Symbiont footprints highlight the diversity of scleractinian‐associated Zanclea hydrozoans (Cnidaria, Hydrozoa)

Abstract

Hydrozoans of the genus Zanclea have been acknowledged only recently as a fundamental component of the highly diverse fauna associated with reef-building scleractinian corals. Although widely distributed in coral reefs and demonstrated to be important in protecting corals from predation and diseases, the biodiversity of these hydrozoans remains enigmatic due to the paucity of available morphological characters, incomplete morphological characterisations and the possible existence of cryptic species. Recently, molecular techniques have revealed the existence of multiple hidden genetic lineages not yet supported by diagnostic morphological characters. In this work, we further explore the morpho-diversity of three genetic lineages, namely Zanclea associated with the coral genera Goniastrea (clade I), Porites (clade II) andPavona (clade VI). Aside from providing a complete classical characterisation of the polyp and medusa stage of each clade, we searched for new potential taxonomic indicators either on symbiotic hydroids or on host corals. On the hydroids, statistical analyses on almost 7,000 nematocyst capsules revealed a significant difference in terms of nematocyst size among the three Zanclea clades investigated. On each host coral genus, we identified peculiar skeletal modifications related to the presence of Zanclea symbionts. Lastly, we discussed the potential diagnostic value of these footprints in the characterisation of Zanclea-scleractinian associations. K E Y W O R D S cnidome, coral reefs, coral skeleton, coral-associated invertebrates, symbiosis 400 | MANCA et Al.

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

“…For instance, by combining observations on the presence, localization and size of euryteles, and the general appearance of polyps and medusae, it is possible to distinguish the analyzed bryozoan-associated species. By contrast, scleractinian-associated species showed a very similar morphology, as already documented in previous studies [18,21,34].…”

“…A similar situation was observed for Celleporaria-Zanclea associations, where the base of polyps was occasionally surrounded by bryozoan skeletal structures. Additionally, scleractinians hosting Zanclea showed micro-alterations related to the presence of symbionts, as already observed in Goniastrea, Pavona, and Porites corals [21]. The presence of these modifications may support the hypothesis that at least some Zanclea species are mutualistically associated with their hosts, since they may provide additional protection and competitive advantages to their hosts and in turn benefit from being partially enclosed in hard carbonatic structures [36,37].…”

“…The cnidome is considered a useful character to discriminate among zancleid species, due to the variation of type and size of nematocysts in different species [20]. For instance, the statistical treatment of nematocysts measurements of three Zanclea cryptic species resulted in significant differences between the taxa [21], further supporting the importance of the cnidome as a reliable taxonomic character. Another useful character to distinguish closely related symbiotic species is the host specificity, since some species or lineages are specifically associated with one or a few invertebrate taxa (e.g., scleractinian corals) [16,18].…”

“…Another useful character to distinguish closely related symbiotic species is the host specificity, since some species or lineages are specifically associated with one or a few invertebrate taxa (e.g., scleractinian corals) [16,18]. Moreover, some coral-associated Zanclea species were found to induce modifications of the host skeletons that could be taxonomically informative [21].…”

Evolution and biogeography of the Zanclea-Scleractinia symbiosis

“…More recently, the use of novel approaches in hydrozoan taxonomy has shown promising results in discriminating among Zanclea species. For example, Manca et al (2019) found significant differences in the size of the nematocysts among three coral-associated clades and described the presence of symbiont-related peculiar modifications of the coral skeleton that may have taxonomic value. The latter corresponds to skeletal overgrowths that surround the base of Zanclea polyps, and their size was different among the three investigated clades.…”