1.9 Adaptation of marine organisms to live in the sea

Biology adaptation “is any alteration in the structure or function of an organism or any of its parts that results from natural selection and by which the organism becomes better fitted to survive and multiply in its environment”.

For a living organism, adaptation to the aquatic life involves changes that affect not only its shape but also its internal physiology to solve the physical and chemical problems affecting this particular environment, although this proves to be a little more stable than the air environment.

One of the problems to be faced in the water is sinking and to face it we try to increase the frictional forces or with the diminution of the dimensions as in most of the plankton or increasing the size of the body as in the whales. Other strategies are body shapes that help to remain in suspension without sinking like the jellyfish’s parachute shape or lighten the body with mechanisms like gas production; for example the Portuguese man of war (Physalia physalis), has a floating bag of air. In other cases lightening is ensured by various forms of fat in the body: drops in phytoplankton, oily liver in sharks, skin fat in marine mammals.

Each marine species adapts to the various habitats to reach an equilibrium, trying to solve problems that may compromise its survival such as the regulation of temperature, salinity, pressure, the provision of oxygen for respiration, food, locomotion, how to defend oneself and how to perpetuate the population.

Many are the marine organisms that carry out their lives in the absence of light. In response to this they have developed various types of adaptation, among these the main one is the production of light through the bioluminescence. Bioluminescence is a phenomenon present among algae, such as the Dinoflagellate of the genus Noctiluca (FIG1_SES1.9), as well as in animals for example in the jellyfish Pelagia noctiluca (FIG2_SES1.9).

FIG1_SES1.9 Noctiluca scintillans.
By © Hans Hillewaert, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=10711494
FIG2_SES1.9 Pelagia noctiluca.
By © Hans Hillewaert, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=3966390

Most organisms do not tolerate large variations in salinity (they are called stenohaline) and only a few can survive when it changes (euryhaline), as can happen in coastal lagoons. Very few are the species able to switch from fresh to salt water or vice versa during their life. Salmon and eel are the most famous examples.

The evolutionary history of prey-predator relations has led to a wide variety of morphological and chromatic adaptations. The aim of these adaptations is to increase the chances of survival of the species in the environment. Many types of mimicry have been described, among them defensive, aggressive and reproductive mimicry.

1.9.1 Biology Adaptation: a brief history of the theory.

The biology adaptation is a changing in the structure and in the function of organisms, and it happens because of natural selection. The organism becomes more fitted on surviving and multiplying itself on the environment where it lives.
For an organism, the adaptation to the aquatic life is more complex.
For example, one of the problems the organism has to face is sinking and to solve that problem, we try to increase the frictional forces.
Other strategies are body shapes that help to remain in suspension, like the jellyfish’s parachute shape or lighten the body with mechanisms like gas production.
Lots are the marine organisms that live in absence of light. In fact, to solve this, they have developed various types of adaptation, which they can create light by themselves, throw bioluminescence. That is a phenomenon among algae, such as Dinoflagellate of the genus Noctiluca and in jellyfish Pelagia Noctiluca.
The majority of organisms don’t tolerate large variations of salinity (such as stenohaline) and only a few can survive (such as euryhaline). That happens in coastal lagoons. Only few species are able to switch from fresh to salt water and vice versa. For example, salmon and eel can do this.
The relation between prey and predator in the time has changed a lot, bringing to a wide variety of adaptations. The purpose of the adaptation of these species is to increase their chances of living and reproducion. A lot of species of mimicries have been described as aggressive and defensive.

