3. The Migrant Biota
3.1 What is a Lessepsian Migrant?
The term "Lessepsian" was first used by Por (1964) to characterize the new
phase into which the Eastern Mediterranean had entered with the opening of the
Suez Canal. Por (1969a, 1971 b) coined the term "Lessepsian migrant" for Red Sea
species which have passed through the Suez Canal and settled in the Eastern
Mediterranean.
In zoogeography the phenomena are usually studied post factum and the
argumentation is circumstantial. The opening of the Suez Canal and the first
reports on erythraean immigrants in the Mediterranean kindled the enthusiasm
of zoologists. The Suez Canal became a link to justify the presence of tropical
species even in the Black Sea or the finding of Atlantic species in the southern Red
Sea.
A primary task for anyone who studies this subject is to weed out the considerable number of would-be migrants, according to a set of criteria as reliable as
possible. This has been attempted on two previous occasions (Por, 1971 b, 1973 b).
I shall try to point out the different categories of reasoning I have used to
reject would-be migrants, and to differentiate between presumable and certain
Lessepsian migrants. It would be presumptuous to claim that the judgements
proposed are ofabsolute value. However, every phenomenon can be studied only
on the basis of a set ofworking principles.
Circumtropical Species. These are present ab initio on both sides of the Suez
Canal. This is true in the case of many planktonic species: Kimor (1973) writes
that the microplankton ofthe Mediterranean and the Red Sea "is characteristic of
all warm-water seas in the world and therefore alm ost circumtropical in distribution". Even in cases like the Tintinnidae, in which according to Kimor (Komarovsky, 1959) the resemblance between the Red Sea fauna and the Eastern Mediterranean fauna is greater than between the Eastern and Western Mediterranean;
Kimor (1973) attributes this to the effect of resembling abiotic factors rather that
to a migrational effect. Alvariiio (1974) assures that the presence of the siphonophorans Chelophyes appendiculata and Sulculeolaria chuni on both sides of the
Suez Canal is due to cosmopolitan distribution and that "the source of the populations in the regions adjacent to the Suez Canal may be in the adjacent regions".
Circumtropical, or tropicopolitan species among algae have frequently been considered as Lessepsian migrants, e.g. the species of Caulerpa found in the Eastern
Mediterranean (HameI, 1930, 1931; Rayss, 1941). Many of the weed-haunting
3.1 What is a Lessepsian Migrant?
The term "Lessepsian" was first used by Por (1964) to characterize the new
phase into which the Eastern Mediterranean had entered with the opening of the
Suez Canal. Por (1969a, 1971 b) coined the term "Lessepsian migrant" for Red Sea
species which have passed through the Suez Canal and settled in the Eastern
Mediterranean.
In zoogeography the phenomena are usually studied post factum and the
argumentation is circumstantial. The opening of the Suez Canal and the first
reports on erythraean immigrants in the Mediterranean kindled the enthusiasm
of zoologists. The Suez Canal became a link to justify the presence of tropical
species even in the Black Sea or the finding of Atlantic species in the southern Red
Sea.
A primary task for anyone who studies this subject is to weed out the considerable number of would-be migrants, according to a set of criteria as reliable as
possible. This has been attempted on two previous occasions (Por, 1971 b, 1973 b).
I shall try to point out the different categories of reasoning I have used to
reject would-be migrants, and to differentiate between presumable and certain
Lessepsian migrants. It would be presumptuous to claim that the judgements
proposed are ofabsolute value. However, every phenomenon can be studied only
on the basis of a set ofworking principles.
Circumtropical Species. These are present ab initio on both sides of the Suez
Canal. This is true in the case of many planktonic species: Kimor (1973) writes
that the microplankton ofthe Mediterranean and the Red Sea "is characteristic of
all warm-water seas in the world and therefore alm ost circumtropical in distribution". Even in cases like the Tintinnidae, in which according to Kimor (Komarovsky, 1959) the resemblance between the Red Sea fauna and the Eastern Mediterranean fauna is greater than between the Eastern and Western Mediterranean;
Kimor (1973) attributes this to the effect of resembling abiotic factors rather that
to a migrational effect. Alvariiio (1974) assures that the presence of the siphonophorans Chelophyes appendiculata and Sulculeolaria chuni on both sides of the
Suez Canal is due to cosmopolitan distribution and that "the source of the populations in the regions adjacent to the Suez Canal may be in the adjacent regions".
Circumtropical, or tropicopolitan species among algae have frequently been considered as Lessepsian migrants, e.g. the species of Caulerpa found in the Eastern
Mediterranean (HameI, 1930, 1931; Rayss, 1941). Many of the weed-haunting