Lessepsian migration

The Suez Canal, across which marine species migrate in the so-called Lessepsian migration

The Lessepsian migration (also called Erythrean invasion) is the migration of marine species across the Suez Canal, usually from the Red Sea to the Mediterranean Sea, and more rarely in the opposite direction. When the canal was completed in 1869, fish, crustaceans, mollusks, and other marine animals and plants were exposed to an artificial passage between the two naturally separate bodies of water, and cross-contamination was made possible between formerly isolated ecosystems. The phenomenon is still occurring today. It is named after Ferdinand de Lesseps, the French diplomat in charge of the canal's construction.

The migration of invasive species through the Suez Canal from the Indo-Pacific region has been facilitated by many factors, both abiotic and anthropogenic, and presents significant implications for the ecological health and economic stability of the contaminated areas; of particular concern is the fisheries industry in the Eastern Mediterranean. Despite these threats, the phenomenon has allowed scientists to study an invasive event on a large scale in a short period of time, which usually takes hundreds of years in natural conditions.

In a wider context, the term Lessepsian migration is also used to describe any animal migration facilitated by man-made structures, i.e. one which would not have occurred had it not been for the presence of an artificial structure.

Background[edit]

The opening of the Suez Canal created the first saltwater passage between the Mediterranean Sea and the Red Sea. Constructed in 1869 to provide a more direct trade route from Europe to India and the Far East, the canal is 162.5 km (101.0 mi) long, with a depth of 10–15 m (33–49 ft) and a width varying between 200 and 300 m (660 and 980 ft).^[1]

Construction of the Suez canal

Because the surface of the Red Sea is slightly higher in elevation than the Eastern Mediterranean, the canal serves as a tidal strait by which Red Sea water pours into the Mediterranean. The Bitter Lakes, which are natural hypersaline lakes that form part of the canal, blocked the migration of Red Sea species into the Mediterranean for many decades, but as the salinity of the lakes gradually equalized with that of the Red Sea, the barrier to migration was removed, and plants and animals from the Red Sea have begun to colonize the eastern Mediterranean.^[2] The Red Sea, an extension of the Indian Ocean, is generally saltier and less nutrient-rich than the Mediterranean, an extension of the Atlantic Ocean, so Red Sea species, able to tolerate harsh environments, have advantages over Atlantic species in the conditions of the Eastern Mediterranean. Accordingly, most migrations between the two bodies of water are invasions of Red Sea species into the Mediterranean, and relatively few migrations occur in the opposite direction. The construction of the Aswan High Dam across the Nile River in the 1960s reduced the inflow of fresh water and nutrient-rich silt from the Nile into the eastern Mediterranean, making conditions in the eastern Mediterranean even more like those of the Red Sea, thereby increasing the impact of the invasions and facilitating the occurrence of new ones.^[2]

The Red Sea is a profusely abundant tropical marine environment sharing species in common with the eastern Indo-Pacific region, while the Mediterranean is a temperate sea with much lower productivity; the two ecosystems are extremely different in terms of structure and ecology.^[1] The Suez Canal quickly became the main pathway for the introduction of invasive species into the Eastern Mediterranean, having zoogeographic and ecological consequences far beyond what the designers could foresee. The Lessepsian migration includes hundreds of Red Sea and Indo-Pacific species that have colonized and established themselves in the Eastern Mediterranean system, causing biogeographic changes comparable to that of continental drift.^[3]

To this day, about 300 species native to the Red Sea have been identified in the Mediterranean Sea, and probably others are as yet unidentified. In the late 20th and early 21st centuries, the Egyptian government announced its intentions to deepen and widen the canal, which raised concerns from marine biologists, fearing this would expand the invasion of Red Sea species into the Mediterranean and facilitate the crossing of the canal for additional species.^[4]

Ecological impacts[edit]

Out-competition of natives[edit]

Fistularia commersonii, a Lessepsian migrant^[5]

Native Argyrosomus regius vs. invasive Scomberomorus commersoni[edit]

A wide-ranging species in the eastern Atlantic and Mediterranean, the meagre Argyrosomus regius is a species indigenous to the Eastern Mediterranean which was one of the most common commercial fish in Israel. However, this species has since disappeared from local catches, while the narrow-barred Spanish mackerel Scomberomorus commersoni, a known Lessepsian migrant, has dramatically increased in population. Studies have been done on this occurrence, and due to similar life histories and diets, this may be an example of an invasive migrant outcompeting a native species and occupying its niche.^[6]

Native Melicertus kerathurus vs. invasive prawns[edit]

Eight species of invasive prawns from the Erythraean Sea have been recorded in the Eastern Mediterranean. These prawns are considered highly prized in Levantine fisheries, and compose most of the prawn catch off the Mediterranean coast of Egypt, being 6% of total Egyptian landings. However, this high abundance of invasive prawns has led to the decline of a native penaeid prawn, Melicertus kerathurus, which supported a commercial Israeli fishery throughout the 1950s. Due to out-competition and its habitat being overrun by these migrants, this native species has since disappeared and had detrimental impacts on the commercial fishery.^[7]

