Origin and Age of the Marine Stygofauna

Five species of troglobitic crustaceans, previously known only from the Jameos del Agua marine lava tube in Lanzarote, Canary Islands, have been collected from wells in other, geologically older areas of the island. Most, and possibly all, of the endemic species inhabiting the Jameos del Agua probably entered the cave from adjacent crevicular groundwater habitats. The endemic hypogean fauna of Lanzarote can be divided into 2 groups: (1) relict species with affinities to the cave fauna of other oceanic, primarily Western Atlantic, islands and (2) species with close relatives from the deep sea. The origin of the first group can be correlated to Mesozoic plate tectonics. The species of the second group are probably derived from widely spread deep sea ancestors and may have colonized the crevicular system of Lanzarote at different times. S. 223-230 Band 83 Introduction Hamburg, November 1986 ISSN 0072-9612 Lanzarote, at the eastern end of the Canary Islands, possesses one of the world's longest lava tubes, the Cueva de 10s Verdes Jameos del Agua system, having a total length of more than 8 km (HALLIDAY 1972; WILKENS & PARZEFALL 1974). The seaward-most segment of the tube forms a partially to completely flooded marine cave, about 2 km in length. Water in this section of the cave is of fully marine salinities and is subjected to the tides. Except for some 100 m at the two James del Agua entrances, the cave is in total darkness. Although the lava tube was formed by eruptions of the volcano Monte Corona during the Holocene, the volcanic history of the island of Lanzarote is much older. The first marine flows began about 40 m. y. ago, with subaerial flows commencing 11 m. y. (ARANA & CARRACEDO 1979). The eastern pair of Canary Islands, Lanzarote and Fuerteventura, may have been continuously or periodically connected by land bridges or shallow water straits with Africa between the late Cretaceous and the Mio-Pliocene (ROTHE & SCHMINCKE 1968). The marine cave biotope in Lanzarote is characterized by a troglobitic fauna with partial or complete absence of body pigment and eyes reduced to varying degrees. To date, 9 endemic species have been described from this biotope. About half of these, such as the crustacean Morlockia ondinae VALDECASAS ') Address of the authors: Profrs. Dres. HORST WILKENS and JAKOB PARZEFALL, Zoologisches Institut und Zoologisches Museum der Universitat Hamburg, Martin-LutherKing-Platz 3, D-2000 Hamburg 13, West Germany; Dr. THOMAS M. ILIFFE, Bermuda Biological Station for Research, Ferry Reach 1-15, Bermuda. 224 HORST WILKENS, JAKOB PARZEFALL and THOMAS M. ILIFFE which belongs to the class Remipedia (YAGER 1981), show a relict chara'cter (Table 1, ILIFFE et al. 1984). To clarify this seeming contradiction of relict troglobitic species inhabiting a marine cave of recent geologic origin, further studies on the occurrence of such species in different and older areas of Lanzarote were carried out. Material and methods Specimens in this study were collected from both active and inactive wells supplying saline groundwaters for salinas (Saltworks). These wells, situated at a maximum distance of about 300 m from the coast, are approximately 1 m in diameter and extend from 2 to 8 m below the land surface. The water is of. normal marine salinity (35 %o) and oscillates with the tides. Seawater penetrates inland from the coast through a 2-3 m thick layer of porous, highly fissured rock that underlies 5 m of rather impermeable cap rock. A total of 29 wells from 12 salinas were examined, although animals were found in only 13 of these (Fig. 1). Sampling was performed with plankton nets (55 ym) and baited traps. Benthic specimens were also collected. Studies were carried out between 1 and 23 February 1985 and between 16 February and 23 March 1986. All material is deposited in the Zoological Museum Hamburg.