A commented check-list of the Balearic Branchiopoda (Crustacea)

43 species of branchiopoda have so far been recorded from the Balearic islands, including species mentioned here for the first time. Chorologic grouping hows an important stock of species associated with semiarid regions of continental inland basins, both from fresh and atalassohaline waters. Their biogeographic significance and age is discussed in relation to the neotectonics of the islands and the possible existence of barren conditions in part of the insular landscape in the past.


INTRODUCTION
The Iberian region is noted for its interesting freshwater entomostracan zoogeography, the study of which was originated at the end of the past century, but with its exponential development cooccuring with that of the regional limnosociology in the forties and fifties. The Balearic Islands were not an exception to this rule, and they were one of the areas studied by MARGALEF (1951bMARGALEF ( , 1952, who started the taxonomy and biogeography of the freshwater crustacea of the islands (MARGALEF, 1948(MARGALEF, , 1958. Recently work on the subject was recommenced, mainly due to separate situations which have arisen: One, the appearance of botanists and zoologists interested in and centred on the aquatic systems of the Balearic region, which has led to a greater geographic exploration and recognition of aquatic habitats unknown so far; this situation has allowed different authors to collect and study a varied spectrum of crustacean samples (JAUME, 1989;PRETUS, 1985PRETUS, , 1987. Two, the existence of faunistic synthesis on different groups of branchiopoda, from severa1 Mediterranean countries (GAUTHIER, 1928;Limnetica, 6: 157-164 (1990) O Asociación Española de Limnología, Madrid, Spain NEGREA, 1983;ALONSO, 1985a, b;MARGARITORA, 1985;RAMDANI, 1986;THIÉRY, 1987) resulting in an easier and more detailed comparison of new zoologic data. And three, the progressive consolidation of the vicariant evolutionary model for several genera of Chydoridae (FREY, 1982;ALONSO, 1987). This model is revolutionising the classic approach which consists of a supposed generalized cosmopolitanism. This change has affected the Balearic area . The result of these combined features is to change our view of the Balearic branchiopod biota, to a point at which a first check-list recording the new materials is now possible.

MATERIAL AND METHODS
The study area concerns the four main islands of the Balearic archipelago. The Check-list includes the existing bibliographic data and unpublished data collected by the author. Samples were taken in Majorca during May and June 1987, dunng January and February 1988and in March 1989 Ibiza is poorly endowed with freshwater environments. Samples were also taken in both Ibiza and Formentera in February 1988. Minorca has been studied since 1984, with the largest effort made in fieldwork taking place between March and May 1984, July 1986, from December to April 1987, and in January 1988 General characteristics of the Balearic regional limnology are compiled in previous works, mainly Margalef's papers on the hydrobiology of the islands. It is also interesting to comment here that these islands, especially Majorca and Minorca, are furnished in some degree with wetlands, mainly near the coastal areas, connected with rather irregular rushing streams. Temporary muddy and transparent freshwater pools are well developed in the south of Mallorca (SASTRE, 1987), and we refer to this as the Lluchmajor area. Ecologically similar aquatic systems, although not so frequent, but nonetheless still of importance, are also present in the west part of Minorca, namely in the Ciutadella area, and at severa1 points near the coast in the same island. Finally, they are also present in Formentera island. Springs are numerous in the mountainous area of Mallorca. Temporary pools and phreatic permanent ponds are developed in the north part of Minorca, where a complex and varied superposition of permeable and impermeable, limestone, dolomitic and silicic sandstone materials, make it a rich area of aquatic diversification. The hyporheic is poor, but well developed locally in Mallorca. No true atalassohaline lagoons are present.

RESULTS
A commented Check-list follows (see also table 1). Concise descriptive notes are presented for each island separately, because of the distinct sampling timespan spent by different authors on each one. Species cited by previous authors are also incorporated in separate form. Species not followed by references are first records for their corresponding island. First records for overall Balearic islands are shown in comments. Data on localities recorded are accompanied by a reference to date and the observed conductivity rank; accidental extreme high values of conductivity are indicated separately, as placed outside a more or less continuous rank. Marine cladocera are not treated here. The systematic groups at the leve1 of orders are str.uctured following FRYER (1987).

