Bryocamptus zschokkei (Schmeil, 1893)

Novikov, A. A., Sharafutdinova, D. N. & Fefilova, E. B., 2024, A new species of Bryocamptus Chappuis, 1929 (Copepoda: Harpacticoida: Canthocamptidae) from the hydrothermal seep of Lake Baikal, Arthropoda Selecta 33 (1), pp. 36-52 : 47-51

Bryocamptus zschokkei (Schmeil, 1893) species group

The new species is included in the group B. zschokkei (Schmeil, 1893) (sensu Lang [1948]). Lang [1948] and Wells [2007] included this group in subgenus B. ( Bryocamptus ). Borutzky [1948b, 1952] separated this

group, along with the B. pygmaeus (Sars, 1863) group, into a new subgenus B. ( Rheocamptus ) with the type species B. zschokkei . The same scheme is followed by most subsequent authors [ Dussart, Defaye, 1990]. In general, the entire genus Bryocamptus requires revision. In our opinion, the subgenus B. ( Rheocamptus ) is hardly monophyletic, however, at the moment, it is impossible to revise the genus. The main difficulty lies in collecting a sufficient number of species from different groups.

All the ten species of endemic Baikalian B. ( Rheocamptus ) [ Evstigneeva, Okuneva, 2001] belong to the B. zschokkei group: B. albidus Okuneva, 1983 ; B. baikalensis Borutzky, 1931 ; B. brevipes Borutzky, 1948 ; B. crassipes Borutzky et Okuneva, 1972 ; B. cristatus Borutzky et Okuneva, 1972 ; B. denticulatus Borutzky et Okuneva, 1972 ; B. littoralis Borutzky et Okuneva, 1972 ; B. rylovi Borutzky, 1931 ; B. saxicola Borutzky et Okuneva, 1972 ; and B. sitnikovae sp.n. Together with the non-Baikalian B. zschokkei , B. spinulosus Borutzky, 1934 , B. alosensis Apostolov, 1998 and B. madarensis Apostolov, 1969 , the group is characterised by a set of characters which are primitive for the entire genus: a two-segment mandible palp, two exopodal and three endopodal setae of the maxillule and a complete composition of setae and spines on P1–P5. There are few reliable synapomorphies of the species of the group; they include two-segment endopods P1–P2, the ratio of the lengths of setae on the baseoendopod P5 of the female (from the longest to the shortest: IV-II-I-III-V-VI or IV-II-III-I-V-VI), as well as the ratio between the apical setae of the P3 endopod of the male, where one of the setae is much longer and more massive than the other. The Baikalian species of the B. zschokkei group most likely represent a monophyletic lineage. They are similar in basic characters, such as leg armature, abdominal somite and anal operculum ornamentation, but almost all of these characters are plesiomorphic for the B. zschokkei group. Unfortunately, we were unable to detect significant synapomorphies for Baikalian species.

Several species such as B. balcanicus (Kiefer, 1933) , B. pyrenaicus ( Chappuis, 1923) and B. zschokkei kalinae Petkovski, 1956 can probably also be classified as a group of B. zschokkei , however, they all differ in the reduction in the number of setae on the female P5 (3–5 setae on baseoendopod) [ Chappuis, 1923; Petkovski, 1956]. Bryocamptus mirus Petkovski et Karanovic, 1997 also has similar characters, as two-segmented P1 endopod, but differs from the group of species under consideration in the ratio of P5 setae, in the male P3 endopod with two unmodified apical setae, and in the presence of an exopodal mandible seta [ Petkovski, Karanovic, 1997].

In the B. zschokkei group endopods P2–P3 of females are often incompletely fused. In B. zschokkei and B. spinulosus , endopods have sometimes been described as three-segmented [ Kiefer, 1960; Sterba, 1967; Caramujo, Boavida, 2009], which may also reflect a lack of fusion or incomplete fusion of the segments. In B. sitnikovae sp.n., the boundary between the ancestral segments of the P3 endopod is clearly visible. So, it can be described as both 3-segmented and 2-segmented. Such an incomplete fusion of segments can also be seen in other species of Bryocamptus , such as B. putoranus Novikov, Sharafutdinova et Chertoprud, 2023 [ Novikov et al., 2023], or in species of other genera, for example in Attheyella nordenskioldii (Lilljeborg, 1902) [Novikov et al., unpublished].