1.9.2 Lose weight to float: how fat really helps.

Each marine species adapt to the various habitats to reach an equilibrium, trying to solve problems that may compromise its survival. One of the problems to be faced in the water is sinking. To face it we try to increase the frictional forces with the diminution of the dimensions or increasing the size of the body as in the whales. Other strategies are body shapes that help to remain in suspension without sinking (like the jellyfish’s parachute shape) or lighten the body with mechanisms like gas production.
In other cases lightening is ensured by various forms of fat in the body.
Sharks rely upon a huge, oily liver to provide some buoyancy. The oil in the liver, called squalene, is lighter than water and gives the shark some buoyancy, but it is still heavier than water and will sink if it does not actively swim. Oil levels vary in sharks depending on where they prefer to swim; the bigger the livers are, more oil exists to help the sharks stay buoyant.
The marine mammals use a particular skin fat that is called "blubber". Blubber is a thick layer of fat, also called adipose tissue, directly under the skin of all marine mammals. It covers the entire body of animals such as seals, whales, and walruses. Blubber is an important part of a marine mammal's anatomy. Its main function are to increase buoyancy, to store energy and insulate heat. Blubber helps marine mammals stay buoyant, or float. It is generally less dense than the ocean water surrounding it, so animals naturally float.

1.9.3 Dinoflagellates: description and characteristics.

Dinoflagellates, also known as pyrophytes, peridines or dinoficee, are mostly unicellular and flagellate microscopic algae, which represent one of the most important marine and freshwater phytoplankton groups with more than 2000 living species.
The cell has a peculiar structure, the anfiesma consists of a periplasto, in the region below this can be present as cellulose veil. The amphithema is formed by vesicles, the alveoli, which may be empty or contain glucans, in this case form plaques that cover the cell, this is referred to a case.

There are two flagella, both provided with lateral hairs, different from each other for structure and orientation.

There are two main morphotypes: the Dinoconte and the Desmoconte. In the Dinoconte the cell has two grooves, an equatorial (cingulum) and a longitudinal (furrow). The latter divides the cell into two parts, called epicone (or epiteca) and hypocono (or mortgage). The two flagella emerge in the ventral position at the intersection between the cingulum and the furrow. In the Desmoconte the tecal plaques are organized to form two distinct valves and the two flagella emerge in apical position.
Dinoflagellates are very abundant in all oceans, particularly in tropical regions. In temperate areas they have their maximum development typically in summer, under conditions of stability of the water column. In some cases they proliferate intensely reaching very high abundances, in the order of millions of cells per liter, giving rise to the phenomenon of "red tides".

Some species produce biotoxins, compounds that have toxic activity for humans and other vertebrates.
They usually reproduce by vegetative means: the cell divides longitudinally, transversely or obliquely. Apart from a few exceptions, the dinoflagellates are haploid and present a life cycle alone, with meiosis zygotic. Sexual reproduction occurs through the production of gametes that are not distinguishable from vegetative cells.

1.9.4 Living in the dark: Noctiluca Scintillans(description and characteristics).

Noctiluca scintillans is a variety of microscopic algae and is one of the most commonly occurring bioluminescent organisms in the world. Noctiluca is a single-celled organism large about 200-2000 μm in diameter, spherical and gelatinous. There are several vacuoles and a network of cytoplasmic strands. It has only one transverse flagellum and a striated tentacle extending posteriorly, which aid in movement and in the capture of food. Noctiluca feeds on various minute marine organisms: it engulfs its victims through a kind of phagocytosis. In nature, it has been found in two different forms called "red" and "green" respectively. The "red" form is heterotrophic. The other form is called "green" because of the symbiosis with photosynthetic species, while continuing to feed on plankton. Noctiluca scintillans produces bioluminescence when its cells are subjected to an external mechanical stimulus, like the agitation of water through the action of waves. Bioluminescence is a phenomenon present among algae: in response to the absence of light, many species of marine organisms have developed various types of adaptation, including the production of light through the bioluminescence. Noctiluca scintillans reproduce both asexually through binary fission and sexually through isogamy. The species prefers the coastal areas of the tropical regions. It abounds in places where ocean current rises on the surface and in areas rich in food. The temperature range in which it occurs varies from about 10°C to 30°C. Toxicity on other marine organisms does not happen through the formation of toxins, but through the excretion of ammonia and the consumption of oxygen.

