Aggregated Clumps of Lithistid Sponges: A Singular, Reef-Like Bathyal Habitat with Relevant Paleontological Connections

Abstract

The advent of deep-sea exploration using video cameras has uncovered extensive sponge aggregations in virtually all oceans. Yet, a distinct type is herein reported from the Mediterranean: a monospecific reef-like formation built by the lithistid demosponge Leiodermatium pfeifferae. Erect, plate-like individuals (up to 80 cm) form bulky clumps, making up to 1.8 m high mounds (1.14 m on average) on the bottom, at a 760 m-deep seamount named SSS. The siliceous skeletal frameworks of the lithistids persist after sponge death, serving as a complex 3D substratum where new lithistids recruit, along with a varied fauna of other sessile and vagile organisms. The intricate aggregation of lithistid mounds functions as a “reef” formation, architecturally different from the archetypal "demosponge gardens" with disaggregating siliceous skeletons. Leiodermatium pfeifferae also occurred at two additional, close seamounts (EBJ and EBS), but, unlike at SSS, the isolated individuals never formed accretive clumps. The general oceanographic variables (temperature, salinity, dissolved nutrients, chlorophyll, and oxygen) revealed only minimal between-seamount differences, which cannot explain why sponge abundance at SSS is about two orders of magnitude higher than at EBJ or EBS. Large areas of the dense SSS aggregation were damaged, with detached and broken sponges and a few tangled fishing lines. Satellite vessel monitoring revealed low fishing activity around these seamounts. In contrast, international plans for gas and oil extraction at those locations raise serious concerns over the need for protecting urgently this unique, vulnerable habitat to avoid further alteration. Modern lithistids are a relict fauna from Jurassic and Cretaceous reefs and the roots of the very genus Leiodermatium can be traced back to those fossil formations. Therefore, understanding the causes behind the discovered lithistid aggregation is critical not only to its preservation, but also to elucidate how the extraordinary Mesozoic lithistid formations developed and functioned.

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

“…The interpretation is of a radiolarian driven DSi depletion of the surface ocean. The link between sponge distributions and DSi stems from the implicit assumption that siliceous sponges are unable to thrive at low DSi (e.g., Maldonado et al, 1999;Ritterbush et al, 2015), although siliceous sponge reefs in the modern oceans are still forming in some areas despite the relatively low DSi concentrations (10-40 µM) (Uriz, 2006;Chu and Leys, 2010;Maldonado et al, 2015). The modern day global distribution of sponges suggests that sponges are adapted to a great range of DSi conditions and the presence of sponges in the stratigraphic record must be treated cautiously (Alvarez et al, 2017).…”

“…Maldonado et al (1999) suggested declines in reef building sponges occurred at the Cretaceous-Tertiary boundary because of DSi limitation as diatoms evolved. However, highly siliceous sponge reefs are still forming in selected areas of the modern oceans despite the relatively low DSi concentrations (Uriz, 2006;Chu and Leys, 2010;Maldonado et al, 2015). There are certainly other factors, such as food availability and ocean temperature (Kahn et al, 2012) that control the abundance and biosilicification of sponges (Alvarez et al, 2017).…”

Sponge Grounds as Key Marine Habitats: A Synthetic Review of Types, Structure, Functional Roles, and Conservation Concerns

“…Although lithistids were significant members of Paleozoic and Jurassic reefs, they appeared unable to develop analogous aggregations in the modern ocean. This view has been challenged by the discovery of a monospecific reef-like formation around the top of a deep seamount at 800 m in the Mediterranean Sea (Maldonado et al 2015). Individuals of the foliose species Leiodermatium pfeifferae, which grows as erect, contorted, 0.3-0.9 cm thick plates, up to about 80 cm in height, and 100 cm in width (Fig.…”

Rock sponges (lithistid Demospongiae) of the Northeast Atlantic seamounts, with description of ten new species

“…In the Northeast Atlantic (NEA), the current state of knowledge on lithistid sponges is mainly restricted to the Macaronesian islands. So far, 17 species have been described and recorded from the Azores (Carvalho & Pisera, 2019;Gray, 1859;Topsent, 1928Topsent, , 1904Topsent, , 1898Topsent, , 1892, Madeira and Selvagens (Bowerbank, 1869;Carter, 1873;Carvalho & Pisera, 2019;Johnson, 1863), Canary Islands (Carvalho & Pisera, 2019;Cruz, 2002;Topsent, 1892), Portugal mainland (Schmidt, 1870) and Morocco (Lendenfeld, 1907), whereas in the Mediterranean Sea, 15 species have been reported (Maldonado et al, 2015;Manconi, Serusi & Pisera, 2006;Manconi & Serusi, 2008;Perez et al, 2004;Pisera & Vacelet, 2011;Pulitzer-Finali, 1972;Vacelet, 1969). They are commonly found on hard substrate at 110-1,700 m depth (Carter, 1873;Carvalho, Pomponi & Xavier, 2015;Topsent, 1928), whereas in the Mediterranean Sea they usually occur in shallower waters or in cave systems (Manconi & Serusi, 2008;Pisera & Vacelet, 2011).…”