154
The Migrant Biota
different families and subregions or provinces by vieariant genera. Ekman (1940)
proposed bis formula for comparing two faunas:
d _ Specificity of area A + Specificity of area B
-
Similarity of A and B
.
The different hierarchie taxa have, according to Ekman, different "species-age
values": e.g. one species-age value for a difference on the species level, three
species-age values for generie-Ievel difference, 18 species-age values for familylevel differences.
This formula works on the assumption that we are dealing with two faunas
which have evolved separately, and the amount of diversity between them reflects
a difference in their history. However, in the case of Lessepsian migration we are
witnessing a sudden change, i.e. there is no vicariance involved but rather an
addition of species to the preexisting inventory.
Nevertheless, this excursion into the field of Ekmanian quantitative zoogeography can be of use in one point. The species of Lessepsian migrants represent
perhaps 10% of the Levant biota; however, they contain many genera and even
numerous families which were alien to the Atlanto-Mediterranean region. To take
the example of fishes, the Holocentridae, Leiognathidae, Theraponidae, Platycephalidae, Tetraodontidae and Monacanthidae were not previously represented in
the Mediterranean fauna. Among the Mollusca there are also several cases of new
families. Among the Decapoda there are no extra-mediterranean families, but
many genera (Metapenaeus, Myra, Leucosia, Charybdis, Atergatis etc.). It seems
beyond doubt that the area occupied in the Mediterranean by the Red Sea migrants can be considered as a Lessepsian province or subregion within the Mediterranean.
What the geographieallimits of this province will be and the fmal proportion
of the Lessepsian taxa in it is a matter for the future, on whieh only a few
conjectures can be made. Although the great majority of the Lessepsian migrants
are at present concentrated along the Levant coast, it is reasonable to assume that
in due course they will occupy the whole of the Levant Basin and the Sidra Sea
(the sea situated between the Tunisian and Cyrenian coast). The westward migration along the African coast was probably delayed by the opposing current,
the structure of the outlet at Port Said whieh is isolated westward by a 6.S-km
long pier, and perhaps also by the Nile water influx. Deficient information may
also in part be the reason for the apparent paucity of Lessepsian migrants on the
African shores (Stirn, 1973). There is, however, little doubt that the expansion of
the migrants in the Mediterranean is a time-distance function within frames set
by temperature limits. This temperature limit has to be sought at the 16° C
isotherm of minimum winter surface temperature. Therefore, most of the Eastern
Mediterranean, excluding the Aegean Sea proper and the Adriatic Sea, has to be
considered within the radius ofLessepsian migration.
Indeed, there are differences between the power of expansion of the various
migrant taxa. Fishes can appear seasonally far westward during the summer and
The Migrant Biota
different families and subregions or provinces by vieariant genera. Ekman (1940)
proposed bis formula for comparing two faunas:
d _ Specificity of area A + Specificity of area B
-
Similarity of A and B
.
The different hierarchie taxa have, according to Ekman, different "species-age
values": e.g. one species-age value for a difference on the species level, three
species-age values for generie-Ievel difference, 18 species-age values for familylevel differences.
This formula works on the assumption that we are dealing with two faunas
which have evolved separately, and the amount of diversity between them reflects
a difference in their history. However, in the case of Lessepsian migration we are
witnessing a sudden change, i.e. there is no vicariance involved but rather an
addition of species to the preexisting inventory.
Nevertheless, this excursion into the field of Ekmanian quantitative zoogeography can be of use in one point. The species of Lessepsian migrants represent
perhaps 10% of the Levant biota; however, they contain many genera and even
numerous families which were alien to the Atlanto-Mediterranean region. To take
the example of fishes, the Holocentridae, Leiognathidae, Theraponidae, Platycephalidae, Tetraodontidae and Monacanthidae were not previously represented in
the Mediterranean fauna. Among the Mollusca there are also several cases of new
families. Among the Decapoda there are no extra-mediterranean families, but
many genera (Metapenaeus, Myra, Leucosia, Charybdis, Atergatis etc.). It seems
beyond doubt that the area occupied in the Mediterranean by the Red Sea migrants can be considered as a Lessepsian province or subregion within the Mediterranean.
What the geographieallimits of this province will be and the fmal proportion
of the Lessepsian taxa in it is a matter for the future, on whieh only a few
conjectures can be made. Although the great majority of the Lessepsian migrants
are at present concentrated along the Levant coast, it is reasonable to assume that
in due course they will occupy the whole of the Levant Basin and the Sidra Sea
(the sea situated between the Tunisian and Cyrenian coast). The westward migration along the African coast was probably delayed by the opposing current,
the structure of the outlet at Port Said whieh is isolated westward by a 6.S-km
long pier, and perhaps also by the Nile water influx. Deficient information may
also in part be the reason for the apparent paucity of Lessepsian migrants on the
African shores (Stirn, 1973). There is, however, little doubt that the expansion of
the migrants in the Mediterranean is a time-distance function within frames set
by temperature limits. This temperature limit has to be sought at the 16° C
isotherm of minimum winter surface temperature. Therefore, most of the Eastern
Mediterranean, excluding the Aegean Sea proper and the Adriatic Sea, has to be
considered within the radius ofLessepsian migration.
Indeed, there are differences between the power of expansion of the various
migrant taxa. Fishes can appear seasonally far westward during the summer and