New records of sponges associated with siliquariid molluscs from the Indo-Pacific Ocean

Siliquariidae family represents a small group of Caenogastropoda molluscs that can be found fixed on hard substrates or, as in the genus Tenagodus Guettard, 1770, obligatorily dwelling within the body of sponges of the class Demospongiae. Despite this obligatory interaction, references to symbiosis in literature are relatively scarce, due to the rarity and elusiveness of these partnerships. The association does not appear to be strictly species-specific, although only a limited number of sponge families are known to host these molluscs. Pansini et al. (1999) and Bieler (2004) are the only two authors who accurately described the details of this symbiosis. Both highlighted the perfect connection between the sponge’s aquiferous system and the filter-feeding apparatus of Siliquariids. This allows the mollusc to convey and discharge self-drained water into the sponge incurrent canal system. The aim of this work was to investigate the association, with a particular focus on the involved sponge's characterization. A total of 29 samples from the Philippines and New Caledonia, collected between 1978 and 1993 at depths ranging from 90 m to 550 m, were analyzed. Sponge morphological characterization was carried out using the standard method. We reported the presence of Demospongiae belonging to 5 genera: Topsentia Berg, 1899; Spongosorites Topsent, 1896; Erylus Gray, 1867; Penares Gray, 1867 and Discodermia du Bocage, 1869. These genera align with those already reported by Pansini, while Bieler also mentions Siliquariaspongia Hoshino, 1981 and Thrombus Sollas, 1886, not present in our samples. Such sponges are mainly encrusting or with a massive habit, nearly completely covering the shell. As far as porifera are concerned, symbiosis is not obligatory and, in fact, all sponge species involved can also lead to a free-living existence. Additionally, a literature analysis suggests that the growth form and color of sponges may be strongly influenced by the presence of Siliquariids.