Disrupted spiculogenesis in deep-water Geodiidae (Porifera, Demospongiae) growing in shallow waters

Environmental conditions can affect the morphology and distribution of sponges. In particular, depth is known to influence the morphology of shallow-water sponges; however, the influence of depth on deep-water sponges has never been investigated. Although boreal Geodiidae (Demospongiae, Astrophorida) are deep-water species, in fjords and along the Norwegian coast Geodia barretti, G. simplicissima, and Pachymatisma normani can occasionally be found at shallow depths (20-40m). In this study, we examine new shallow specimens from the Norwegian coast to compare their morphological and molecular characteristics with those of their deep-water counterparts. Morphology was studied at the level of the organism, skeletal organization, and the spicules, and a fragment of the cytochrome oxidase 1 gene was sequenced for shallow and deep specimens. Twelve specimens of Geodia spp. and five specimens of P. normani were collected in shallow waters. The majority of the Geodia spp. were identified as G. simplicissima, a species that has not been reported since its original description in 1931. However, we propose that G. simplicissima, only found in shallow waters, is a junior synonym of G. barretti. When comparing shallow and deep-water specimens of G. barretti and P. normani, we found phenotypic differences with respect to color, external morphology, cortex organization, and, above all, spicule morphology. In shallow specimens, microrhabds, sterrasters, and triaenes were smaller and irregular or underdeveloped. Oxyasters and strongylasters were normal in form, but smaller. We hypothesize that the lower silica concentration in shallow waters is primarily responsible for the disruption of spiculogenesis in shallow-water specimens of G. barretti and P. normani. The underdeveloped sterrasters observed in shallow-water specimens provide new insights into the formation of these particular microscleres. Finally, we discuss how the colonization of shallow waters by deep-water sponges may have strongly influenced spicule evolution and speciation.