Lithistid Demosponges of Deep-Water Origin in Marine Caves of the North-Eastern Mediterranean Sea

Andrzej Pisera,Vasilis Gerovasileiou

doi:10.3389/fmars.2021.630900

Abstract

Desmas-bearing demosponges known as lithistids have heavily silicified skeleton and occur typically in bathyal environments of warm and tropical areas but may be found in certain shallow marine caves. Here we report, for the first time two lithistid species, i.e., Neophrissospongia endoumensis, and N. cf. nana, that were earlier known from Western Mediterranean marine caves, from four marine caves in the north-eastern Mediterranean, and their congener Neophrissospongia nolitangere from deep waters (ca. 300 m) of the Aegean Sea. All marine caves, and sections within these caves, where lithistids occur, have freshwater springs. We interpret this surprising association between lithistids and freshwater input by elevated concentration of silica in water in cave sections where such springs occur, being 8–11 times higher in comparison with shallow water outside caves, and comparable to that of deep waters, that promoted lithistids’ development. One of the studied caves harbored an abundant population of N. endoumensis which formed large masses. The age estimation of these lithistids, based on known growth rate of related deep-water sponges, suggest that they could be approximately 769–909 years old in the case of the largest specimen observed, about 100 cm large. These sponges could have colonized the caves from adjacent deep-water areas not earlier than 7,000–3,000 years ago, after the last glaciation, because earlier they were emerged. High variability of spicules, especially microscleres, and underdevelopment of megascleres may be related to silicic acid concentration.

Highlights

Lithistids are those demosponges of various systematic position with “rocky” choanosomal skeleton [ their name derives from the Greek word “λιθoς” meaning stone], that consist of articulated, more or less regular and usually strongly sculptured spicules called desmas
This study aims at (a) describing for the first time lithistid demosponges collected from shallow marine caves of the northeastern Mediterranean Sea; (b) investigating environmental factors that potentially control their distribution and occurrence; and (c) comparing congeneric lithistids from marine caves and deep waters of the north-eastern Mediterranean Sea
Remarks There are four Neophrissospongia species reported from the Mediterranean Sea: N. nolitangere (Schmidt, 1870), N. endoumensis Pisera and Vacelet, 2011, N. radjae

Summary

Introduction

Lithistids are those demosponges of various systematic position with “rocky” choanosomal skeleton [ their name derives from the Greek word “λιθoς” (lithos) meaning stone], that consist of articulated, more or less regular and usually strongly sculptured spicules called desmas. They inhabit mostly waters deeper than 100 m to about 2000 m, but some species occur in shallowwater settings (Zea, 1987), including coral reefs (Vacelet and Vasseur, 1965; Hall et al, 2014), and marine caves (Pisera and Vacelet, 2011; Gerovasileiou and Voultsiadou, 2012). The marine cave environment seems to be favorable for lithistids, harboring 77% of the Mediterranean lithistid diversity, including six endemic species which are known only from a single marine cave but not from other habitats – at least so far – and could be considered cave-exclusive sensu lato (Gerovasileiou and Voultsiadou, 2012)

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