Colloquial Meeting of Marine Biology I
Essay about the phenomenon of
Headed by: dr. A. Goldschmid
Author: Pierre Madl
Salzburg, April 1999 (revised in Nov. 2001)
|Lessepsian Migration (LM)||Definition: Migration across the
Suez Canal (planned by Ferdinand de Lesseps); generally from the Red Sea
to the Mediterranean, rarely the other way. Favored by two distinct man
|For 100 years, the colonization of the Suez
Canal and the movement through this passage has been going on, but surprisingly
little is known of its results. The following report, compiled from numerous
sources, is an attempt to show in a preliminary way, what is known today
of the outcome of the 2-way migration via the Suez Canal. Verification
of records has not been made at this stage, rather records have been adopted
for the present purpose.
The occurrence of Atlantic or Indo-Pacific species not previously recorded in the Mediterranean Sea is not necessarily evidence of recent immigration. Such new records are usually based on only one or few specimens, indicating their rarity in the Mediterranean Sea. These are more likely to be old inhabitants of the Mediterranean that have not come to the attention of taxonomist until now.
When the Suez Canal was opened in 1869 the waters of the Mediterranean and the Red Sea, which had been separated by the Isthmus of Suez, came into direct contact. For plants and animals, elements of the Atlanto-Mediterranean and the Indo-Pacific-Erythrean flora and fauna, an opportunity was thus created to penetrate into new regions. Like many other man-made environmental changes, Lessepsian Migration is imprinted on a general pattern of historical processes.
Although, contact with the Indian Ocean has not been established until recently, fossil specimens of marine fishes found in Italian Messinian deposits reveal that episodic contacts with the Indian Ocean must have taken place previously (some 12mio years ago).
|The Suez Canal - Historical Framework
Construction of the canal initiated on April 25th 1859; officially opened on April 25th 1869. Although an international agreement ratified in 1888 granted free and unhindered passage of commercial and military vessels of all countries, Egypt denied any usage of the canal for ships under Israeli banner which culminated in a 6-day war between those two states.
Fig.1 The Canal from Port Said to Suez
Fig.2 Suez Canal
During the occupation of the Sinai peninsula by Israel's armed forces, traffic through the canal came to a halt and did not resume for eight years. Only after the completion of the 1979 peace treaty between those two states free traffic was once again established. Today, the canal provides a rapid shortcut to/from the Indo-Pacific for container vessels up to 150E3 metric tons which can easily maneuver through the canal within 15 hours. The Isthmus of Suez bridges the Asian with the African continent and separates the Mediterranean from the Red Sea. This strait geologically represents a low-laying sand and stony desert, in which swampy terrain surrounds lake Bitter settled at its lowest depression.
There is almost no fresh water present which would otherwise alter the overall salinity of the Suez Canal. The canal is an open gate with slightly unbalanced water levels at either ends - low enough to operate the canal without appropriate gates and channels. It covers a total length of 165km, its mean depth is 14.5m and 325m in width - minimal surface width of 160m.
|Effects aiding in LM:
Let us now turn to a Mediterranean crisis from the 20thcentury, which indicates what a small rise in sea level can mean.
Cessation of Nile floods after the completion of the Aswan Dam cut the river's freshwater flow to less than 10% of the previous level; with the concomitant effects of:
Shortly after the opening of the canal, the salty bottom of the lakes gradually dissolved over a period of 60 years. This enormous surge in salinity along the various parts of the canal (hypersaline Lake Bitter, 68%0 ) with the extra high temperature of the Canal water during the summer half-year (about 10°C warmer at the northern than at the southern end), made the passage problematic for adult animals, and probably impossible for larvae. By now, the salty bottom has probably already completely disappeared (from 52%0 in 1924 to a current average 49% 0 in the summer and 44%0 in winter - although evaporation rate should be sufficient to maintain these levels). With the deepening of the lakes and reduced salt concentrations - equalizing density differences, an exchange of water along with migrant species is further facilitated.
||Fig.3 Schematic representation
of decreasing salinity and dredging events in the Suez Canal, versus gradual
increase in species diversity.
||Fig.4 The Suez Canal is not in
stable equilibrium with the surrounding seas and the pattern of temperature
and salinity such as these are constantly changing with the seasons.
