FAMILY
Stygiomysidae
TAXONOMY
Stygiomysis cokei Kallmeyer and Carpenter, 1996, Temple of Doom Cave (328 ft [100 m] in from cave entrance), 3.7 mi (6 km) northwest of Tulum, Quintana Roo, Mexico, at a depth of 32.8–65.6 ft (10–20 m).
OTHER COMMON NAMES
None known.
PHYSICAL CHARACTERISTICS
The size of this mysid ranges from 0.35 to 0.86 in (9.0–22.0 mm). The length of the colorless wormlike body is 7.0–7.2 times the width. The length of the carapace is about one-fifth the length of the body. The abdominal somites are smooth and rounded on the dorsal surface. Antenna 1 is about one-half the length of the body. Four paired ventral lamellae or oostegites (typical for Stygiomysis females and a number unique among the Mysida) extending medially and anteriorly from the proximal part of pereiopods 3–6; each oostegite single flexible membranous flap, rounded and elongated anteriorly. The length of the telson is about 1.7–2.0 times its width, or one-sixth the length of the body. It has 15 spines arranged in five groups of three on its posterior margin.
DISTRIBUTION
Coastal inland caves, Quintana Roo, Yucatan Peninsula, Mexico; the type locality, Mayan Blue Cave; Carwash Cave; and Naharon Cave.
HABITAT
These mysids are stygobites, or cave dwellers. All caves from which this species was collected are completely underwater and entered through water-filled limestone sinkholes. The specimens were collected at depths of 32.8–65.6 ft (10–20 m) in the freshwater layer, occasionally in the upper part of the halocline (a well-defined vertical gradient of salinity). Conditions remained relatively constant with temperatures around 76.1°F–77.9°F (24.5°C–25.5°C), pH 6.8–7.0, low oxygen (near2.0 ppm), and high carbon dioxide levels (44–864 ppm).
BEHAVIOR
The behavior of S. cokei was observed in a laboratory setting. Open containers of cave water readily lost carbon dioxide, causing the pH to rise. The mysids kept their tails almost straight up at a right angle when the pH was comfortably low. As the pH rose, their tails gradually dropped in proportion to the increase; in extreme conditions, their tails were nearly horizontal. They also lowered their tails to a nearly horizontal position while walking. When the animals were forced off their substrate in the caves or the laboratory, they displayed frantic and ineffective swimming movements. The uropods spread away from the telson to make a wide tail fan when S. cokei is walking. In healthy specimens, the respiratory beating of pereiopods 1–7 occurred in sequences of 3–9 seconds, followed by rest periods that lasted 2–45 seconds. As carbon dioxide levels dropped and the pH rose, their rest periods increased to as long as 50 minutes.
FEEDING ECOLOGY AND DIET
Primarily filter feeders.
REPRODUCTIVE BIOLOGY
Nothing is known.
CONSERVATION STATUS
Not listed by the IUCN.
SIGNIFICANCE TO HUMANS
None known.
