Zoogeographical Consequences and the Future of Lessepsian Migration
155
retreat to warmer waters in the winter. To some extent, different decapods can
develop similar migrational patterns. Molluscs, however, are tied to their substrate, and therefore show also less tendency to explore the periphery of the
Lessepsian distribution radius. However, some molluscs, e.g. Brachidontes, Malleus, Pinctada may reach protected and warm gulfs or portuary areas in the West
by passive transport.
One can also assurne that the depth distribution, limited along the Israel coast
by the 70-80 m isobath, is gradually changing towards the west and the Lessepsian migrant species are restricted to a more and more limited bathymetric range;
a decreasingly thick sublittoral belt is still able to provide the more or less
uniform and warm temperatures required.
Since the colonizing success ofthe immigrants within the local communities is
also influenced by the temperature sensitivity of the intruder, versus that of the
local competitor, the chances of success will decrease from East to West, with the
lower isotherm values. The very carrying capacity of the environment is also
decreasing westward, being presumably inversely proportional to the increasing
species diversity ofthe local Mediterranean fauna.
This seems to be a fairly static picture. Several authors, chiefly Tortonese
(1951) and Kosswig (1956), have assumed a rapid physiological evolution of the
Lessepsian migrants which leads to speciation, or hybridization with local congeneric species. I myself have raised this possibility in an introductory chapter of
this book, and the eventuality of such phenomena cannot be exc1uded. However,
nothing in the direction of an evolutionary divergence of the Lessepsian migrant
species could until now be externally and descriptively observed. This is also
s u pported by T ortonese (1973 b) regarding fishes and echinoderms. There are diff erences in size between the specimens ofthe Red Sea population of origin and those
ofthe "Lessepsian diaspora". According to Ben-Tuvia (1966) there is a shift in the
reproduction period of some migrant species of fish. These differences however,
belong very much to the infra-subspecific level.
It is regrettable that no experimental work has as yet been done to check the
degree of interfertility of Lessepsian migrant populations with their populations
of origin. It is also possible that differences in tolerance of preference versus
environmental factors can already be experimentally discerned when two conspecific populations are compared. Evolution of new taxa even on the subspecific
level, however, requires time. I have already (Por, 1975a) expressed the view that
the frequent climatic changes during the Pleistocene resulted in the sea in fluctuating advances and retreats ("pulsations") of species which did not change their
ecological requirements. The time-stability dimension necessary for evolution of
a new taxon with a different ecological valency simply did not exist during the
eventful history of the Pleistocene mediterranean seas. Therefore, I considered
(Por, 1975a) that the Lessepsian influx of species into the Mediterranean became
possible since it fitted present interglacial oceanographic conditions in that sea.
Under the present conditions, the cold water spe~ies complex which flourished
during the glacial oceanographic conditions is on the retreat, leaving a vacuum in
the subtropical warming-up Levant Sea.
155
retreat to warmer waters in the winter. To some extent, different decapods can
develop similar migrational patterns. Molluscs, however, are tied to their substrate, and therefore show also less tendency to explore the periphery of the
Lessepsian distribution radius. However, some molluscs, e.g. Brachidontes, Malleus, Pinctada may reach protected and warm gulfs or portuary areas in the West
by passive transport.
One can also assurne that the depth distribution, limited along the Israel coast
by the 70-80 m isobath, is gradually changing towards the west and the Lessepsian migrant species are restricted to a more and more limited bathymetric range;
a decreasingly thick sublittoral belt is still able to provide the more or less
uniform and warm temperatures required.
Since the colonizing success ofthe immigrants within the local communities is
also influenced by the temperature sensitivity of the intruder, versus that of the
local competitor, the chances of success will decrease from East to West, with the
lower isotherm values. The very carrying capacity of the environment is also
decreasing westward, being presumably inversely proportional to the increasing
species diversity ofthe local Mediterranean fauna.
This seems to be a fairly static picture. Several authors, chiefly Tortonese
(1951) and Kosswig (1956), have assumed a rapid physiological evolution of the
Lessepsian migrants which leads to speciation, or hybridization with local congeneric species. I myself have raised this possibility in an introductory chapter of
this book, and the eventuality of such phenomena cannot be exc1uded. However,
nothing in the direction of an evolutionary divergence of the Lessepsian migrant
species could until now be externally and descriptively observed. This is also
s u pported by T ortonese (1973 b) regarding fishes and echinoderms. There are diff erences in size between the specimens ofthe Red Sea population of origin and those
ofthe "Lessepsian diaspora". According to Ben-Tuvia (1966) there is a shift in the
reproduction period of some migrant species of fish. These differences however,
belong very much to the infra-subspecific level.
It is regrettable that no experimental work has as yet been done to check the
degree of interfertility of Lessepsian migrant populations with their populations
of origin. It is also possible that differences in tolerance of preference versus
environmental factors can already be experimentally discerned when two conspecific populations are compared. Evolution of new taxa even on the subspecific
level, however, requires time. I have already (Por, 1975a) expressed the view that
the frequent climatic changes during the Pleistocene resulted in the sea in fluctuating advances and retreats ("pulsations") of species which did not change their
ecological requirements. The time-stability dimension necessary for evolution of
a new taxon with a different ecological valency simply did not exist during the
eventful history of the Pleistocene mediterranean seas. Therefore, I considered
(Por, 1975a) that the Lessepsian influx of species into the Mediterranean became
possible since it fitted present interglacial oceanographic conditions in that sea.
Under the present conditions, the cold water spe~ies complex which flourished
during the glacial oceanographic conditions is on the retreat, leaving a vacuum in
the subtropical warming-up Levant Sea.