1.9.5 Living in the dark: PelagiaNoctiluca(description and characteristics).

The Pelagia Noctiluca is a jellyfish commonly known as the luminous jellyfish, belonging to the Pelagiidae family, the name Noctiluca derives from the green iridescence, of which it is endowed.
The Pelagidae have a relatively simple form: a bell without a ring channel, from the margins of which the tentacles depart, in which the gastrovascular cavity is separated into uniform pockets and with oral "arms" extending like tentacles more thick.
They are commonly in the Mediterranean Sea and from eastern Atlantic Ocean to the North Sea, during autumn and spring approaching the coast.
The jellyfish features are: pinkish-brown or pink-violet umbrella of about 10 centimeters in diameter, translucent, composed of 16 lobes from which 8 long retractable tentacles, very urticant and semi-transparent, depart from the edges and can extend up to 2 Meters. The oral arms, of the same color of the umbrella, are long up to about 30 centimeters.
It feeds on plankton and small fishes that catch by the tentacles equipped with urticating nematocysts (urticating organs enclosed in some ectodermal cells of the coelenterates that serve the animal for defense and to paralyze the prey).
A peculiarity of this jellyfish is that it is one wich that does not cross the polypoid stage during maturation. The adults are separate sexes: the female lays the eggs in the sea, which are fertilized by the sperm of the males. From the zygote the Planula is born, a larva equipped with eyelash for movement and dispersed at the planktonic level. However, it does not cross the Scifistoma stage, anchoring itself to the ground, but it divides directly into Efira, a young jellyfish that will then grow to form the adult jellyfish.

1.9.6 Eels: a versatile species (description and characteristics).

The eels are marine animals that settle in most of the Mediterranean and the Atlantic Ocean. They are bony fish that can survive safely in both fresh and salt water. They are considered an endangered species, due to their prized meat. In fact, their presence has greatly diminished over the course of fifty years. They are cylindrical and elongated fish, similar to crawling reptiles. In addition, their ability to cover certain stretches of land outside the water makes them traceable to amphibian-like animals. The eels have a smooth skin, completely covered with mucus and in appearance without scales but actually present, even if very small. The freshwater eels have a long anal fin that joins first to the caudal fin and then extends up over the back; the pectoral fins are rather reduced. The gills of the eel are thin and the eye looks quite underdeveloped. Eels reach different sizes based on gender. In fact, the female can reach and exceed the meter in length by 2 kilograms of weight, while the male usually does not exceed the size of 50cm for 200g. On the issue of the sexes there were conflicting opinions. In fact, until recently, a hermaphrodite capacity was attributed to the eels. Currently, however, the distinction between the sexes of these is considered more plausible.The behavior of eels is quite complex, in fact the female spends most of her life in fresh water but once ripe for maturation they go to the open sea. Speaking of male behavior, they do not have a migratory behavior, but settle at the places of maturation.

1.9.7 Species mimicry: a defence mechanism (find an example).

We speak of cryptic mimicry (or cryptoism) to indicate the assumption of forms, colors and behaviors that make the individual similar to the surrounding environment or parts of it, of fanerical mimicry (or ostentation) to indicate the imitation of another species, toxic or dangerous, with aposematic colors. This is called mimicry in the strict sense. Batesian mimicry occurs when an animal species, harmless and helpless in the face of predators, exploits its resemblance to an aposematic species that lives in the same territory, coming to imitate their color and behavior. In this way, in the predators' mind, the Batesian species is associated with the aposematic one and therefore increases its chances of survival. A necessary condition, for the development of the Batesian mimicry, is that, the helpless species shares the same type of predators as the aposematic one. It has also been observed that the Batesian species are less numerous and live less long than the aposematic species that occupy the same environment. It is also believed that this is a strategy developed by Batesian animals to statistically reduce the odds of being, eaten by mistake by inexperienced predators.
Examples of Batesian mimicry are represented by different tropical species of diurnal butterflies of the families Papilionidae and Nymphalidae, which include both aposematic species and harmless species that imitate them; also among the Epicopeiidae there are Batesian species, while among the aposematic species that are imitated, there are many Danainae and Uraniinae. The Seseidae lepidopterans, completely harmless and widespread even in temperate zones, imitate different species of Hymenoptera. The latter are also imitated by other flowering insects: different dipterans and some cerambycids of the subfamilies Cerambycinae and Lepturinae. Among the dipterans the species Rhagoletis zephyria imitates jumping spiders.