Parasitic invaders[edit]

The invasion of new Red Sea species into the Mediterranean has also facilitated the invasion of their associated parasites, for example the copepod Eudactylera aspera, which was found on a spinner shark, Carcharhinus brevipinna, taken off the coast of Tunisia. The copepod had originally been described from specimens taken from C. brevipinna off Madagascar and its finding in the Mediterranean has arguably confirmed the previously disputed status of C. brevipinna as a Lessepsian migrant. In addition, parasites originating in the Red Sea have shown an ability to use related native Mediterranean fish species as alternative hosts; e.g. the copepod Nipergasilus bora was known to parasitize the grey mullets Mugil cephalus and Planiliza carinata in the Red Sea, both taxa having been recorded as Lessepsian migrants, and was subsequently found parasitizing the native Mediterranean mullets Chelon aurata and Chelon labrosus.^[8]

On the contrary, the invasion of these parasites may have the effect of reducing the competitive advantages that Red Sea invaders have in the Mediterranean. For example, the Indo-Pacific swimming crab Charybdis longicollis was first recorded in the Mediterranean in the mid-1950s and became dominant in silty and sandy substrates off the coast of Israel, making up to 70% of the total biomass in these habitats. Until 1992, none of the specimens collected was infected with the parasite Heterosaccus dollfusi, but in that year, a few infected crabs were collected. The parasite is a barnacle which desexes its host. Within three years, the parasite had spread to southern Turkey and 77% of the crabs collected in Haifa Bay were infected. This rapid increase and high infection rate is attributed to the extremely high population density of the host and the year-round reproduction of the parasite. One effect of this was that the population of the Mediterranean native swimming crab Liocarcinus vernalis recovered somewhat.^[9]

Species displacements[edit]

The goldband goatfish, Upeneus moluccensis, was first recorded in the Eastern Mediterranean in the 1930s and has since established an abundant population. Following the warm winter of 1954–1955, it increased to 83% of the Israeli catch, replacing the native red mullet, which also affected the Egyptian fishery, being 3% of their total landings.^[10] The high water temperatures of this unusually warm winter may have resulted in the poor survival of red mullet juveniles, which may have allowed the goatfish population to expand into the opened niche.^[11] Native mullet have since been displaced into deeper, cooler waters, where Lessepsian migrants consist of only 20% of the catch, whereas in shallower, warmer waters, this invasive species takes up a staggering 87% of the catch.^[11] From these data, the Lessepsian migrants apparently have not adapted to the more temperate environment of the deeper areas of the basin, but have established dominant populations in the habitats most similar to the tropical sea habitats from which they came.

Food web phase shift[edit]

The marbled spinefoot (Siganus rivulatus) and dusky spinefoot (Siganus luridus), both indigenous Red Sea rabbitfish, were first recorded off the coast of Israel in 1924. In only a few decades, these schooling, herbivorous fish were able to settle in a range of habitats forming abundant populations, to the extent that George and Athanassiou, in a paper published in 1967, reported: "The millions of young abound over rocky outcropping grazing on the relatively abundant early summer algal cover".^[12] By 2004, a study on these species found that they comprise 80% of the abundance of herbivorous fish in the shallow coastal sites of Lebanon.^[13] They have been able to create marked phase shifts within the food web on multiple levels. Prior to the arrival of these Lessepsian migrants, the herbivores filled a small ecological role within the Eastern Mediterraneansystem. Therefore, with such a high influx of herbivorous species in a small period of time, this phenomenon has normalised the food web, increasing the rate at which algae are consumed and serving as a major prey item for large predators.^[13] This increased rate of grazing has, in turn, proliferated the settlement and colonisation of a nonindigenous species of mussel from the Indo-Pacific that is now able to attach to rocky substrate that was once covered with algae. This mussel, which has a thicker shell than that of the native mussel, has created a change in predation patterns, as well, since they are more difficult to consume.^[13] Not only are these Red Sea migrants having a huge impact on this ecosystem, they also are affecting fisheries, as well, by out-competing native fish of high commercial value, such as the seabream Boops boops.^[13]

Anti-Lessepsian migration[edit]

European seabass: one of the few anti-Lessepsian migrants

Only a comparatively few species have colonised the Red Sea from the Mediterranean, and these are referred to as anti-Lessepsian migrants. As the predominant flow of the canal is from south to north, this acts against the southward movement of Mediterranean species, and as stated above, the Red Sea has higher salinity, fewer nutrients, and a much more diverse biota than the Eastern Mediterranean. Some of the anti-Lessepsian migrants such as the sea star Sphaerodiscus placenta are found only in specialised habitats such as the lagoon of El Bilaiyim, which lies 180 km (110 mi) south of the southern entrance to the Suez Canal, but is much more saline than the surrounding waters of the Gulf of Suez.^[2]

