Necton: Floating And Swimming

‭NECTON: FLOATING AND SWIMMING‬ ‭The second large group of marine organisms leading pelagic life is the nekton. Even though they‬ ‭share the same habitat as plankton, nekton organisms have developed locomotion systems and‬ ‭mechanisms that make them able to oppose the motion of currents. In the nekton there are‬ ‭excellent swimmers capable of moving independently over long distances. While plankton is‬ ‭mainly made up of invertebrates, nekton is mainly made up of vertebrates. In practice, the only‬ ‭invertebrates of the nekton can be considered cephalopod molluscs, among the vertebrates there‬ ‭are bony fish and cartilaginous fish, then there is the component of marine tetrapods and among‬ ‭these there are reptiles such as turtles and sea snakes (typically pelagic ) or iguanas and‬ ‭crocodiles (presence restricted to the coastal environment), some birds such as penguins are‬ ‭linked to the coastal environment while other bird species are linked to the coastal environment‬ ‭only at the time of reproduction. Then there are many marine mammals which include Cetaceans‬ ‭such as Odontocetes and Mysticetes and pinniped carnivores such as seals, fur seals and‬ ‭walruses. Then there are the Sirens, therefore dugongs and manatees, large herbivores that live in‬ ‭coastal environments, sometimes also in estuarine environments. Also coastal are otters which‬ ‭are carnivores of the Mustelidae family.‬ ‭Among fish, we can identify different components that are distributed at different depths: there are fish that‬ ‭we will define as holo epipelagic as they spend their whole life in the epipelagic zone, have floating eggs and‬ ‭epipelagic larvae, and are particularly abundant in tropical and subtropical regions. Metro Epipelagic fish, on‬ ‭the other hand, spend their adult lives in the epipelagic zone but reproduce in coastal waters, a typical case‬ ‭of herring, sometimes in fresh waters as happens with salmon. Then we have the component of mesopelagic‬ ‭fish that live in the mesopelagic pelagic zone, among these there is an important component which is given‬ ‭by lantern fish. These fish migrate at night towards the more superficial layers of the epipelagic zone,‬ ‭essentially to feed, thus contributing to the formation of a phenomenon known as deep reflective layers or‬ ‭deep scattering‬ ‭layers (DSL). The last component is that of the abyssal fish which are those that populate the‬ ‭bathypelagic and abyssopelagic areas. Some examples of oleo epipelagic fish: tuna, marlin,‬ ‭sunfish, some sharks. Among the metro epipelagic fish there are the whale shark, which is mainly‬ ‭pelagic and approaches the coast during the reproductive period, the garfish which live in deep‬ ‭waters and form very dense schools, however in summer and early autumn they can often be‬ ‭found in large quantities in tens or hundreds of meters at most from the beach, sometimes even‬ ‭in the first sections of the mouth of a river or a few meters from the coastal shallows or near the‬ ‭tips of the cliffs or a little further offshore than the ends of the piers that delimit the ports. The‬ ‭area of diffusion of this fish is very wide and goes substantially from the area from the Atlantic to‬ ‭the North Sea, it is widespread throughout the Mediterranean and is common along all our‬ ‭coasts, the favorite food of the garfish is the juvenile Clupeidae like herring. Herring are also‬ ‭metro epipelagic fish that move from deeper waters to‬ ‭coastal waters for reproduction, thus attracting garfish. Dolphinfish is another‬ ‭fish that appears on the coasts only during the spawning period, genaerally in‬ ‭autumn. The salmon, on the other hand, definitely changes its environment and‬ ‭goes up the rivers to reproduce. Another important component of the nekton is‬ ‭given by cephalopods, therefore octopuses, cuttlefish, squid which have‬ ‭different species distributed overall throughout the water column. Two particular‬ ‭species among cephalopods are equipped with an external shell like the‬ ‭Nautilus‬‭And the Argonauts which is essentially similar to an octopus, is an‬ ‭open-sea species and swims mostly at surface level. A particular feature of the‬ ‭Argonauts is that of having a particularly accentuated sexual dimorphism: the‬ ‭males are a few centimeters in size and have no shell unlike the females who‬ ‭instead have the shell and can even reach about thirty centimeters in length.‬ ‭Then there is a component of the nekton that we can more correctly define as nektobenthos, it‬ ‭includes species that move by swimming near the seabed and are above all species that belong‬ ‭to families such as the Gadidae, Sparidae and Serranidae. These nektobenthic forms should not‬ ‭be confused with other species which are instead typically benthic, therefore they have much‬ ‭closer and more constant relationships with the bottom. Benthic fish are therefore the rays, many‬ ‭species of pleuronectidae (therefore sole, turbot, flounder, halibut), there are scorpenids (such as‬ ‭scorpion fish) and mullins (such as mullet). All these nektobenthic and benthic forms can be‬ ‭indicated with the name of demersal species, so when this definition is used we mean all the‬ ‭species that have more or less close relationships with corals. Then there are some species that‬ ‭have particular behaviors, they are nektobenthic during the day and then rise up the column‬ ‭during the night and become nectonic. We talked about adaptations made by plankton to deal‬ ‭with life in the water column.