Leptothecata, Cornelius, 1992

Galea, Horia R. & Maggioni, Davide, 2024, On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species, Zootaxa 5428 (1), pp. 1-57 : 45-47

Leptothecata : Incertae sedis

Boissin et al. (2018) assessed the genetic diversity of Nemalecium from the Indo-Pacific, including specimens belonging to N. lighti ( Hargitt, 1924) and an unnamed congener (identified as Nemalecium sp. ), and found a high genetic diversity in the latter, hypothesizing the presence of multiple cryptic species. We provide an updated 16S rRNA phylogenetic hypothesis for the genus, including sequences of N. caeruleus sp. nov., Nemalecium sp. and N. lighti from Bali ( Fig. 30 ). Nemalecium caeruleus clusters within a clade comprising sequences of Nemalecium sp. 1 sensu Boissin et al. (2018), even though that clade shows low statistical support. The overall genetic distance within the clade composed of N. caeruleus and Nemalecium sp. 1 sensu Boissin et al. (2018) is relatively high (2.5 ± 0.4%), and so is the genetic distance between N. caerulues and Nemalecium sp. 1 from all western Indian Ocean localities (3.5 ± 1.0%). These relatively high genetic distance values, together with the absence of any morphological and ecological information on Nemalecium sp. 1 , leave open the question whether they belong to the same species or not. Specimens morphologically indistinguishable from N. caeruleus surprisingly clusters with sequences of Nemalecium sp. 2 sensu Boissin et al. (2018) in a well-supported clade (BPP = 1, MLBS = 98), showing a genetic distance of 7.3 ± 1.0% from the clade composed of N. caeruleus and Nemalecium sp. 1 , and an intra-clade distance of 2.0 ± 0.4%.

Finally, the sequence of the Balinese N. lighti clusters in a fully supported clade together with other conspecific sequences from the Indian Ocean and Caribbean Sea. It is evident that a great genetic diversity, with genetic distance values much higher than typical intra-specific values for most hydrozoan species (i.e., pairwise genetic distances between the supposedly cryptic species ranging 7.1–7.6 %), occurs among Indo-Pacific Nemalecium specimens studied herein and by Boissin et al. (2018), even between the morphologically indistinguishablesamples analyzed in this study. However, the absence of taxonomical accounts for the specimens dealt with by Boissin et al. (2018), together with the lack of any information about their habitat, hamper a clear understanding of the diversity of this group and, similarly, a correlation with the hydroid inhabiting coral crevices ( Gravier-Bonnet & Mioche 1996) could not be established with certainty for all their clades.

Figures

FIGURE 30. Phylogenetic hypothesis of the genus Nemalecium based on the 16S rRNA region. Numbers at nodes represent BPP and MLBS, respectively.