Stygiomysis cokei Kallmeyer & Carpenter, 1996
Taxonomic Characterization: Unpigmented, vermiform, body length 7.0-7.2 x width. Carapace length one-fifth body length. Ventrally, carapace housing mouthparts, anterior 3 pairs of pereiopods, and 1 pair of ostegites, if present, on pereiopod 3. Dorsally, pleonite segments smooth and rounded. Telson length 1.7-2.0 x width, length one-sixth of body length; posterior margin with 15 spines in 5 groups of 3 each (Kallmeyer & Carpenter, 1996).
Ecological Classification: Stygobitic
Size: Adult females to 22.0 mm in length; adult males to 15.3 mm
Number of Species in Genus: Seven, all stygobitic
Species Range: Reported from Cenote Mayan Blue, Cenote Carwash and Cenote Naharon, Quintana Roo, Mexico
by Kallmeyer & Carpenter (1996). Additional sites reported by Pesce & Iliffe
(2002) include Cenote Pabakal, Cenote San Eduardo, Cenote Kankirixche, and
Cenote Dzonot-ila in Yucatan, and Actun Ko in Quintana Roo.
Closest Related Species: Entire family appears to be remarkably uniform in many features. S. cokei shares characteristics with each of the other six Stygiomysis species.
Habitat: Anchialine limestone caves
Ecology: Collected at depths of 10-20 m, in the freshwater layer or occasionally in the upper part of the halocline. Conditions remained relatively constant with temperature 24.5-25.5oC, pH 6.8-7.0, low oxygen near 2.0 ppm, and high carbon dioxide 44-864 ppm). All specimens were collected from remote areas of the caves where there is little disturbance from SCUBA divers. Associated fauna include atyid shrimps, thermosbaenaceans, amphipods, harpacticoid copepods, ostracods, cirolanid isopods, anthurid isopods and remipedes (Kallmeyer & Carpenter, 1996).
Life History: Ten specimens have been collected - 9 females and 1 male. Of the female specimens collected, the 6 largest had developing oostegites. Paired ventral oostegites extended medially and anteriorly from coxal plates between pereiopods 3-6. Each oostegite single flexible membranous flap, rounded and elongated anteriorly (Kallmeyer & Carpenter, 1996).
Behavior: Behavior was observed in a laboratory setting. Open containers of cave water readily lost CO2 causing the pH to rise. The mysids kept their tails almost straight up at a 90o angle when the pH was comfortably low, but as pH rose, their tails gradually dropped in proportion (e.g. 75, 60, 45, 30, 15o, to near horizontal in extreme conditions). They also lowered their tails to near horizontal while walking. When animals were forced off their substrate, in either the caves or in the laboratory, they displayed frantic, ineffective swimming movements. During walking, the uropods spread away from the telson to make a wide tail fan. In healthy animals, respiratory beating of pereiopods 1-7 occurred in 3-9 second sequences, followed by rest periods from 2-45 seconds. As CO2 levels dropped and pH rose, rest periods increased to as much as 50 minutes (Kallmeyer & Carpenter, 1996).
Evolutionary Origins: According to Pesce (Groundwater Mysids of Italy), "The remarkable distribution of Stygiomysis undoubtedly points to a Tethyan origin, all the species of this genus being actually known from the Caribbean area and South Italy."
Conservation Status: Restricted to anchialine caves of Quintana Roo, Mexico.
Contributor: Giuseppe L. Pesce, University of L'Aquila, Italy
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