148
The Migrant Biota
Two facts need to be emphasized: A. wega is a species which reproduces
asexually by fission and subsequent regeneration, and Achituv found little indication that the species reproduces sexually in the Mediterranean. This is a circumstance which was no doubt most favorable for the spreading and colonizing
success of A. wega. Besides, this species is now found only north of 'Atlit, and the
various populations show clear differences in sizes and in the regeneration coefficient. One may assurne that a favorable year, or period of years, has enabled
several small populations ("propagula") to proliferate rapidly and independently
from a general gene pool.
The nearshore waters offer several other examples in which the biology of
colonizing can be investigated: for instance, the interrelation between the local
purpie snail Thais haemostoma and the Lessepsian migrant Thais carinifera first
reported in 1956 from Tel-Aviv. The migrant species has been fairly common
since and the analysis of the relation between the two species is easy to follow
because they leave behind clearly distinguishable egg capsules. In two other molluscs common along the rocky shores of Israel-the key-hole limpet Diodora
rüppelli and the hammer oyster Malleus regula-there is no obvious evidence that
their settling success was detrimental to any local species.
Finally, the meiobenthos offers another interesting case of niehe occupation
by a Lessepsian migrant species. I have shown (Por, 1964) that the bathymetric
succession of the Copepoda Harpacticoida of the Canuellidae is different, compared with European waters, because of the appearance of the Lessepsian migrant
Scottolana longipes. Usually Canuella perplexa occupies the predominantly sandy
bottoms and Canuella jurcigera follows deeper on the predominantly sandy
muddy bottoms. Along the Levant shores, Scottolana longipes clearly occupies the
belt in which usually C. jurcigera is found. It is interesting that S. longipes descends to such depths as 430 m and occurs everywhere from 20 m downwards in
great numbers.
A few conclusions can be drawn from the analysis of the relatively scarce data
that we have:
1. The colonization by the Lessepsian migrants is not of the type studied in
various cases of invasions of terrestrial animals. There is a build-up of populations, sometimes even explosive, but this is not followed by a population crash but
by constant and relatively high numbers.
2. Following Gilat's words (1969), it looks as if the Lessepsian migrants first
have small populations, propagula, which are "waiting" for the favorable environmental episode to build up a significant population.
3. In agreement with Ben-Tuvia (1973), I consider temperature to be the most
important single factor in triggering the build-up of a successful migrant population, for example the many successful populations of Lessepsian migrants whieh
appeared after the warm winters of 1955-1956 and 1960-1961.
4. The Lessepsian migrants are most successful at intermediary depths, between 20-40 m on mixed sandy-muddy bottoms. The rock dwellers prefer the
infralittoral. The reason is the strong cooling of the shallow waters in the winter
and the exposure to cold at higher intertidallevels. Usually, the depth limit of the
Lessepsian migrants is 70-80 m: this is the depth whieh according to Oren (1970),
has a year-round temperature limit of 18° C.
The Migrant Biota
Two facts need to be emphasized: A. wega is a species which reproduces
asexually by fission and subsequent regeneration, and Achituv found little indication that the species reproduces sexually in the Mediterranean. This is a circumstance which was no doubt most favorable for the spreading and colonizing
success of A. wega. Besides, this species is now found only north of 'Atlit, and the
various populations show clear differences in sizes and in the regeneration coefficient. One may assurne that a favorable year, or period of years, has enabled
several small populations ("propagula") to proliferate rapidly and independently
from a general gene pool.
The nearshore waters offer several other examples in which the biology of
colonizing can be investigated: for instance, the interrelation between the local
purpie snail Thais haemostoma and the Lessepsian migrant Thais carinifera first
reported in 1956 from Tel-Aviv. The migrant species has been fairly common
since and the analysis of the relation between the two species is easy to follow
because they leave behind clearly distinguishable egg capsules. In two other molluscs common along the rocky shores of Israel-the key-hole limpet Diodora
rüppelli and the hammer oyster Malleus regula-there is no obvious evidence that
their settling success was detrimental to any local species.
Finally, the meiobenthos offers another interesting case of niehe occupation
by a Lessepsian migrant species. I have shown (Por, 1964) that the bathymetric
succession of the Copepoda Harpacticoida of the Canuellidae is different, compared with European waters, because of the appearance of the Lessepsian migrant
Scottolana longipes. Usually Canuella perplexa occupies the predominantly sandy
bottoms and Canuella jurcigera follows deeper on the predominantly sandy
muddy bottoms. Along the Levant shores, Scottolana longipes clearly occupies the
belt in which usually C. jurcigera is found. It is interesting that S. longipes descends to such depths as 430 m and occurs everywhere from 20 m downwards in
great numbers.
A few conclusions can be drawn from the analysis of the relatively scarce data
that we have:
1. The colonization by the Lessepsian migrants is not of the type studied in
various cases of invasions of terrestrial animals. There is a build-up of populations, sometimes even explosive, but this is not followed by a population crash but
by constant and relatively high numbers.
2. Following Gilat's words (1969), it looks as if the Lessepsian migrants first
have small populations, propagula, which are "waiting" for the favorable environmental episode to build up a significant population.
3. In agreement with Ben-Tuvia (1973), I consider temperature to be the most
important single factor in triggering the build-up of a successful migrant population, for example the many successful populations of Lessepsian migrants whieh
appeared after the warm winters of 1955-1956 and 1960-1961.
4. The Lessepsian migrants are most successful at intermediary depths, between 20-40 m on mixed sandy-muddy bottoms. The rock dwellers prefer the
infralittoral. The reason is the strong cooling of the shallow waters in the winter
and the exposure to cold at higher intertidallevels. Usually, the depth limit of the
Lessepsian migrants is 70-80 m: this is the depth whieh according to Oren (1970),
has a year-round temperature limit of 18° C.