Introduction
Species whose entire lifecycle takes place in lagoons and estuaries are often called "sedentary fish". These fish show some peculiarities as much on a morphoanatomical, genetic and bio-ecological level as in their behavior. Here, we will sketch the broad outlines of this guild's characteristics before giving detailed descriptions species by species.
Sedentary ichthyofauna in lagoons and estuaries, which differ depending on the geographical location of their habitat, are characterized by low species diversity (a total of 25 species, excluding Lessepsian species). This relative paucity of these species results from the values of several factors, including temperature (-2°C to +35°C) and salinity (0 to 70%), which vary over space and time, so much so that lagoon waters are considered "extreme environments". As a result, only species that show some euryvalence have been able to adapt and establish themselves in these spaces.
Based on 97 lagoon fish surveys carried out in 45 lagoons, the most common species found are Atherinidae (80.4%), Syngnathidae (67%), Blenniidae and Gobiidae (62.8% each).
Overall, the best represented families include Gobiidae (at least 11 species) and Syngnathidae (at least ten species). These are followed by Cyprinodontidae (two species), Atherinidae (one or two species), Blenniidae (one species), Poeciliidae (one species), Labridae (one species) and Gasterosteidae (one species).
Disparity between environments
Some disparity is found between large, deep lagoons and laminar lagoons, with the latter having a relatively low species diversity. The geographical distribution of these species also shows some peculiarities. Iberian lagoons and estuaries are home to endemic species such as Aphanius iberus and Valencia hispanica; those in the Gulf of Lion are glacial relicts such as Pomatoschistus microps, P. minutus and perhaps P. pictus. The brackish waters along the Adriatic region possess Knipowitschia panizzae, Padogobius sp. and Pomatoschistus canestrinii. The lagoons of the eastern Mediterranean are enriched by Lessepsian immigrants. Furthermore, the lagoons of both Bardawil (Egypt) and El Bibane (Tunisia) contain Gobiidae (Coryogobius (Monishia) ocheticus, Papillogobius melanobranchus, Silhouettea aegyptia) and Atherinidae (Atherinomorus forskalii (A. lacunosus)). Finally, Pomatoschistus tortonesei is only present in Marsala (Sicily), Farwah (Libya) and six other Tunisian lagoons. We note that the most common species are Atherina lagunae (boyeri) (80.4%), Syngnathus abaster (67%), Salaria pavo and Gobius niger (62.8% each).
In a single geoclimatic sector, the diversity of sedentary fish in lagoons and estuaries is linked strongly to the history of navigation routes that link them to the sea; these may have been long term or temporary (graus, inlets, intermittent channels) and show dimensions (length, section, depth) that lead, via a "threshold effect" and "channel effect", to the selection of species with "lagoon affinities" present on the shore. To these factors can be added the significance of the currents crossing the channel, lagoon and estuary inlets and the quality of the environment on arrival: surface and depth of the lagoon and nature and diversity of the habitat.
To interpret the current ichthyic lagoon landscape, in particular some of its specifics in terms of population, biogeographers and geneticists need to know the age of the lagoon. Indeed, at a given geographical point, this landscape can particularly differ depending on whether it is tectonic or indeed sedimentary in origin. In general, tectonic lagoons (e.g. Berre, Diane, Urbino) are older than sedimentary lagoons (the lagoons of Languedoc). Other events such as the Mediterranean Messinian crisis (about -5.5 Ma) should be considered, in which despite repeated phases of drying, long-term estuary systems were used as refuges by some small coastal sea fish. The last Würm glaciation and the Flanders marine transgression (10,000 to 17,000 years ago) may also have played a role in the distribution and diversification of laguno-estuarian fish, not to forget some more local hydrodynamic events. In the majority of cases, Mediterranean lagoons are relatively recent. Those of Venice and Tunis appeared at least 5,000 years ago, but others are more recent. For example, the lagoons of Languedoc (France), which are of potamothalassic origin, formed essentially from the 14th Century. They then underwent notable natural or anthropic fragmentation. From the end of the 20th Century, they could be considered "domesticated" and "frozen" in the coastal landscape.
