Pleurobrachia pileus (O.F. Müller, 1776)

Baiandina, Iu. S., Kuleshova, O. N. & Krivenko, O. V., 2025, The Black Sea Pleurobrachia (Ctenophora): P. rhodopis or P. pileus?, Zoosystematica Rossica (China) 34 (1), pp. 18-27 : 21-22

Pleurobrachia pileus (O.F. Müller, 1776)

( Fig. 1)

Material. Black Sea, env. of Sevastopol: 44°13′01″N 33°41′27″E, 7 June 2022, 11 specimens; 44°24′69″N 34°24′45″E, 10 June 2022, 14 specimens.

Morphological characters. We analysed the diagnostic characters of the Black Sea Pleurobrachia ( Fig. 1) based on Chun (1879) and Licandro & Lindsay (2017). We observed the animals with a body length (from statocyst to mouth) ranging from 9.6 to 22.4 mm (average size 16.7 mm) ( Table 1). The length of ctene rows varied among specimens: some were less than three-quarters of the body length (n = 9), while the others were longer ( Table 1). The length of the stomodeum was about half of the body length ( Table 1). The tentacular bulbs were far from the stomodeum. The adradial canal opened onto the meridional canal above the infundibulum (relative to the axis of the statocyst-mouth). The tentacles were very long, supplied with numerous tentilla. The body shape was nearly spherical. The body was transparent, without pigmentation or coloration.

A living specimen of the Black Sea Pleurobrachia is presented in our video (Addenda: Electronic supplementary material 2).

Molecular phylogeny. The lengths of sequenced DNA fragments for the Black Sea Pleurobrachia were 681 bp and 932 bp for the COI gene and 2511 bp for the 18S rRNA gene. The length of the resulting alignment was 1167 bp for the COI and 1753 bp for the 18S rRNA genes, respectively. The BLAST algorithm ( McGinnis & Madden, 2004) implemented in NCBI revealed the highest percentage identity match with P. pileus for both genes, with 98.68–100% identity for the COI and 99.89–99.44% identity for the 18S rRNA. The second-best matches for the COI gene are P. globosa (Moser, 1903) with 86.39–87.62% and P. bachei (Agassiz, 1860) with 82.86–83.69% identity; for the gene 18S rRNA are P. bachei with 99.83% identity and P. globosa with 99.19%. The molecular data on P. rhodopis are not available in the NCBI. We calculated the Kimura two-parameter (K2P) distances within and between P. pileus , P. globosa and P. bachei . K2P distances among all studied P. pileus specimens (including our own material) ranged from 0 to 1.30%, while within the Black Sea P. pileus , the level of genetic divergence ranged from 0 to 0.44%. K2P distances between P. pileus and P. globosa ranged from 13.02 to 15.41%; between P. pileus and P. bachei ranged from 18.45 to 20.64%.

A phylogenetic reconstruction based on the COI sequences showed that the Pleurobrachia samples from the Black Sea clustered within the P. pileus clade in the ML tree with high supports (ultrafast bootstrap and SH-aLRT values equal to 100, aBayes equal to 1) ( Fig. 2A). The Black Sea samples (including the specimen from Turkey, MW 735824) formed a separate subgroup (the distance between the Black Sea clade and the other P. pileus samples is 0.008 nucleotide substitutions per site) with 100% ML support, 92.9% SH-aLRT support, and aBayes support equal to 1. Phylogenetic reconstruction for the 18S rRNA gene showed the same clustering pattern of the Black Sea specimens, though with a ultrafast bootstrap support of 90% ( Fig. 2B). The supporting values obtained using SH-aLRT and aBayes methods were low (0 for SH-aLRT and less than 0.4 for aBayes), indicating that the optimal reconstruction conditions were not met. At the same time, the clade of Pleurobrachia has high support in the ML tree (ultrafast bootstrap value equal to 96%, SH-aLRT equal to 93.9%, aBayes equal to 1). Therefore, despite the fact that phylogenetic reconstructions based on the COI and the 18S rRNA gene fragments have a similar topology, in our conclusions we rely on the phylogeny obtained for the COI gene fragment.

Iu.S. Baiandina et al. The Black Sea Pleurobrachia : P. rhodopis or P. pileus ?