THE RESPIRATORY SYSTEM OF GLOBONAUTES MACROPUS (RATHBUN, 1898), A TERRESTRIAL FRESHWATER FROM LIBERIA (GECARCINUCOIDEA, )

BY

NEIL CUMBERLIDGE Department of Biology, Northern Michigan University, Marquette, Michigan 49855, U.S.A.

RÉSUMÉ

Le système respiratoire de Globonautesmacropus (Rathbun, 1898) crabe d'eau douce arboricole de l'Ouest-Africain, est adapté pour des échanges gazeux aussi bien aériens qu'aquatiques, ce qui reflète un mode de vie terrestre. Les premiers, seconds et troisièmes maxillipèdes montrent des modifications pour la respiration de l'air. Globonautesa sept paires de branchies pleinement fonctionnelles, quoique ces structures soient réduites en taille et en nombre comparées à celle des crabes brachyoures aquatiques. La chambre branchiale dorsale élargie abrite des poumons extrêmement bien développés dans les espaces épibranchiaux au-dessus des branchies. La sur- face interne de la membrane qui double la chambre branchiale dorsale est composée de grandes alvéoles de forme irrégulières qui forment une structure complexe spongieuse en couches, et offrent une large surface pour les échanges gazeux. L'échange des gaz aériens chez les crabes hors de l'eau est facilité par un flux d'air relativement lent vers l'avant, à travers les chambres branchiales. Les crabes en partie immergés dans l'eau peu profonde montrent une alternance entre le flux d'eau vers l'avant et le flux d'air inverse. Les crabes complètement immergés en eau profonde présentent une prédominance du flux d'eau vers l'avant. Ceux immergés dans une eau juste assez peu profonde pour qu'ils atteignent occasionnellement la surface, manifestent une manoeuvre d'émersion, ce qui montre que les crabes sous l'eau respirent de l'air en plus de l'eau. Les modifications de structure et de ventilation du système respiratoire de G. macropus ressemblent à celle des crabes d'eau douce et de terre adaptés à un mode de vie terrestre.

INTRODUCTION

Most members of the three superfamilies of freshwater -the Pseudo- thelphusoidea, , and Gecarcinucoidea-are adapted to an aquatic existence and have only a limited ability to live on land. A few species, however, can survive in terrestrial habitats and are capable of prolonged respiration in air. The ability to breathe air has apparently evolved several times in freshwater crabs since there are amphibious and terrestrial species of crabs represented in all three superfamilies. Air-breathing freshwater crabs have a wide distribution throughout the tropics and are found in Central and South America (Valente, 1948; Diaz & Rodriguez, 1977; Innes & Taylor, 1985), Africa (Zoond & Charles, 1931; Cumberlidge, 1986), Asia, and Australia (Greenaway & Taylor, 1976; Taylor & Greenaway, 1979; Greenaway, Taylor & Bonaventura, 1983). 70

In the majority of freshwater crabs studied the modifications for aerial gas exchange generally include the vascularization of the branchial chamber lin- ings to form functional lungs, and the enlargement of the epibranchial spaces in the branchial chambers to form air stores (Valente, 1948; Diaz & Rodriguez, 1977; Innes & Taylor, 1985; Cumberlidge, 1986). The lung tissue consists of either a flat, vascularized epithelium of varying complexity (, Potamonautidae, Parathelphusidae), or a vascularized epithelium plus an ad- ditional invaginated spongy structure (: Diaz & Rodriguez, 1977; Innes & Taylor, 1986). The sizes of the epibranchial air stores are reflected in the degree of carapace expansion (both outward and upward), and most species of air-breathing crabs possess a bulbous, rounded, carapace. The exact pattern of ventilation of the branchial chambers in species of air- breathing freshwater crabs is usually complex, since all air-breathing freshwater crabs studied so far have also retained fully-functional gills and are capable of sustained aquatic respiration in addition to aerial respiration. Most species of freshwater crabs when breathing air ventilate their branchial chambers by means of a slow, forward air flow (Valente, 1948; Cumberlidge, 1986; Innes & Taylor, 1986). Notable exceptions to this are the Australian parathelphusid freshwater crabs which ventilate their lungs with a slow, tidal air flow (Greenaway & Taylor, 1976; Taylor & Greenaway, 1979). Freshwater crabs are often found in habitats where shallow water is available from timc to time and patterns of simultaneous mixed aquatic and aerial ventilation have also been reported for crabs in such circumstances (Cumberlidge, 1986). The anatomy of the respiratory system and the patterns of branchial chamber ventilation have been described for only two species of African freshwater crabs, both of which belong to the Potamoidea in the family Potamonautidae Bott, 1970 (Zoond & Charles, 1931; Cumberlidge, 1986). There have been no similar accounts for any of the African members of the Gecarcinucoidea of the family Gecarcinucidae Rathbun, 1904, since gecar- cinucids are poorly represented in Africa, and until recently all of the seven taxa were known only from preserved type material (Bott, 1969, 1970). This situation changed when populations of the gecarcinucid Globonautes macropus (Rathbun, 1898) were discovered in the rain forests of Liberia, West Africa (Cumberlidge & Sachs, 1989). Globonautes macropus, the subject of the present study, inhabits rain-filled cavities in trees-an extremely unusual niche for a . This species is found in localities far from permanent water sources in the forest such as the rivers, streams or swamps where freshwater crabs are usually found. The largely terrestrial existence of G. macropus is made possible by a number of morphological and physiological adaptations, including modifications of the respiratory system to breathe air in addition to water. The present work presents an account of the anatomy of the respiratory