The structure of the P 3 male endopod is rather unusual in this group of species. In the new species, the outer seta is massive, evenly thickened, and has two rows of spinules in the distal part. Such an interesting structure could be attributed to the peculiarity of the species, given that the structure of the P 3 male endopod can often be species-specific both in Bryocamptus [ Novikov et al., 2023] and in other genera of Canthocamptidae and genera from families, close related to Canthocamptidae , for example, Lourinia C.B. Wilson, 1924 [ Karaytuğ et al., 2021], Mesochra Boeck, 1865 [ Soyer, 1977] or Cletocamptus Schmankevitsch, 1875 [ Gómez et al., 2017]. However, an almost identical structure is found in both the Baikalian endemic species [ Borutzky, Okuneva, 1972], B. zschokkei from Korea [ Lee, Chang, 2006] and B. spinulosus [ Borutzky, 1934]. Taking into account the low quality of the description of the last century, such a modification of the seta may also be found in other species of the B. zschokkei group. Among the Baikalian species, B. rylovi [ Okuneva, 1989] and B. denticulatus [ Borutzky, Okuneva, 1972] have the same seta, something similar can be seen in B. brevipes [ Borutzky, 1948b], B. crassipes , B. cristatus , B. saxicola and B. littoralis [ Borutzky, Okuneva, 1972]. Only in one species, B. albidus , is this seta depicted without features [ Okuneva, 1983]; however, given the low quality of the figures and the existing taxonomic errors in this work (the description of A. nordenskioldii as a new species of Canthocamptus gibba Okuneva, 1983 ) [ Novikov, Sharafutdinova, 2022], most likely, all Baikalian species have this brush-like seta.

The structure of the caudal rami of females and the antennules of males

Bryocamptus sitnikovae sp.n. has an interesting modification of caudal rami with an expanded base of seta IV. At the same time, there are also modifications of the male antennules. On segment 8, the laminar setae are considerably enlarged, one of them is elongated and its apical end extends beyond the base of segment 9. Previously, Novikov et al. [2023] suggested that the coevolution of male antennules and female caudal rami is associated with a sexual arms race. Since it is more difficult for males to grasp a female with modified caudal rami, males with the most successful form of antennules leave more offspring. Boxshall & Evstigneeva [1994] suggested as well that the main evolutionary vector in the Baikalian Canthocamptidae is reproductive isolation through the transformation of female caudal rami. However, they consider specific mate recognition systems to be the reason for this (mainly studied in the genus Moraria Scott et Scott, 1893 ). These two hypotheses have in common that the main factor of speciation in both is reproductive isolation; however, the role of the males differs. In the second hypothesis, the male recognises a female of a different species and does not continue mating. On the contrary, in the first hypothesis, the male can and would begin to fertilise a female not of his own species; however, due to the structural features of the antennules, he would not be able to complete the process of fertilisation. At the moment, it is impossible to say which hypothesis is more correct. Moreover, both of them can be true to one degree or another for different groups of species. So, for further conclusions, it is necessary to conduct experiments with living individuals.

Differences from other Baikalian Bryocamptus (Rheocamptus) species

First of all, when studying the Baikalian Canthocamptidae , it is necessary to pay attention to several main problems. First, the descriptions are of poor quality for most species. It should be taken into account that most of the species of Bryocamptus and Moraria are included in species flocks [ Boxshall, Evstigneeva, 1994], the species of which differ little from each other. Given the high variability of Baikalian species, any characters may be important for identification. The authors, however, often cited only drawings of diagnostic characters in the descriptions [ Borutzky, 1947]. Also, despite the indication that species are represented by a large number of individuals, the authors rarely mentioned the variability of species [ Borutzky, 1952; Borutzky, Okuneva, 1972].

The second problem is the difficulty of matching males and females of the same species in a sample. Most samples may contain several species from the same species flock. According to the available descriptions, only females can be distinguished (for example, genus Moraria : [ Borutzky 1952]); it is almost impossible to correlate males purely morphologically. Therefore, it is often necessary to rely on samples where representatives of only one species from any species flock are numerous, as is the case with the description of B. sitnikovae sp.n.

Be sure to keep in mind the problems described above as we consider the group of species. The 10 species of Baikalian B. ( Rheocamptus ) listed earlier can be distinguished by several features, including:

1. Segmentation of the female antennule. Only one species, B. crassipes , has a 7-segmented antennule [ Borutzky, Okuneva, 1972].

2. Number of inner setae on the P 3 female endopod. All species have three, except for B. crassipes which does not have any inner seta and B. saxicola which has two inner setae [ Borutzky, Okuneva, 1972].

