BIODIVERSITY AND INVASION – HOW RESILIENT IS THE LEVANT SEA?
By Bella S. Galil
Israel Oceanographic and Limnological Research
P.O. Box 8030, Haifa 31080, Israel
In 1977, a single specimen of an unknown species of jellyfish was collected off the coast of Israel. Preserved but unidentified, it remained unnoticed until the mid-1980s when large aggregations of jellyfish along the Israel coast prompted efforts to study the phenomenon. The jellyfish was identified as a new species, Rhopilma nomadica, originating in the Red Sea. The largest specimen of the nomadic jellyfish, R. nomadica, measured 80 cm. across its umbrella, though most specimens surveyed measured between 20-60 cm. Mass swarming of R. nomadica occurs in the summer when marine and coastal surveys point to a "jellyfish belt" at a distance of 1-5 kms offshore where the maximal density was estimated at 25 specimens per cubic meter. The mass aggregations of the jellyfish exert a significant impact on fisheries, coastal installations and tourism. Similar phenomena were reported from Egypt, Lebanon, Turkey and Cyprus.
The presence of this jellyfish served as an impetus to reappraisal of some long established views concerning the migration of Red Sea biota into the Mediterranean and led to re-evaluation of its percepts with regard to population dynamic patterns, competition between migrant and indigenous species and its course.
The construction of a sea-level waterway between the Eastern Mediterranean and the northernmost tip of the Gulf of Suez linked the Atlantic-Mediterranean marine province with the Indo-Pacific. The deep-water communication between the Mediterranean and the Indian Ocean was breached during the early Miocene. Early in the Pliocene, the isthmus of Suez was elevated and the two seas separated completely. Thus, the present day Suez Canal is at once an iteration of an earlier connection and a repeal of a palaeobiogeographic process.
In 1869, the Suez Canal was inaugurated, inadvertedly launching a remarkable faunal movement. Despite physical and hydrological impediments, hundreds of Red Sea species traversed the canal and settled in the Mediterranean.
The population dynamics pattern of the Red-Med migrants was considered to be one of unremitting expansion. Earlier researchers believed that there is evidence that there is no regress in the building up of populations of the new immigrants and no known examples of reversals, i.e., of a disappearance or radical depletion of the lessepsian population. Certainly, hundreds of Red-Med migrants became established and founded thriving populations along the Levantine coasts. Some species are even of economic importance and being exploited commercially. Examination of fishery statistics underscores the migrants’ prominence in the local communities, migrant fish constituting a third of the total trawl catches off the Israeli Mediterranean coast and immigrant penaeid prawns making up most of the shrimp catches along both the Egyptian and Israeli coasts. A massive "build up" of populations is also found in the swimming crab, Charybdis longicollis. First recorded from the Mediterranean in the mid-1950s, it has since dominated the macrobenthic fauna on silty sand bottoms, forming up to 70% of the biomass at places. A more recent case in point is that of Strombus decorus persicus, first discovered in Mersin Bay, Turkey in 1978 and then in quick succession off southern Turkey, Rhodes, Cyprus and Israel. The mass appearance of the migrant jellyfish, unlike the seasonal proliferation or cyclic fluctuation of the indigenous Mediterranean jellyfish, is again the exponential phase of an intruder. However, among the colonists there are several well-documented cases of "explosive growth" followed by recession. In the late 1940s, the migrant goldband goatfish made up 10-15% of total mullid catches. Following the exceptionally warm winter of 1954-1955, its percentage in the catch increased to 83%. Its share has since been reduced to 30% of the catch. Following the same winter, the brushtooth lizardfish became a commercially important fish and its proportion in trawl fisheries catches rose to 20% of the volume in the late 1950s. The population then diminished and catches stabilized at about 5% of the total trawl catch. Pony fish was extremely common on trawling grounds in depths of 20-100 m in the early fifties, but its populations were decimated during the sixties. A more dramatic case is that of the gastropod Rhinoclavis kochi. It was first reported in Haifa Bay in the mid-1960s and has since spread rapidly and became one of the dominant species between 20-60 m, peaking in the late 1970s. Collections made a decade later brought up mostly empty shells.
The precipitate merger of disjunct faunas may ensue in competition for different resources or direct interference between the newcomers and the autochthonous species, the latter sometimes is outcompeted in a part of their habitat space. Indeed, the disappearance of the indigenous sea star, Asterina gibbosa, from the Israel coast paralleled the rapid advent of its Red Sea congener A. wega. Only rarely is sufficient data at hand regarding the life histories and ecological relationships of both indigenous and Red Sea species to determine which are the competing species. There are documented instances where we have evidence of a drastic change of abundance that can be attributed to the new competition. A native penaeid prawn, Penaeus kerathurus, was very commonly caught by trawlers on an Israeli coastal shelf, especially on a sandy or sandy-mud bottom and supported a commercial fishery throughout the 1950s. It has since become nearly extinct and its habitat overrun by the migrant penaeid prawns. As the migration proceeds, we might perceive replacement of one immigrant by anotherthe southern rough shrimp is now found in smaller quantities where it is replaced with the similarly sized Egyptian shrimp.
A less drastic competition might take the form of depth adjustment. Such competitive displacement probably occurs between the local red snapping shrimp and the intruding congener Alpheus rapacida; the former appears mainly between 35-145 m and the latter between 15-50 m. The local red mullet and the native hake were both displaced into deeper, cooler water by their respective Red Sea competitors: the goldbank goatfish and the toothbrush lizardfish. Depth-shifting among migrants could also be ascribed to the physical environmental characteristics. Red Sea species that are rock and rubble dwellers are confined in the Mediterranean to infralittoral regions for want of hard substrate in deeper waters. Among Red Sea species inhabiting soft bottoms, some occupy a depth range similar to that in the Red Sea while others seem to shift their range to water 20-50 m deep. As temperature is considered to be the most important single factor for migrants, it is assumed that the winter cooling of the shallow waters is the cause for the shifting.
Most migrants concentrate off the southern Levant coast. A considerable number reaches the southern Turkish coast and Cyprus, whereas only a few species have been recorded as west as Malta, Tunisia and Sicily.
That the Levantine fauna evolves under migration pressure influence is evident; the number of Red-Med migrants grow continuously and modification of the composition and structure of the Levantine biota is already underway. The presence of migrants augments the tropical affinities which, quite noticeably, distinguish the Levantine fauna. The infusion of Indo-Pacific elements and the concomitant change in the indigenous fauna enhance the tropical character of the Levant compared with the more temperate character of the sea as a whole, causing disruption of the zoogeographic unity of the Mediterranean fauna.
The sustainable exploitation of living resources and conservation of biological diversity is limited by our failure to comprehend the origins and ecological function of biodiversity. In order to understand the connection between biodiversity and the function of ecosystems, their productivity, stability and resilience, we must know how food webs and population control mechanisms react to changes in species diversity and loss or introduction of keystone species. The invasion of red Sea biota and its establishment in the Levant Sea presents us with a unique opportunity to examine these elements of ecological dynamics and wonder how much longer the Levant biota will remain precariously balanced between migrant and indigenous species.