Characterization of Iophon spp. (Porifera, Demospongiae) in association with the brittle stars genus Ophioplinthus (Echinodermata, Ophiuroidea) in the Southern Ocean

Sponges represent one of the most important components of the Antarctic zoobenthos, with over 350 known species found on hard and soft substrata, where they increase habitat heterogeneity. Due to hard substrates scarcity in the Antarctic sea bottom, sponges often show peculiar morpho-functional adaptations required to thrive on soft bottoms or to colonize biotic secondary substrates. In these cases, the associations are referred to as ectosymbioses. Symbiotic interactions represent important ecological and evolutionary drivers, often promoting speciation through host shift. Extensive investigations into benthic biodiversity in the Southern Ocean and the ecological role of symbiotic interactions has been recently re-evaluated. In this context, particularly intriguing is the ectosymbiontic relationships established between brittle-stars of the genus Ophioplinthus Lyman, 1878 and the demosponge genus Iophon Gray, 1867. Even though this interaction is widely reported in the Antarctic scientific literature, detailed information regarding the partners is still limited. In this work we analysed the samples available at the Italian National Antarctic Museum (MNA, Section of Genoa) to classify the partners of the association at the finest possible level. Thanks to these new materials we report the first record of I. flabellodigitatum Kirkpatrick, 1907 on O. brevirima (Mortensen, 1936) and of I. unicorne Topsent, 1907 on O. gelida (Koehler, 1901). Out of the 166 Ophioplinthus specimens examined in this work, the significant proportion of 57.8% were found to be engaged in symbiotic associations, highlighting the importance of this interactions within the Antarctic brittle star population. In the specimens studied, the presence of Iophon has different degrees of development but numerous specimens of O. gelida did not exhibit this association, while all the O. brevirima specimens present the symbiosis. Our analysis also reported a difference between the external morphology of the two Iophon sponge compared to the original description by Kirkpatrick of 1907.