Left margin: southern end of canal
Right margin: northern end of canal
Currently, lower winter temperatures are apparently the main factor preventing the spreading of the bulk of the Red Sea fishes into the Mediterranean Sea, much in the same way as they prevent the penetration of a multitude of other tropical organisms, for example the reef-building corals.
Hydrostatic Pressure: The present level of the Red Sea is about 1.2m higher than the Mediterranean; since the canal is not equipped with channel gates , the gravitational gradient pushes water unhindered northward (and even more so during prevailing winds from the south-east).
The actual dynamics of the passage of animals are boosted also by tidal currents which usually move rapidly from the Red Sea into the Bitter Lakes to spread in the large basin where animals flushed along must spend a comparatively great part of their total passage time through the Canal in hypersaline conditions.
|The Magnitude of LM:
The colonization of the (eastern) Mediterranean by Red Sea species may
be regarded as a large-scale experiment by which the processes of colonization
and adaptation can be observed, and the source and colonizing populations
may be compared.
Several species of fish parasites (Protozoa and Monogenea) have accompanied their hosts. Consequently an equally migrant bivalve must be mandatory to enable his nutritional survival in this new distributional area.
Invertebrates: More up to dated census extensively list migrant species of Polychaeta (Nereidae), several among the Serpulidae, species of Indo-Pacific mollusks and more un-doubtful species as new settlers; the influx of Crustacea seems to be somewhat slower than compared to the others. Increase in polychaete, mollusks, and decapods populations are significant - to a lesser extent towards the western basin of the Mediterranean. At the moment when the first census on exotic species within the east-Mediterranean was done (early 70's), some 45 Red Sea species (26 among Gastropods and 19 among Bivalves) have been identified - which amounts to about 9% of the known mollusc fauna. Among decapods, some 43 species out of 19 families (some 20% of the total fauna) have been identified as Lessepsian migrants.
Especially ship-borne expansion represents a useful means of transportation for "low probability migrants" such as juvenile stages of several organisms with bi-phasic lifecycles (pelagial larvae drifting as plankton).
Migrating fish: Of the 430 (or more) species of marine fishes identified from the Mediterranean coast of Israel and its immediate neighborhood, 100 species are circumtropical-cosmopolitan, the remaining species belong to the Atlanto-Mediterranean fauna. Many of them are found also in the Indo-Pacific and Red Sea regions. Thus summing up the Red Sea and cosmopolitan fishes in the eastern Mediterranean, there are 56 species (among 39 families) which constitute about 13% of all the ichthyofauna identified in this region;
Commission for the Scientific Exploration of the Mediterranean Sea):
exotic Fishes in the Mediterranean
exotic Decapods in the Mediterranean
exotic Molluscs in the Mediterranean
The origin of Immigrants
70% of the immigrant species belong to the wide Indo-Pacific region, ranging from northern Australia and southern Japan to the eastern coast of Africa and the Red Sea. A meager 17% is known to be of western Indian origin, 4% are circumtropical, and none of the immigrants belong to endemic Red Sea species.
The following factors might expose a limiting effect upon a northward migration:
2. Both corals and coral reef fishes are thermophilic and cannot survive the low winter temperatures of the Mediterranean Sea.
3. Lack of suitable habitat prevents coral reef fishes from entering the Suez Canal.
4. Coral fishes are mostly bottom spawners with non-existing or short-lived pelagic stages; therefore, they lack sufficient opportunities of drifting with currents to the northern end of the Suez Canal.
5. The shallow Gulf of Suez contains good trawling grounds of muddy bottom depths of 20-50m. The main stream of colonizers come from this demersal stock, being able to find a similar habitat in the Mediterranean.
|The conquest of new territories:
Lessepsian province: Due to the consistent influx of migrant species into the eastern basin, one can delimitate a biogeographic province within the Mediterranean. The limits of the province are, for the time being, the entrance to the Aegean Sea on hand and the Eastern coast of Sicily on the other (excluding the Adriatic). The boundaries of today will certainly expand or shrink according to the climatic evolution in our area.
