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.

The epikarstic waters of a restricted sector of the Ojo Guareña cave (north Iberian Peninsula) were investigated to characterizethe physico-chemical variation in an annual cycle, to improve the scarce knowledge of the aquatic cave fauna on the IberianPeninsula, to look for distribution patterns of species per habitat along the annual cycle, and to search for the environmentalbasis (either physico- or hydro-geochemical) that could explain species distribution in the epikarst. The habitats studied includedall puddles and gours present in the sector selected making a total of 51 chemical and 42 biological samples for the entirecycle. The waters show no appreciable contaminationand exhibit small chemical variations throughout the year which arepatently affected by external weather conditions. The 53 taxa found belong to ten higher taxonomic groups (Oligochaeta,Turbellaria, Mollusca, Copepoda, Ostracoda, Isopoda, Bathynellacea, Tardigrada, Acarina and Cnidaria), and consist mainly ofcrustacea with a total of 27 species. Fourteen species were stygobionts (belonging to ten genera), nine of which are new to scienceand ten of which are endemic. Cave pools that appeared to be more stable in terms of water volume and mineralization, hada lower pCO2 and were carbonate oversaturated, harboured the greatest number of taxa. It is these pools that can maintain strictlycave dwelling species. Pools with lower levels of mineralization and greater water volume fluctuations had a lower diversityof fauna and in general lacked stygobiotic species.