Phylum Arthropoda
Subphylum Crustacea
Class Copepoda
Order Calanoida
Family Ridgewayiidae

Exumella tsonot Suárez-Morales & Iliffe, 2005

Exumella tsonot: after Suárez-Morales & Iliffe, 2005

Taxonomic Characterization: This species shares the following characters with other member of the ridgewayiid genus Exumella: body not compressed laterally, female urosome three-segmented, male with four urosomites, a strong pointed rostrum with a pair of filaments, a well-developed maxilliped with two extremely elongate setal elements on the endopod, and both rami of the fifth legs of the female three-segmented, whereas the male has a two-segmented, elongated fifth leg exopod with long spines. It is distinguished from other species in the genus by a combination of characters, including a relatively large anal somite, the presence of 24 antennular free segments in the female and 19 in the male right antennule, and by peculiar details of the structure of the male and female fifth legs (Suárez-Morales & Iliffe, 2005).

Disposition of Specimens: Collection of Zooplankton of El Colegio de la Frontera Sur, Chetumel, Mexico, catalog numbers ECO-CHZ-02903, ECO-CHZ-20904, ECO-CHZ-20905.

Ecological Classification: Stygobitic

Size: Female body length, excluding caudal setae, measures between 0.38-0.425 mm. Adult male length, excluding caudal setae, measures 0.345-0.365 mm.

Number of Species in Genus: Four, including two from anchialine caves.

Genus Range: This genus is mostly found in the Caribbean region with only one species found in the Mediterranean. The general distribution of the genus Exumella suggests a Tethyan pattern.

Species Range: Found only in the Ponderosa Cave System on the Caribbean coast of Quintana Roo, Mexico.

Closest Related Species: E. mediterranea Jaume & Boxshall, 1995.

Habitat: Anchialine limestone caves

Ecology: Epibenthic hypogean species. Found below the halocline in fully marine waters at depths of 10-16 meters. Additional fauna include cyclopoid copepods with only one other record of a calanoid from the genus Balinella.

Life History: Seven specimens were collected including one adult female and 6 adult males (Suárez-Morales & Iliffe, 2005).

Evolutionary Origins: The marine ancestors of the cave-dwelling calanoid copepods were probably hyperbenthic forms that became adapted to the oligoxic conditions present in anchialine, hypogean systems (Jaume & Boxshall, 1995). Under the continent-island vicariance theory of Rosen (1976),  the ancestors of ridgewayiids were present in the shallow seas around the Maya Block, which came in contact first with the Florida-Bahamas Block and later on with the drifting proto-Antilles. The coastal edges of these ancient islands were colonized by marine benthic or epibenthic marine forms that were tectonically transported eastwards to their current position. Hence, for Exumella and probably for other ridgewayiids as well, the oldest regional populations might have been those connected to the continental mainland of the Yucatan Peninsula. Remipedes, the oldest crustaceans known, are present in the same area of the Yucatan Peninsula where this new ridgewayiid was found. This also suggests an old origin of this species in the Caribbean (Suárez-Morales & Iliffe, 2005).

Conservation Status: Restricted to a single cave in the Yucatan Peninsula.

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Contributor: Eduardo Suárez-Morales, El Colegio de la Frontera Sur (ECOSUR)