouching in the basin. This includes lakes in the delta of the Volga, Don and Dnieper rivers, which are of fresh water. The 1950s–1960s were a time of dam construction on almost all Ponto-Caspian rivers, and the species spontaneously expanded to the reservoirs thus created. It is euryionic and tolerates salinities of up to 10 0 / 00. It crossed the Volga Baltic gap in 2003 ( Panov et al., 2007) but was not observed in the Baltic afterwards (Aladin, pers. com.) and is still not seen there today (Van Damme, pers. com.). Its expansion to the Baltic must be considered a failure.

Morphologically, C. maeoticus bears divergent horns on the head, springing from a common peduncle. Long, curved horns are typical of maeoticus , short horns define C. bicornis , but there is considerable within-population variability and overlap in this character. The trunk bears two divergent spines, but again, their size and shape are variable, rendering separation from the next two “species” subjective.

C. bicornis ( Zernov, 1901) is thus hard to differentiate from C. lacustris and C. maeoticus , and shares a tolerance to freshwater and an upper salinity limit of about 10 0 / 00 with it. It also expanded into the newly created reservoirs on the major rivers.As the two occur mixed across their full range and overlap in morphology, one cannot trust identification in the literature, and it seems best to synonymize them.

C. lacustris differs hardly from the preceding, by having short horns and ‘less’ divergent trunk thorns, set by spinules. Such characters may be taxonomically meaningful, but they may also be adaptive morphologies and their development may depend on age, temperature and strain.

The fourth species, C. arvidi Mordukhai-Boltovskoi 1967 , is distinct by the horns on the head not springing from a common but from two distinct stalks on the head. This species is restricted to the salinity of the middle-south caspian, 5–14 0 / 00. It is not found in the fresher northern third of the lake and in the entire Black Sea, and rarely co-occurs with the preceding species. Therefore, morphology as well as ecology support its species status, and we conclude that the genus is composed of two species, well segregated spatially and ecologically.

COI. The five sequences from Lake Hazar were identical, repeating a single haplotype. Within-population differences in both the Caspian Lake and Black Sea were about 0.3 %. Differences between Black Sea and Caspian Lake animals ( Table 1 ) were few ( Table 1 and Figure 1 ) but more numerous (genetic distance 1.57 %) than between Black Sea and Hazar Lake (distance ca. 0.9 %). The largest, but still small, genetic distance was between Lake Hazar and the Caspian Lake (1.89 %). A Bayesian tree illustrates well the main point ( Figure 2 ): the three populations are all close, but the Hazar lake population is genetically closer to the Black Sea than to the Caspian Lake.

Figures

FIGURE 1. Estimate of relatedness of Cornigerius maeoticus – Cornigerius lacustris calculated by bootstrap neighbor joining method, showing that the population from Hazar Lake (lacustris) is closer to that of the Black Sea than to that of the Caspian Lake (maeoticus).

FIGURE 2. Haplotype map for the COI barcoding fragment of three geographically distinct populations of Cornigerius maeoticus – Cornigerius lacustris.

Tables

TABLE 1. Genetic distance, estimated by K2P method, between Cornigerius maeoticus and C. lacustris from the Black Sea (BS) and Caspian Lake (CL), and from Lake Hazar (LH), Turkey.

		C. maeoticus AY189510 BS	C.maeoticus AY189513 BS	C. maeoticus AY189512 BS	C. maeoticus AY189511 BS	C. lacustris 12805 LH	C. lacustris 12892 LH	C.lacustris 12807 LH	C. lacustris 12814 LH	C. lacustris 12816 LH	C. maeoticus AY189515 CL	C. maeoticus AY189516 CL
C. maeoticus AY189513	BS	0.0016
C. maeoticus AY189512	BS	0.0016	0.0031
C. maeoticus AY189511	BS	0.0015	0.0031	0.0031
C. lacustris 12805	LH	0.0078	0.0094	0.0094	0.0094
C. lacustris 12892	LH	0.0078	0.0094	0.0094	0.0094	0.0000
C. lacustris 12807	LH	0.0078	0.0094	0.0094	0.0094	0.0000	0.0000
C. lacustris 12814	LH	0.0078	0.0094	0.0094	0.0094	0.0000	0.0000	0.0000
C. lacustris 12816	LH	0.0078	0.0094	0.0094	0.0094	0.0000	0.0000	0.0000	0.0000
C. maeoticus AY189515	CL	0.0125	0.0141	0.0141	0.0141	0.0173	0.0173	0.0173	0.0173	0.0173
C. maeoticus AY189516	CL	0.0141	0.0157	0.0157	0.0157	0.0189	0.0189	0.0189	0.0189	0.0189	0.0016
C. maeoticus AY189514	CL	0.0141	0.0157	0.0157	0.0157	0.0189	0.0189	0.0189	0.0189	0.0189	0.0016	0.0031