The sea slug Chelidonura fulvipunctata was originally described from waters around Japan and is widespread in the eastern Indian Ocean and western Pacific. It was first identified in the Mediterranean in 1961, and was seen in the Red Sea in 2005, most likely as a result of anti-Lessepsian migration.^[14] In addition, a survey of polychaete worms in the southern Suez Canal found six species that were regarded as anti-Lessepsian migrants.^[15] Among the fish species that have been confirmed as anti-Lessepsian migrants are Solea aegyptiaca, Mediterranean moray Muraena helena, the rock goby (Gobius paganellus),^[16] comber Serranus cabrilla, European seabass Dicentrarchus labrax, and spotted seabass Dicentrarchus punctatus.^[17]

Chelidonura fulvipunctata

Factors facilitating Lessepsian migrant colonization and expansion[edit]

Anthropogenic stressors: Aswan Dam[edit]

Impact of Lessepsian migrants on system may be heavier due to a major anthropogenic factor: the construction of the Aswan Dam. Before construction, the Nile River was able to deeply influence the marine environment of the Eastern Mediterranean, discharging high tonnage of nutrient-rich water. This resulted in a high abundance of phytoplankton in the delta which had a beneficial influence on the productivity in the surrounding sea, and attracted large schools of sardines, resulting in a highly lucrative commercial fishery. After the dam’s completion in 1964, this productivity diminished, resulting in the cessation of nutrients in the Mediterranean, leading to a sharp decrease in fish populations, namely sardines, which ultimately lead to the collapse of the sardine fishery. As a result, the Egyptian purse-seine fishing industry today takes only 10% of the pre-dam catch, which may have been due to the influence of dispersion of the Red Sea invasives. The freshwater discharge of the Nile could have been a natural barrier for some of the migrants in their movement through the Eastern Mediterranean.

Natural stressors: climate change[edit]

With climate change and the warming of seawater temperature, the thermophilic Lessepsian migrants may find it easier to reproduce, grow, and survive and give them a distinct advantage over native temperate Mediterranean taxa. Both processes, global warming and the influx of Lessepsian migrants, may impact the already teetering fisheries by displacing commercially important native species, causing a phase shift in coastal ecosystems and changing seascape patterns. Furthermore, deepening of the warm surface layer is causing massive mortalities of organisms that do not tolerate high temperatures. Through various studies, species have been shown to be now restricted to deeper levels and thrive for shorter periods than in the past. Climate change is also one of the reasons for another stressor on this system, the decline of natural barriers that were once in place to prevent many Red Sea natives to migrate to the Mediterranean. Due to global warming, the Eastern Mediterranean is experiencing an increase in temperature and salinity, which is decreasing the hydrological barrier between the two seas, favoring the migrants from the tropic Indo-Pacific which have a warm-water affinity and causing mortality in the temperate Eastern Mediterranean natives. The Bitter Lakes created a natural salinity barrier within the Suez Canal due to their high deposits of salt, preventing many species from migrating. However, due to the freshening of these lakes, this natural barrier is weakening, allowing a higher mitigation of invasive species.

Other examples[edit]

North America[edit]

The sea lamprey reached Lake Ontario from the Atlantic Ocean through shipping canals and was recorded for the first time in Lake Ontario in the 1830s, but Niagara Falls was a barrier to their further spread. The deepening of the Welland Canal in 1919 allowed the sea lamprey to bypass the barrier created by the falls, and by 1938, sea lampreys had been recorded in all of the Great Lakes.^[18]

The alewife (Alosa pseudoharengus), a species of shad from the Western Atlantic, also invaded the Great Lakes by using the Welland Canal to bypass Niagara Falls. They colonized the Great Lakes and became abundant mostly in Lake Huron and Lake Michigan, reaching their peak abundance by the 1950s and 1980s.^[19]

Europe[edit]

The white-eye bream (Ballerus sapa) has invaded the Vistula River basin by migrating along the Dnieper–Bug Canal in Belarus, which connects the Vistula drainage basin with that of the Dnieper River.^[20]

Panama[edit]

A small number of species have used the Panama Canal to move from the Atlantic Ocean to the Pacific Ocean, and vice versa. Six species of Atlantic fish were recorded on the Pacific side of the canal. The Atlantic fish included Hypleurochilus aequipinnis, Barbulifer ceuthoecus, Oostethus lineatus, Lophogobius cyprinoides and Omobranchus punctatus, while among the Pacific species moving to the Atlantic included Gnathanodon speciosus and Lupinoblennius dispar. The Gatun Lake's freshawater environment proving a barrier to the interchange of marine species.^[21]

See also[edit]

• List of Lessepsian migrants

References[edit]