The predominantly northward route of the immigration can be explained by the following factors:
Fig.8 The Lessepsian Province of the Mediterranean featuring selected species (based on ciesm, 2001).
|A colonizer species
is considered to be a species that has established a population in an area
previously unoccupied by that species. In studying the immigration of organisms
(mostly fishes) trough the Suez Canal, three zoo-ecological areas must
be taken into consideration:
Fig.9 Bardawil Lagoon
|The prevailing hydrographic conditions differ
in these areas, although the salinities and summer temperatures are to
some extend similar, they are the main abiotic factors influencing the
distribution of organisms over large zoo-geographical areas. Often they
also have a decisive influence on the ecological distribution of species
in various biotopes of an area.
The process of immigration is highly selective. Common species of the home areas are not necessarily successful immigrants in a new region. The adaptation of a species to a new area requires adjustment of its reproductive processes, especially with regard to the correct timing of spawning in order o ensure suitable physical and ecological conditions for the development and survival of the young stages.
Species for whom settlement in the canal proper has been impossible, owing to its adverse physical conditions, and whose pelagic life is too short for a "one jump" dispersal, have probably crossed the Canal using the Bitter Lake as a stepping stone. Having higher temperatures, lower turbidity and less gelatinous mud on its bottom, the lake, despite its extraordinary characteristics, may provide a more favorable environment for settlement and reproduction, than the canal proper.
Evidently, immigration is a continuos process, and over time the probability of suitable species of fish entering the Suez Canal and colonizing the new region increases. Time also plays an essential role in the biological processes of adaptation of the species to the modified conditions of life. More resistant species, endowed with greater plasticity of genetic characters, can form local "races" within a few generations by natural selection in the new environment. But first they need a firm foothold on the other side of the Canal, geographically close to the parental stock and in places where conditions are not drastically different from their normal habitat. Many of the species that successfully colonized the eastern Mediterranean are also dominant species on the trawling grounds of the Gulf of Suez.
High percentage of Red Sea fishes found in the hypersaline Bardawil Lagoon on the northern coast of Sinai indicates that it may serve as a further stepping stone in the immigration of Red Sea fishes into the Mediterranean, especially if we regard it as a part of the system of lakes and lagoons of the Isthmus of Suez.
It is a common feature of invading organisms that after an initial period of successful adaptation to the new and basically favorable environment, they may suddenly increase in number and spread to adjacent areas.
|Coexisting with indigenous species
The impact of Lessepsian migration in the Levant basin ecosystem has
Inter- and Intraspecific Competition:
The ecological process occasioned
by spatial and temporal overlap in the use of resources, is considered in important
force in organizing fish communities. Competition can mediate adaptations which
may include habitat selection and/or resource allocation or extinction; however,
it is not very common in natural communities, since resources (renewable or
non-renewable) usually provide more ecological niches than there are specialists
to fill them. In regards to ecological times, many resources remain under-exploited
even when species exist that are specialized on these resources. Yet, predation
and environmental harshness/variability are known to moderate the effect
of competition. Local extinction by means of competition are probably restricted
to habitats that occur in patches and in islands.
Trophic Characterization of migrant fish:
The feeding habits are divided into four categories, i.e. piscivorous, benthic invertebrates feeders, planktivorous, and herbivirous (both benthic and pelagic feeders were split equally between piscivorous and benthic invertebrate feeders). As can bee seen in the scan on the right, a strong similarity between the trophic composition of source ichthyofauna of the Red Sea and the recipient ichthyofauna of the Levant, the main difference being the expected paucity of herbivorous fishes in the temperate-derived Mediterranean. The relatively high proportion of planktivorous species among the migrants may not necessarily indicate a vulnerable and penetrable site for invasion, but rather the result of their transient behavior and their subsequent experience of a wide range of hydrographic conditions, enabling them to acclimate rapidly to new environments.
Fig.10 Food of Lessepsian fishes as compared to that of the source ichthyofauna (Red Sea) and recipient ichthyofauna (Mediterranean).
|In addition, these species from medium-
to large-sized schools, reducing the probability of random extinction, especially
during the founding stage.
It is commonly agreed upon that bathymetrical distribution of goatfishes (family
Mullidae) is the most important mechanism for maintaining coexistence amongst
migrant and native species (e.g. the migrating species Upeneus moluccensis
dominates the 10-30m range while U.pori as a demersal invader is most
common at 40-55m).
This bathymetrical distribution is indirectly supported by the fact that significantly
higher average Hg-concentration in bottom dwelling fish species closer to the
surface, which coincides with the heavier polluted depths by human activity.