‬ ‭ ven for the nekton we can make a similar series of considerations, even for all the nektonic‬ E ‭forms there is the problem of floating. Floating and maintaining one's position in the water‬ ‭column are also fundamental aspects in nektonic life. Another aspect that requires adequate‬ ‭adaptations concerns the movement which must be efficient and also allow high speeds. Also‬ ‭for nekton, buoyancy means reduction of the average body density in order to reach conditions‬ ‭as similar as possible to the density of seawater. To achieve this goal, some areas of the body‬ ‭(tissues and organs) must have a very low density in order to counterbalance the high density of‬ ‭most tissues.‬ ‭The majority of fish have a swim bladder which constitutes 5-10% of the volume of the fish, the‬ ‭gas content in the swim bladder can be regulated and this allows the fish not only to float and‬ ‭therefore to have the appropriate flotation conditions but also to define the depth at which to stay‬ ‭in the water column according to different needs. We then have other types of adaptations which‬ ‭concern other components of the nekton, for example cavities filled with air such as lungs or‬ ‭accessory air sacs which intervene in allowing pulmonary-breathing vertebrates to float. These‬ ‭animals are able to regulate the floating depth by varying the amount of air contained in their‬ ‭lungs. As already seen in plankton, also for nektonic animals there is the possibility of replacing‬ ‭heavier ions with lighter ions in their body fluids. This solution is actually adopted only in the‬ ‭squid, where sodium ions are replaced by ammonium ions. This system to avoid sinking is‬ ‭necessarily rare in the nekton because it is very difficult to have a quantity of body fluids with a‬ ‭prevalence of ammonium ions sufficient to resolve the problem. buoyancy problem. The‬ ‭accumulation of large volumes of liquids would be at the expense of hydrodynamics which is‬ ‭another essential requirement for most nektonic organisms. Another system aimed at improving‬ ‭buoyancy is given by the presence of significant quantities of fat, fat in fact has a lower density‬ ‭than water, it can be accumulated in the tissues in various parts of the body, in the muscles in‬ ‭body cavities, in some internal organs for example the liver of sharks and then in the‬ ‭subcutaneous tissue of mammals.‬ ‭This slide shows some of these examples, from the presence of the swim bladder in physostomous fish and‬ ‭physoclistous fish, to that of air sacs and the presence of fat. In particular, the importance of subcutaneous‬ ‭fat must be underlined, the important role played by this subcutaneous adipose tissue, the so-called‬ ‭blubber, which is particularly developed in cetaceans and constitutes not only a system for improving‬ ‭flotation but is also important in guaranteeing conditions of thermal insulation. Those just described are‬ ‭essentially systems that we will define as static, designed to maintain and improve buoyancy, systems that‬ ‭do not imply any active regulation by the organisms.‬ ‭However, some nektonic organisms also exploit hydrodynamic mechanisms to float better during‬ ‭swimming; the most common solutions in this sense are represented by the presence of pectoral‬ ‭fins and are configured just like surfaces that rise towards the car in the anterior region of the body‬ ‭and then the other contribution to buoyancy is given by the presence of a heterocercal tail (typical‬ ‭for example of sharks). In general there is an evolutionary trend that sees the transition from‬ ‭hydrodynamic adaptations, which allow you to create a lift of the body, to static or passive‬ ‭systems to obtain flotation. Static systems, therefore not continuously managed by the animal,‬ ‭involve a lower expenditure of energy compared to having to continuously move to keep the body‬ ‭floating in the water column, so in this sense passive systems (static systems) represent an‬ ‭evolutionary choice winning.‬ ‭The adaptations then made to improve the movement of the body in water or rather the most‬ ‭frequent adaptations essentially concern two types: adaptations necessary to create a propulsion‬ ‭force for the movement in water and adaptations that tend to reduce the resistance in the‬ ‭movement of the body in water. Two aspects: how to create a repulsive force and how to reduce‬ ‭resistance in movement in water. The most common approach to improving movement potential‬ ‭is to assume an undulating movement of the body and fins. The undulating movement of the‬ ‭body is achieved thanks to alternating contractions of the muscles first on one side and then on‬ ‭the other, the speed of a fish in the water therefore depends on the frequency with which these‬ ‭waves of muscular contraction follow one another along the body and then obviously also from‬ ‭other aspects relating to body shape.‬