The make-up of sedentary populations in lagoons and estuaries significantly varies in space and time. They can be locally wiped out by dystrophic, anoxic and toxic (H2S, CH4) crises; repopulation occurs via marine or river populations, sometimes from contiguous lagoons. In sedimentary lagoons, a rupture of the lido that isolates them from the sea following sea storms, or the overflow of fresh water following heavy precipitation, can upset the entire local lagoon ecosystem by a sudden influx of sea water, causing a serious run-off of lagoon bottoms and organisms. The duration of this marinization depends on the speed at which the lido is reconstructed and therefore on the arrival of sediment. However, it is often difficult to date the lagoon age of this populating process.
Similar morphologies and behaviors
Sedentary fish display shared morpho-anatomical traits: all of them are small (maximum 15 cm TL, in exceptional cases 20 cm) and display accentuated sexual dimorphism and dichromatism, except the Atherinidae. They have a short lifespan (from a few months to a maximum of 4-5 years), and their growth is so rapid that 80% of their maximum size is reached before sexual maturity. These fish are also remarkable in their reproductive behavior. In fact, the restrictive hydroclimatic conditions that we mentioned previously (temperature, salinity, anoxia, turbidity, etc.) tend to limit their reproductive success. The long reproductive period (sometimes 7 to 8 months) of these species and the "fragmentation of spawning" in females during this period enable them to overcome the negative effects of passing hydroclimatic crises. None of these fish produce planktonic oocytes (eggs). Females lay relatively fat oocytes (with a diameter equal to or greater than 1 mm), which focus on relatively developed "parental care". From this point of view, we can recognize several guilds:
- 1) "attentive layers", which limit the care given to the eggs (1 to 3 mm in diameter) by laying them in areas rich in vegetation (genera Aphanius, Valencia) or attaching them to upright algae and phanerogams (genera Atherina, Atherinomorus), before abandoning them. In both cases, the eggs are isolated from the bed, which is often sandy-muddy, or even muddy and putrid, and develop in waters rich in photosynthetic oxygen which helps in their development. The larvae hatch subplanktonically or planktonically;
- 2) those layers that practice "parental care" at least until the eggs hatch. In this case, we distinguish between species that practice:
- - "external gestation" or nesting species (Gobiidae: Gobius sp. and Pomatoschistus sp.; Blenniidae: Salaria pavo; Labridae: Symphodus cinerus; Gasterosteidae: Gasterosteus aculeatus);
- - "outer body gestation" (Syngnathidae: Nerophis);
- - "internal gestation" or viviparity (Poeciliidae: Gambusia holbrooki; Syngnathidae: Syngnathus and Hippocampus).
One of the particularities shared by the majority of laguno-sedentary fish is to exert control over the management of their gametes and eggs. Thus, a female goby is able to distribute her mature oocytes over several nests depending on their appearance - size, general state, filling rate (nests without eggs, just like overcrowded nests, are not very attractive) - as well as on the appearance of the male owner. As a precaution, she "doesn't put all her eggs in the same basket". The same is true for the females of the genus Syngnathus. In fact, genetic studies have shown that a male's incubator pocket may contain eggs from several females. In this case, it could be the male that limits, for one reason or another (e.g. undesired female), by interruption of coitus, the number of oocytes that a female can transmit to the male. In Poeciliidae, females are able to eliminate spermatozoids from unwanted couplings, especially when they are subject to sexual attacks. Males with nests are also able to increase or decrease the volume of their ejaculate depending on competition between sperm caused by the presence of mature males without nests (called sneakers). These sneakers seek to fertilize oocytes left by a female in the nest of a male owner. Moreover, in gobies and perhaps in blennies, the females fix their oocytes and the males fix their spermatoza to the walls of the nest "wrapped" in a mucus ribbon called a "sperm trail" (which prevents losses caused by dispersion of a current) and ensure that they are protected by guardian males; these factors control the fate of their spermatozoids and oocytes. The spermatozoids are gradually freed from this as the females spawn. Some losses are caused...
