Phylum Arthropoda
Subphylum Crustacea
Class Copepoda
Order Cyclopoida
Family Cyclopidae

Mesocyclops chaci Fiers, 1996

	Mesocyclops chaci: after Fiers et al., 1996

Taxonomic Characterization: One-eyed. Placement of the spiniform seta of leg 5 is medially rather that subterminally on the distal (exopodite) segment. There is lateral extension and rotation of the lateral spine of leg 2 exopodite segment 1. Lack of spines on pediger 5 (Fiers et al., 1996).

Disposition of Specimens: National Museum of Natural History, Smithsonian Institution, catalog numbers USNM 274244; and Invertebrate Collection of the Royal Belgian Institute of Natural Sciences, catalog numbers COP 3811-7.

Ecological Classification: Stygobitic

Size: Largest female measured 0.73 mm in length; largest male 0.66 mm.

Number of Species in Genus: Numerous

Genus Range: This genus has a world-wide distribution.

Species Range: Known only from a permanently covered well at Cheuman and from Gruta Tzab-Nah Cenote, Yucatan Peninsula, Mexico.

Closest Related Species: Closely related to M. yutsil and M. reidae Petkovski, 1986.

Habitat: Freshwater limestone caves and groundwaters

Ecology: Lifestyle is omnivorous, tending toward carnivory. Additional fauna include: cyclopoids, harpacticoids, misophrioids, isopods, shrimps, ostracods, thermosbaenaceans and diaptomids (Fiers et al., 1996).

Life History: Female dominated species. Of the 57 specimens collected, 40 were female, 4 were male and 13 were copepodids. Egg sacs contained 6 to 8 eggs.

Evolutionary Origins: Along with M. yutsil and Keiferiella delamarei (Lescher-Moutoué, 1971; 1976) (known from the saturated zone of a karst system in France), this species is considered a species-flock which has radiated as a specialist within a highly dynamic geomorphological environment. It is as if M. yutsil shared a cervicular living ancestor with M. chaci, but evolved for a planktonic existence in the Yucatan cenotes. It has been suggested several times that ancestral copepods were epibenthic, giving rise to planktonic forms (Fiers et al., 1996).

Conservation Status: Restricted to one well and one cenote of the Yucatan Peninsula.

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