Anchialine caves are characterized by high levels of endemism and extreme conditions. However, few ecological studies have been conducted in these ecosystems. This study integrates biotic and abiotic parameters of two sets of cave systems with contrasting high and low species richness. Seven ecological patterns are used to explain the expected species richness and density in an anchialine cave. In addition, the population size for conspicuous macrofauna was estimated. The high impact that single-events have on anchialine fauna are also reported. These findings reinforce the conclusions of previous studies of the high extinction risk of anchialine cave fauna, and substantiate the necessity of ad hoc conservation strategies for anchialine caves.

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Bermuda is an Eocene age volcanic island in the western North Atlantic, entirely capped by Pleistocene eolian limestone. The oldest and most highly karstified limestone is a 2 km2 outcrop of the Walsingham Formation containing most of the island’s 150+ caves. Extensive networks of submerged cave passageways, flooded by saltwater, extend under the island. In the early 1980s, cave divers initially discovered an exceptionally rich and diverse anchialine community inhabiting deeper sections of the caves. The fauna inhabiting caves in the Walsingham Tract consists of 78 described species of cave-dwelling invertebrates, including 63 stygobionts and 15 stygophiles. Thus, it represents one of the world’s top hotspots of subterranean biodiversity. Of the anchialine fauna, 65 of the 78 species are endemic to Bermuda, while 66 of the 78 are crustaceans. The majority of the cave species are limited in their distribution to just one or only a few adjacent caves. Due to Bermuda’s high population density, water pollution, construction, limestone quarries, and trash dumping produce severe pressures on cave fauna and groundwater health. Consequently, the IUCN Red List includes 25 of Bermuda’s stygobiont species as critically endangered.

Porifera has been relatively well studied from underwater caves worldwide. However, sponges in Mexico are only known from two anchialine caves: La Quebrada and El Aerolito, both in Cozumel Island. An ecological study found that sponges have the second highest species richness and density in La Quebrada. The aim of the present work is to describe the sponge biodiversity inhabiting the dark zone of La Quebrada anchialine cave. A total of ten species were found and are described herein, six are new species to science vis. Calyx maya n. sp. a thinly branched sponge; Haliclona (Reniera) stygobia n. sp. globe-shaped with the thinnest diameter of oxeas in its group; Haliclona (Halichoclona) chankanaabiis n. sp. tubular habit with the smallest oxeas in its group; Neosiphonia microtriaeneae n. sp. characterized by very small dichotriaenes; Svenzea germanyanezi n. sp. a lone conical shape; and Diplastrella cozumella n. sp. for its tri to multilobed tylostyles. The other four species are previously known from open reefs or deep-sea: Discodermia adhaerens, Siphonidium ramosum, Cinachyrella kuekenthali, and Plakinastrella onkodes. This is the only inventory of Porifera in Mexican caves up to now, thus the species here described are the first stygobiont sponges from Mexico with an endemism rate of Porifera in this anchialine cave of 60%. Conservation programs are needed to preserve this unique and special ecosystem.

We record for the first time a nominal species of the genus Microscleroderma in the Western Atlantic, and the first record of the genus Amphibleptula in Mexico. Two new species of ‘lithistid’ Tetractinellida are described: Microscleroderma mexica sp. nov. from crevices of two reefs in Veracruz, Mexico, and Amphibleptula aaktun sp. nov. from the anchialine cave El Aerolito, Cozumel Island, Mexico. Microscleroderma mexica sp. nov. is characterized by its tubular and cup-shaped to wavy laminar habitus, hirsute surface and two size categories of oxea diactines. Amphibleptula aaktun sp. nov. is characterized by its ficiform or lobular habitus with an exhalant area at the flat top of the body, divided into numerous vertical septa-like structures; its desmas have thinner epirhabds, and thinner oxea than those of Amphibleptula madrepora. This is the second species of Amphibleptula that has been described; hence, we propose the redefinition of the genus. Both species described have a shallow distribution (≤21 m depth); they are differentiated from each other by the disjunction between their habitats as well as by their morphology.