DISCUSSION
43 species of Branchiopoda have been recorded in Balearic Islands: 4 Anostraca, 2 Spinicaudata, 36 Anomopoda and 1 Notostraca. One part of the species belongs to a group of generalized forms in the Mediterranean-European area: Simocephalus, Scapholeberis, several Ceriodaphnia, Daphnia subgenus Daphnia, Alonella excisa, Alona affinis, A. guttata, or A. rectangula. Nevertheless, an important sector is a part of a chorologic grouping inhabiting steppic areas in inland zones: Branchinecta ferox, Cyzicus bucheti, Leptestheria mayeti, Daphnia atkinsoni, D. bolivari, or chydorids as Alona cf esteparica, Alona iberica, or Ephemeroporus phintonicus. On the other hand, at least two species from continental atalassohaline waters occur in the hiperhaline coastal marshes of Majorca: Daphnia mediterranea and Moina salina. New environments such as the reservoirs in Majorca, have been colonized by some species such as Bosmina longirostris, Macrothrix laticornis and Daphnia longispina, which are common in the Iberian resewoirs (ARMENGOL, 1978).
The most remarkable data concerns the appearance of steppic species in the islands, where no extreme continental climate is supposed to exist. They frequently inhabit temporary muddy pools, distributed in some areas of the south of Majorca, and also in the west part, and the littoral of Minorca, in accordance with the distribution of the semiarid zones in the islands. The presence of steppic species amongst the Iberian fauna may be due to the persistence since the Tertiary of appropriate conditions, despite the Quaternary climatic oscillations. On the other hand, more recent col-onizations could have occured as the result of passive migrations from dispersal areas situated in North Africa. The value of either hypothesis is difficult to measure precisely (MARGALEF, 1947(MARGALEF, , 1983ALONSO, 1985a;JAUME, 1989). In the case of the Balearic islands, the persisting Quaternary argument is hardly defensible in a preliminary consideration, mainly due to the inherent ecologic instability of small areas, such as limited areas inside the islands. This instabiiity is denved Pom changes in the local hydrography due to the neotectonics, and is favoured by the climatic oscillations. The known biota could have arrived at the islands by means of the present active dispersal mechanisms thought to exist, as discussed by JAUME (1989) for Leptestheria mayeti in Majorca, and this can be extended to the minorcan population of this species, known from a tectonically emerging area in the north littoral.
Minorcan pools where the rarest species occur (the Ciutadella area), are placed on a stable western platform weakly affected by the neotectonics since the late Miocene (BOURROUILH, 1983). Here appears Branchinecta ferox, an extended but not frequent steppic species in the Western Palaeartic region, and Cyzicus bucheti, one locality in Sardinia and common in Morocco (THIÉRY, 1987), where it is found in the «dayas», mainly in the flatlands of the Atlantic region. Alona iberica, a recently described chydorid from Minorca and SW of Iberia, is also found in this area.
The hypothesis of ancient biota cannot be rejected a priori, since arguments concerning Balearic ecological aspects are also favourable. The arguments emerge from sedirnentologic and paleontologic data. A synthesis on the Quaternary of the islands (CUERDA, 1975) shows that glacial periods, owing to the effect of glacioeustatic regressions, are registered along most of the coast by eolic dunes, sometimes intercalated with red soils. Although the age of these soils is yet uncertain, their formation requires the existence of a dry season, which is necessary to explain the reddening due to iron hydroxide that furnishes characteristic terra rossa soils. These eolic formations are known from probably the ancient Plioquaternary to the greater part of the Pleistocene. The geographic extension involved is not only that which is close to the littoral, but also includes far off inland areas. With respect to Minorca, this latter situation precisely defines the stable Ciutadella region, mentioned here because of its outstanding steppic freshwater communities. This area had probably more «continental)) characteristics before a great part of lands at the north disappeared in recent subsidence (BOURROUILH, 1983). Moreover, the disharrnonic mammal composition of Majorca and Minorca during the Pliopleistocene, dominated by the Balearic dwarf goral (Myotragus sspp.), expanding and evolving in the absence of mammal predators, must have aided the development of a particular landscape. Part of the hypothetic impact should concern local barren conditions and the consolidation of temporary pools, in some instances because of the effect of winds, which are considerable as shown by the presence of inland dunes. Similarly, connections between the appearance of the Balearic endemic flora and the evolution of the specific functional anatomy of the Myotragus have previously been pointed out by severa1 authors (see ALCOVER et al., 1981). The fact that Myotragus ossiferous deposits are ubiquitous in Majorca and Minorca, and are not only located in mountainous areas, shows the possible extension involved. The progressive evolution of the Balearic dwarf goral over 6 million years is assumed by paleontologists to be in cause-effect relationship with the denudation of the vegetation due to suc-cessive demographic explosions, which at the same time created specific insular selection pressures to the isolated artiodactyles (op. cit.). If the phenomenon is admitted, its repercussion on the landscape, and particularly on the persistence of temporary ponds, must have been only a question of degree. According to al1 the arguments expressed here al1 the present steppic fauna is not necessarily a postglacial (holocenic) introduction. In other words, the point made is that recent colonizations are still not necessarily the only explanation to a part of the biotic composition of the isles. Some species, once more widely distributed, may have persisted under a local geologic stability due to weak neotectonics, and under assumed barren conditions originated by climatic events and the disharmonic vertebrate biota.

ACKNOWLEDGEMENTS
1 acknowledge the invaluable assistance afforded by B. Sastre and L1. Gradaille, in providing detailed locations of the Lluchmajor pools. This work is part of a series on Balearic crustacea sponsored by the FPI program of the Ministerio de Educación y Ciencia, Madrid (1985).