Systematics and character evolution of capitate hydrozoans

Capitate hydrozoans are a morphologically and ecologically diverse hydrozoan suborder, currently including about 200 species. Being grouped in two clades, Corynida and Zancleida, these hydrozoans still show a number of taxonomic uncertainties at the species, genus and family levels. Many Capitata species established symbiotic relationships with other benthic organisms, including bryozoans, other cnidarians, molluscs and poriferans, as well as with planktonic dinoflagellates for mixotrophic relationships and with bacteria for thiotrophic ectosymbioses. Our study aimed at providing an updated and comprehensive phylogeny reconstruction of the suborder, at modelling the evolution of selected morphological and ecological characters, and at testing evolutionary relationships between the symbiotic lifestyle and the other characters, by integrating taxonomic, ecological and evolutionary data. The phylogenetic hypotheses here presented shed light on the evolutionary relationships within Capitata, with most families and genera being recovered as monophyletic. The genus Zanclea and family Zancleidae, however, were divided into four divergent clades, requiring the establishment of the new genus Apatizanclea and the new combinations for species in Zanclea and Halocoryne genera. The ancestral state reconstructions revealed that symbiosis arose multiple times in the evolutionary history of the Capitata, and that homoplasy is a common phenomenon in the group. Correlations were found between the evolution of symbiosis and morphological characters, such as the perisarc. Overall, our results highlighted that the use of genetic data and a complete knowledge of the life cycles are strongly needed to disentangle taxonomic and systematic issues in capitate hydrozoans. Finally, the colonization of tropical habitat appears to have influenced the evolution of a symbiotic lifestyle, playing important roles in the evolution of the group.

Animal evolution at the ocean's water-air interface

Innovation is a key to evolutionary success and entrance into novel ecosystems.1 Species that float freely at the ocean's surface, termed obligate neuston (also called pleuston, here referred to simply as neuston), include highly specialized taxa from distinct evolutionary lineages that evolved floating morphologies.2 In 1958, Soviet scientist, A.I. Savilov,3 stated that floating animal species are derived from benthic ancestors, rather than species from the adjacent pelagic zone, and that floating morphologies are homologous to benthic attachment structures. To test Savilov's hypothesis, we constructed molecular phylogenies and ancestral states for all major floating groups for which molecular data were available. Our results reveal that four of the five clades examined arose directly from a substrate-attached ancestor, although that substrate was not necessarily the benthos, as Savilov stated, and instead included epibiotic and rafting ancestors. Despite their diverse evolutionary origins, floating animals use gas-trapping mechanisms to remain at the surface,4,5,6 and many of these gas-trapping structures appear to be homologous to substrate attachment structures. We also reconstruct the trophic habits of floating mollusks and their sister species, revealing that prey preference remains conserved upon entering the ocean's surface ecosystem. Colonization of the ocean's surface seems to have occurred through successive evolutionary steps from the seafloor. Our results suggest that these steps often included transitions through epibiotic (where species attach to other living organisms) or rafting (where species attach to floating debris) habits. The water-air interface, despite its unique properties, may, in some ways, be just another substrate.

Integrated characterisation of nine species of the Schistorchiinae (Trematoda: Apocreadiidae) from Indo-Pacific fishes: two new species, a new genus, and a resurrected but 'cryptic' genus

We report nine species of the Schistorchiinae Yamaguti, 1942 (Apocreadiidae Skrjabin, 1942) from Indo-Pacific marine fishes. Molecular data (ITS2 and 28S rDNA and cox1 mtDNA) are provided for all species and the genus-level classification of the subfamily is revised. For Schistorchis Lühe, 1906, we report the type-species Sch. carneus Lühe, 1906 and Sch. skrjabini Parukhin, 1963. For Sphinteristomum Oshmarin, Mamaev & Parukhin, 1961 we report the type-species, Sph. acollum Oshmarin, Mamaev & Parukhin, 1961. We report and re-recognise Lobatotrema Manter, 1963, for the type and only species, L. aniferum Manter, 1963, previously a synonym of Sph. acollum. Lobatotrema aniferum is phylogenetically distant from, but morphologically similar to, Sph. acollum and Lobatotrema is recognised as a 'cryptic genus'. We propose Blendiella n. gen. for B. trigintatestis n. sp. and B. tridecimtestis n. sp. These species are broadly consistent with the present morphological concept of Schistorchis but are phylogenetically distant from the type-species; a larger number of testes and some other subtle morphological characters in species of Blendiella serve to distinguish the two genera. We report three species of Paraschistorchis Blend, Karar & Dronen, 2017: P. stenosoma (Hanson, 1953) Blend, Karar & Dronen, 2017 (type-species), P. seychellesiensis (Toman, 1989) Blend, Karar & Dronen, 2017, and P. zancli (Hanson, 1953) Blend, Karar & Dronen, 2017. Lobatotrema aniferum, P. stenosoma, and Sch. carneus each have two distinct cox1 populations either over geographical range or in sympatry. Available evidence suggests that most of these species, but not all, are widespread in the tropical Indo-Pacific.

