Littorinoidea

Hey, Abbottella!
Published 16 August 2023

Among the world’s many biogeographical curiosities, particular note should be made of the island of Hispaniola. The islands of the West Indies began forming, probably during the early Cenozoic, as the Caribbean tectonic plate was squeezed between the approaching North American and South American plates (Hedges 1996). They have always been separate from the adjoining land masses and, as a result, seem to punch above their weight in the endemism game. Many remarkable animals can be found today in the West Indies and nowhere else—todies, solenodons, hutias—and Hispaniola seems particularly rich in such curiosities. Among the fauna specific to this island are the snails of the genus Abbottella.

Abbottella domingoensis, copyright Marcos Rodríguez Bobadilla.

Abbotella is a genus of the Annulariidae, a family of land-living snails unique to the Caribbean and adjoining regions of Central America. Rather than belonging to the Stylommatophora clade that includes the majority of terrestrial snails, annulariids belong to a different branch of the gastropods, the Caenogastropoda, and are closer relatives of the marine periwinkles of the Littorinidae. Unlike stylommatophorans, annulariids retain a calcareous operculum that may be used to close the aperture of the shell. Abbottella are small snails, at most 15 mm in diameter, with low-spired, widely umbilicate shells. The aperture in mature snails flares out into a broad, more or less undulating skirt. The shell is criss-crossed by axial lamellae and spiral cords that are more or less raised at their intersections into sharp cusps (Watters et al. 2020).

Abbottella and other annulariids are strict calciphiles, almost entirely restricted in their distribution to limestone outcrops. As a result of this specialisation, species often have very restricted ranges. On Hispaniola, annulariids are all but absent from the central mountain ranges which are primarily composed of igneous and metamorphic rocks. Individuals may suspend themselves from mucus threads when at rest. They are also commonly found in mated pairs; Watters et al. (2020) remarked that, “These snails spend an inordinate amount of time making more snails”.

Abbottella crataegus, copyright Claude & Amandine Evanno.

The majority of Abbottella species are found in the eastern part of Hispaniola, in the Dominican Republic. Interestingly, they are not found on the Samaná Peninsula in the island’s northeastern part, though they are found on the facing southern shore of Samaná Bay. Other annulariid genera found on this peninsula were historically included in Abbottella but were treated as distinct by Watters et al. (2020), albeit within the same subfamily Abbottellinae. The absence of Abbottella sensu stricto from this landmass may be due to the Samaná Peninsula being a separate island until quite recently, the dividing channel only silting closed some time during the 1800s. A single species A. haitensis has been recorded far removed from other Abbottella species in Haiti, near the town of Tomazeau. However, this species has seemingly not been recorded since the collection of the holotype in 1920. Watters et al. (2020) remarked that its location might be considered questionable were it not collected by the naturalist William Abbott (after whom the genus was presumably named), whose records were usually reliable and who was known to have been in the right place at the right time. Conversely, A. haitensis is distinct enough from other species in the genus that future studies might lead to its placement elsewhere.

Topographic image of Hispaniola, from NASA. Note the lowlands separating Samaná Peninsula in the northeast and Tiburon Peninsula in the southwest.

One part of Hispaniola in which neither Abbottella nor any other member of the Abbottellinae is found is the Tiburon Peninsula in southern Haiti (Watters 2013). Instead, this region is inhabited by a distinct assemblage of annulariids belonging to a different subfamily. The closest relatives of the Hispaniolan abbottellines lie not among the Tiburon genera but among genera found on Cuba. The Tiburon Peninsula is tectonically distinct from the remainder of Hispaniola, having formed as a separate island before the two collided during the Oligocene. Meanwhile, Cuba and northern Hispaniola would have been connected by a land bridge during this time. Annulariids would have been able to readily disperse across the latter but would have been prevented from crossing between northern Hispaniola and the Tiburon, first by the intervening ocean and then by the inhospitable landscape of the Cordillera Central. Millions of years later, these slow-moving gastropods still demonstrate the influence of ancient boundaries.

References

Hedges, S. B. 1996. Historical biogeography of West Indian vertebrates. Annual Review of Ecology and Systematics 27: 163–196.

Watters, G. T. 2013. New taxa and distributional notes on Abbottella and related taxa (Gastropoda: Littorinoidea: Annulariidae). Zootaxa 3646 (1): 1–22.

Watters, G. T., M. L. Smith & D. J. Sneddon. 2020. The subfamily Abbottellinae (Gastropoda: Annulariidae): origins, associations, and a review of the Hispaniolan taxa. Nautilus 134 (1): 1–34.

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