Abstract
Due to their peculiar and sometimes bizarre morphology, cave fauna (across invertebrates and vertebrates from both aquatic and terrestrial cave habitats) have fascinated researchers throughout history. Despite their success in colonizing most marine ecosystems, the adaptations of cave brittle stars (Ophiuroidea) to a stygobiotic lifestyle have been scarcely examined. Employing comparative methods on a data set of two species belonging to the genusOphionereis, this study addresses whether a cave-dwelling species from Cozumel exhibited similar troglomorphic traits as those of other taxa inhabiting caves. Our work demonstrated that some characters representing potential morphological cave adaptations inO. commutabiliswere: bigger sizes, elongation of arms and tube feet and the presence of traits potentially paedomorphic. In addition, an element of ophiuroid’s photoreceptor system, as well as pigmentation, was observed to be peculiar in this stygobiotic species, plausibly as a result of inhabiting a low light-energy environment. Finally, we add evidence to the statement thatO. commutabilisis a cave endemic species, already supported by demography, distribution and origin of this species, and now by a typical array of troglomorphisms.
Highlights
Several traits are often associated with cave-dwellers, which are known as troglomorphisms, defined as a morphological modification in cave fauna (Culver et al 1995; Romero 2009)
A remarkable arm elongation is observed in O. commutabilis, with arms up to 20 times the disc diameter and a mean of 13.2 in comparison to 6.6 of O. reticulata
We suggest that tentacle scales, Dorsal arm plates (DAP) and ventral arm plates (VAP) shape of O. commutabilis are paedomorphic traits based on the resemblance between mature cave specimen’s ossicles and that of juvenile O. reticulata
Summary
Several traits are often associated with cave-dwellers, which are known as troglomorphisms, defined as a morphological modification in cave fauna (Culver et al 1995; Romero 2009) These modifications or morphological traits could be expressed as reductive or constructive traits, such as the lack of photoreceptors or depigmentation in the case of the former and as the hypertrophy of appendages for the latter (Mejía-Ortíz et al 2006, Gonzalez et al 2018). According to Bishop et al (2015), anchialine caves are “a tidally-influenced subterranean estuary located within crevicular and cavernous karst and volcanic terrains that extends inland to the limit of seawater penetration” These environments are thought to be food-limited, with detritus and dissolved organic matter being the most important nutritional sources (Mejía-Ortíz et al 2013). The limitation of food is related to the fact that caves are completely devoid of sunlight and lack of sources of primary productivity (Culver et al 1995; Gonzalez et al 2018)
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