Being Safe, but Not Too Safe: A Nudibranch Feeding on a Bryozoan-Associated Hydrozoan

Nudibranchs have a mostly carnivorous diet, and they prey on a wide variety of other animal taxa. Many species, mainly belonging to the Cladobranchia suborder, feed on cnidarians, including member of the class Hydrozoa. Several hydrozoan species display a symbiotic lifestyle, being associated with other benthic invertebrates, including for instance bryozoans, corals, octocorals, and sponges. In our knowledge, no record of nudibranch predation on symbiotic hydrozoans has been reported so far, possibly thanks to the protective action by the host towards its symbiotic hydrozoan. Here, we show the unexpected case of a nudibranch belonging to the recently described species Sakuraeolis marhe (Fernández-Simón and Moles, 2023) feeding on Zanclea sp. 2, a hydrozoan associated with the cheilostome bryozoan Celleporaria sp. This trophic association is confirmed by the presence and storage of the nematocysts into the nudibranch cnidosacs. Moreover, the nudibranch appears to selectively store mostly a single type of nematocyst, that is large size stenotele. The observation here reported represents the first well-documented record of a nudibranch feeding on a symbiotic hydrozoan and the first confirmed case of predation on Zanclea polyps. Moreover, we provide additional genetic information and the first description of the internal anatomy of S. marhe.

Cryptic species and host specificity in the bryozoan-associated hydrozoan Zanclea divergens (Hydrozoa, Zancleidae)

Zanclea divergens is a tropical hydrozoan living in symbiotic association with bryozoans and currently reported from Papua New Guinea, Indonesia, and Maldives. Here, we used an integrative approach to assess the morpho-molecular diversity of the species across the Indo-Pacific. Phylogenetic and species delimitation analyses based on seven mitochondrial and nuclear loci revealed four well-supported molecular lineages corresponding to cryptic species, and representing a Pacific clade, an Indian clade, and two Red Sea clades. Since the general polyp morphology was almost identical in all samples, the nematocyst capsules were measured and analysed to search for possible fine-scale differences, and their statistical treatment revealed a significant difference in terms of length and width among the clades investigated. All Zanclea divergens specimens were specifically associated with cheilostome bryozoans belonging to the genus Celleporaria. The Pacific and Indian clades were associated with Celleporaria sp. and C. vermiformis, respectively, whereas both Red Sea lineages were associated with C. pigmentaria. Nevertheless, the sequencing of host bryozoans revealed that one of the Red Sea hydrozoan clades is associated with two morphologically undistinguishable, but genetically divergent, bryozoan species. Overall, our results show that Z. divergens is a species complex composed of morphologically cryptic lineages showing partially disjunct distributions and host specificity. The presence of two sympatric lineages living on the same host species reveal complex dynamics of diversification, and future research aimed at understanding their diversification process will likely improve our knowledge on the mechanisms of speciation among currently sympatric cryptic species.

Green Fluorescence Patterns in Closely Related Symbiotic Species of Zanclea (Hydrozoa, Capitata)

Green fluorescence is a common phenomenon in marine invertebrates and is caused by green fluorescent proteins. Many hydrozoan species display fluorescence in their polyps and/or medusa stages, and in a few cases patterns of green fluorescence have been demonstrated to differ between closely related species. Hydrozoans are often characterized by the presence of cryptic species, due to the paucity of available morphological diagnostic characters. Zanclea species are not an exception, showing high genetic divergence compared to a uniform morphology. In this work, the presence of green fluorescence and the morpho-molecular diversity of six coral- and bryozoan-associated Zanclea species from the Maldivian coral reefs were investigated. Specifically, the presence of green fluorescence in polyps and newly released medusae was explored, the general morphology, as well as the cnidome and the interaction with the hosts, were characterized, and the 16S rRNA region was sequenced and analyzed. Overall, Zanclea species showed a similar morphology, with little differences in the general morphological features and in the cnidome. Three of the analyzed species did not show any fluorescence in both life stages. Three other Zanclea species, including two coral-associated cryptic species, were distinguished by species-specific fluorescence patterns in the medusae. Altogether, the results confirmed the morphological similarity despite high genetic divergence in Zanclea species and indicated that fluorescence patterns may be a promising tool in further discriminating closely related and cryptic species. Therefore, the assessment of fluorescence at a large scale in the whole Zancleidae family may be useful to shed light on the diversity of this enigmatic taxon.