In a study of a non coevolved assemblage of estaurine fishes in California, a
stable coexistence of recently encountered species suggests that in an ecosystem
vulnerable to invasion, coevolution plays a minor role in determining the
|Although no precise surveys about indigenous species and successful colonizers have been made until recently (compared with terrestrial ecology), it is generally assumed that the massive invasion of the Levant basin with Red Sea species do not seem to influence the abundance of native species (observations based primarily upon commercial fishing). So far, the greater number of immigrant species enrich the catch considerably. Although, recent piscivore immigrants reduced some of the flourishing populations of earlier migrants.||
Fig.11 El Bilaiyim
It seems that there are very few species which migrated from the Mediterranean to the Red Sea. This fact cannot be simply ascribed to the circumstance that the number of autochthonous species is so much higher in the Red Sea than in the Mediterranean. As the biota of the Red Sea is several times richer than that of the Mediterranean, these facts and figures indicate that the number of Lessepsian migrants could be three (or more) orders of magnitude higher than of the anti-Lessepsians.
As stated above, many factors facilitates
the transport of organisms through the canal in a predominantly northward direction.
The discovery of Mediterranean species Sphaerodiscus placenta (a sea star)
in the lagoon of El Bilaiyim, situated 180km south of the entrance to the Suez
Canal, is one of the few indisputable evidences of the anti-Lessepsian migration.
The invasion of the eastern Mediterranean by Red Sea species enables an investigations regarding the processes of migration and colonization. Although this invasion of the Mediterranean received scientific attention almost from its outset, until the last two decades most studies were of an inventory or monitoring nature.
In general the lack of knowledge about the faunistics of the Suez Canal system in the last three decades is very unfortunate.
With all the justified willingness to approach the aspects of strategies and the dynamics of Lessepsian Migration, in most of the cases the taxonomic information or even the taxonomic resolution is still insufficient. De facto potential migrants are "R-strategists" (quick populators in which the structure of organism focused on reproduction rather than complexity) involves a smooth gene-flow between con-specific populations on both sides of the canal. This likely possibility has to be taken into account when differentiating between Lessepsian Migrants and indigenous species; e.g. comparative advantages between different reproductive strategists can be established only between strictly congeneric species. Comparison between the autochthonous and migrant populations too, have to be performed by using all the parameters which can be mustered by both classical and modern taxonomy.
For a better understanding of Suez Canal immigration, additional taxonomic and biological investigations are required. Comparison of racial characteristics of immigrant organisms could help to clarify the question of the origin and relationship between the Red Sea and the Mediterranean populations. It is expected that in some cases, exchange of fauna and flora may have taken place before the opening of the Suez Canal as a result of the elevation of seal levels and undulations of the Isthmus during the Pleistocene.
Knowledge of the comparative life histories of the immigrant species in the two areas is essential for understanding the selective mechanisms controlling passage through the Suez Canal and evaluating extensive ecological changes that the invading species may produce in the new areas of their distribution.
Whether the success of the Lessepsian migrants is the result of their
occupation of an "unsaturated niche" or whether it is due to an efficient
competition with local species on the same resources is yet to be determined.
Regardless of the cause, it is anticipated that the number of Red Sea fish
species which will successfully colonize the (benthic) Levantine habitat
will increase in the coming years rather than flatten asymptotically.
It seems that the eastern Mediterranean ecosystem allows additional links
in the food web. This indicates that communities are loosely packed and
competition at present plays an important role (as compared to predation
and/or environmental harshness/variability) in shaping the eastern
Mediterranean communities and will continue to do so until equilibrium in terms
of resident species is reached (immigration balances extinction) and communities
become tightly packed with a regular spacing between species (refering to
|References:||Material kindly provided by Dr. Goldschmid|
Monaco 1990 - FRA
91904 Jerusalem, 1989 - ISR
1991 - UK
F.S. of the British Isles, 1989 - UK
Heidelberg, 1978 - FRG
Berlin 1994 - FRG
London 1992 - UK
Haifa, 1970 - ISR
Spektrum Verlag; Heidelberg - FRG
|Some Sites on the WWWeb:
Exotic Species in the Mediterranean Sea
The Mediterranean Crises
A Case Study of the Maltese Islands
Jellyfish Sting Newsletter
School of Biological Sciences
Israel Journal of Zoology
De Lesseps Lives
The Introductions Table
Role of the Mediterranean in the Global Change system