3. Shape of the outer spine of the P 4 male endopod. In most species, this spine is not modified, as in B. sitnikovae sp.n. However, a modified spine is present in B. cristatus (curve spine) and B. littoralis (bifurcate blunt spine) [ Borutzky, Okuneva, 1972].

4. Spinular ornamentation of the P 5 female exopod. The exopod may have spinules on the outer, inner and apical margins; one species, B. denticulatus , has a group of spinules on the anterior surface of the exopod [ Borutzky, Okuneva, 1972].

5. Length of setae on the P 5 female exopod. Setae I and V are the most variable. The first seta is shorter than the exopodal segment in B. albidus , B. baikalensis , B. cristatus and B. rylovi . In other species, this seta is longer than the exopod [ Borutzky, 1952; Borutzky, Okuneva, 1972; Okuneva, 1983].

6. Presence of ventral and lateral spinules on the anal somite of females and males. All studied females of B. sitnikovae sp.n. did not have both groups of spinules, which may indicate the diagnostic nature of this character. However, in the earlier descriptions, unfortunately, this feature was not given attention. It can only be assumed from the figures that B. rylovi females do not have lateral spinules, and B. littoralis females do not have ventral spinules [ Borutzky, 1952; Borutzky, Okuneva, 1972].

7. Length of caudal rami. In species of the group, the ratio of length to width of the caudal rami ranges on average from 1 to 1.5. Only one B. cristatus has an elongated caudal rami; their ratio is almost 2 [ Borutzky, Okuneva, 1972].

8. Ornamentation of the caudal rami is a traditional diagnostic feature widely used in keys [Borutzky, Okuneva, 1972; Okuneva, 1989]. However, in the taxonomy and identification of the Baikalian species of B. ( Rheocamptus ), one should be very careful about the groups of spinules of the caudal rami. For instance, in B. sitnikovae sp.n., these characters are highly variable; B. baikalensis and B. cristatus , judging by the descriptions, have no spinules, except for the spinules at the base of setae I–III; in B. denticulatus and B. saxicola , dorsal spinules are also observed to the ventral and inner groups of spinules [ Borutzky, 1952; Borutzky, Okuneva, 1972].

9. Shape of caudal setae in female. In our opinion, this is one of the most useful characters since it is associated with reproductive isolation and is unlikely to be subject to significant intra-species variability. Seta IV is modified in B. crassipes , B. denticulatus , B. littoralis and B. sitnikovae sp.n. [ Borutzky, Okuneva, 1972]. A study of new material shows that, for example, in B. littoralis the modification of the seta is exactly as shown in the description and is not similar to that in B. sitnikovae sp.n. However, the descriptions of other species are not detailed enough to understand what kind of modifications these species exhibit and whether they are similar to the modification in B. sitnikovae sp.n.

It is also worth noting some of the features mentioned in the descriptions and keys are most likely deformities or errors:

1. Three setae on the distal segment of the P1 endopod in B. albidus [ Okuneva, 1983]. Most likely, the small apical seta was ovelooked.

2. Three inner setae on the P 2 female endopod in B. rylovi [ Borutzky, 1952; Okuneva, 1989]. In the genus Bryocamptus , as a whole, one of the inner setae of the P2 endopod is always reduced. This character is present in different species groups, for example, B. minutus species group [ Novikov, Sharafutdinova, 2022], B. jejuensis Lee et Chang, 2016 [ Lee, Chang, 2016], species of the subgenus B. ( Echinocamptus ) [ Lee, Lee, 2010]. So, this is most likely either an abnormality, or an error in the interpretation of setule/seta, or the P2 and P3 were confused.

3. Seven setae on the P 5 male exopod in B. brevipes [ Borutzky, 1948b]. In the entire family, exopods of males have a maximum of six setae. Additional setae are not uncommon in females and males, but in all cases, they are individual deviations.

Finally, the spinular ornamentation of the abdominal somites can certainly be taxonomically important, but it is currently unclear how variable it is, and it is also rather poorly described in most species. Most do not have drawings, and in B. saxicola , the description and drawing do not match [ Borutzky, Okuneva, 1972], which further adds to the doubts about the use of this character.

Abbreviations for groups of spinules: O — outer; A — apical, near setae II–IV of P5Exp; I — inner; F — frontal; L — lateral; V — ventral.

In conclusion, it can be noted that the taxonomy of Baikalian representatives of the genus Bryocamptus is quite confusing. It requires the study of new material and a redescription of described species. The new species is one of the exceptions; it is quite stable morphologically and has good distinctive characters. And given that other species often exhibit significant variability, a full revision is probably impossible without the use of molecular genetic methods to determine species boundaries.