Hydrobiologia 467: 57–62, 2002. J. Alcocer & S.S.S. Sarma (eds), Advances in Mexican Limnology: Basic and Applied Aspects. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 57 The external micro-anatomy of the cephalon of the asellotan isopod Craseriella anops Elva Escobar 1 , Luis Oseguera 2 , Gerardo H. V´ azquez Nin 2 & Javier Alcocer 3 1 Unidad Acad´ emica en Sistemas Oceanogr´ aficos y Costeros, Instituto de Ciencias del Mar y Limnolog´ ıa, UNAM, A.P. 70-305, M´ exico D.F. 04510, Mexico E-mail: escobri@mar.icmyl.unam.mx 2 Facultad de Ciencias, Laboratorio de Microscop´ ıa Electr´ onica, UNAM, Mexico 3 Limnology Laboratory, Environmental Conservation and Improvement Project, UIICSE UNAM- Campus Iztacala, Mexico Key words: anchialine, mechanoreceptor, scales, setae, sensory hair, turbulence, Yucatan Abstract The micro-anatomy of the cephalon is described in the troglobic asellotan isopod Craseriella anops from the Nohoch Nah Chich anchialine cave system in southeast Mexico. The cephalon is entirely covered by cuticular scales bordered by marginal spines. The anterior end of the cephalon is bordered by a carina that is wider medially. The isopod is eyeless. The distal seventh portion of the cephalon is characterized by the presence of two sutures and six setae. A suture is found on each side of the distal margin of the cephalon. Each suture is bordered by microtrichs. Two simple setae with a sensory hair, articulated on the base by a socket, are found one on each side of each of the sutures. Two additional setae, similar in shape and size, occur medially on the cephalon. A terminal pore is absent on the sensory hairs of all setae. These setae are suggested to be mechanoreceptors that provide directional sensitivity and enhance the sensibility of turbulent motion, viscosity and changes of hydrostatic pressure. Introduction Adaptation is a central problem in evolutionary bio- logy and an extraordinary topic for research in sub- terranean and deep-sea ecology (Christianen, 1992). Crustacea have been recognized as one of the most diversified and abundant groups inhabiting the anchi- aline cave systems in the tropics, the deep-sea and other interstitial habitats and have been used as mod- els to exemplify how animals adapt to a life in total darkness with short food supply (Kaufman, 1994). Among the adaptations recorded in troglobic fauna are the reduction of size (Holsinger, 1986), the loss of pigment and of visual organs and the development of specialized sensorial structures (George, 1981). Underwater cave systems remain in total darkness. The absence of light limits photosynthesis, which is the basis of the food web in other aquatic habitats. The food sources sustaining the cave system faunal assemblages depend on the import of debris from the neighboring terrestrial and aquatic environments through the sinkholes. Organic material generally enters in dissolved or particulate form and is sub- sequently processed by bacteria (Eichem et al., 1993) and detritovores. The relationship between the food resources entering the cave systems and the consumers in the caves are not well documented; neither is the prey–predator relationship. The cuticle of crustaceans is a dynamic system that has been recognized to perform diverse functions. In habitats where light is absent, structures on the epi- cuticle may provide a way to link the external environ- ment with the sensory function and behavior. Isopods have been recognized as scavengers and predators that, in shallow habitats in presence of light, can utilize visual cues to locate their prey. An extensive termin- ology has been developed for describing the nature of crustacean cuticle. Studies on decapods date back to last century and early this century and have extensively described mechanosensory and chemosensory struc-