1.8 Food web and its actors

Every living organism depends on its existence both from its environment and from other living beings: animals that depend on other animals, animals that depend on plants, plants that depend on nutrients and light. Plants are the ones that produce, the animals those who consume, and they are both tied to the nets of the food web (FIG1_SES1.8) that transfers substances and energy from one organism to another. Even removing only one component of the network can lead to the destruction of the whole structure. Not only living organisms are important but also dead ones; in fact, organisms that decompose dead matter allow it to be recycled, making it again available.

1.8.1 Plankton and Necton: move along water column
One of the key elements in the marine food network is plankton. Organisms that are defined as planktonic are both animals (zooplankton, FIG2_SES1.8) and plants (phytoplankton, FIG3_SES1.8). The characteristic that distinguishes them is that they have no tools to counteract the current. Generally the size of the planktonic organisms is microscopic, but there are also cases of large animals, such as some jellyfish.

All animals able to move independently and with their own propulsion structures, such as fins or siphons, are included in the group called necton.

They are part of this set, for example, among the Vertebrates, the adults of many fishes, marine mammals, sea turtles and, among the Invertebrates, squid.

1.8.2 Benthos: live near and on the bottom
Plants and animals that live on the backdrop, whether or not attached to it, are called benthos. Nearly 16% of all living animals are benthic species. Anemones, sponges, corals, crabs are some examples of benthic animals. There are not only animals that creep or walk on the bottom, but also those that are buried in the sand or that are attached to the rocks. Many of these animals however, pass the first part of their life (the larval stage) as floating animals between the waves, belonging to the plankton.

1.8.1 Decomposing organisms: who are they?

Decomposing organisms: who are they?
When we hear the word decomposition most of us think of rotting corpses, awful smells and other unpleasant sights. But in nature decomposition is everywhere and it’s essential for life on Earth.
In charge of decomposition are many organisms, most of which are small in size but large in numbers. They range from beetles, larvae, flies, maggots, woodlice, fungi, bacteria, slugs and snails, millipedes, earthworms and many others. Usually over long periods of time, they slowly break down organic matter into things they themselves or other organisms can use.
The main decomposers in plants are fungi. Using their hyphae they absorb nourishing materials from the plants allowing them to grow more and decompose quicker. The process is aerobic, meaning air (oxygen) is in its presence. This in turn makes the process odorless. Other organisms present here are bacteria and in the later stages slugs, snails and earthworms. The rate of decomposition is dependent on the type of plant. Plants that are less woody tend to decompose quicker.
Tree decomposition is similar but takes longer. There are also many species that grow in the dead wood such as beetle larvae. The fungi feed on the cellulose in the trees causing them to decompose quicker and allow more access to other organisms. In this stage woodlice and millipedes start to eat the wood.
The decomposition of animals is lead mostly by other animals and bacteria. Larger animals such as foxes, vultures, etc. are vital to this type of decomposition. After them flies, maggots and larvae are in charge. This process is anaerobic (without air) and has a bad smell. Their rate of decomposition depends on the decomposing material.
Decomposition and the organisms which carry it out are some of the most complex and interwoven processes that take place in nature. They are also vital to the ecosystem and the circle of life.
https://treesforlife.org.uk/forest/forest-ecology/decomposition-and-decay/

1.8.2 Food Chains vs Food Webs: are they the same?

Food chains vs. Food webs. Are they the same?

To describe feeding relationships between species, humans have, since the 9th century, learned to create graphical visualisations to have a better understanding of energy transfers throughout nature. In the 20th century, that concept has been expanded and a newer, more accurate representation has been created – Food Webs.
One of the reasons food webs were created was to more precisely display food relations not only within species, but within communities. Food webs generally consist of several food chains. There exists three types of food webs: The Connectedness Web, which simply shows feeding relationships between organisms using arrows, Energy Flow Webs, which more precisely describe amounts of energy in transfer by using different thicknesses of arrows and finally, Functional Webs, which emphasize on growth rate relations between species. One of the most useful aspects of food webs is the possibility to analyze them from bottom-up or from top to bottom. The structure of food webs suggests that productivity of populations is controlled by trophic life forms below them. This is called bottom-up control. If the population density of a consumer can control populations below them, the term that should be used is top-down control. The best example of this would be the ability of predators to control the population density of their prey.
Still, food webs are limited representations of real ecosystems as they roughly sort organisms into trophic species, which may or may not be accurate. These simplifications are mostly used in quantitative models that represent ecological dynamics.

Excerpted from Nature's article:
"Food Web: Concept and Applications”, by Dafeng Hui
https://www.nature.com/scitable/knowledge/library/food-web-concept-and-applications-84077181

1.8.3 Zooplankton: a definition and an example of your choice.

Zooplankton are small floating or weakly swimming organisms that drift with water currents and make up the planktonic food supply upon which almost all oceanic organisms are ultimately dependent. Many animals, from single-celled Radiolaria to the eggs or larvae of herrings, crabs, and lobsters, are found among the zooplankton. The word zooplankton is derived from the Greek "zoon", meaning "animal", and "planktos", meaning "wanderer" or "drifter". Individual zooplankton are usually microscopic, but some (such as jellyfish) are larger and visible to the naked eye.

Zooplankton are classified by size and/or by developmental stage. Size categories include: picoplankton that measure less than 2 micrometers, nanoplankton measure between 2-20 micrometers, microplankton measure between 20-200 micrometers, mesoplankton measure between 0.2-20 millimeters,macroplankton measure between 20-200 millimeters, and the megaplankton, which measure over 200 millimeters (almost 8 inches). There are two categories used to classify zooplankton by their stage of development: meroplankton and holoplankton. Meroplankton are actually larvae that eventually change into worms, mollusks, crustaceans, coral, echinoderms or insects.

Many types of zooplankton migrate deeper into the water during the day and come up at night. The migration of species appears to be dependent on location rather than particular species types. All plankton migrate differently based on factors like age, sex and the season. The amount of light is probably the major factor in the extent of migratory behavior. It seems that zooplankton move around most in low light and least in higher light situations. It’s possible that zooplankton migrate to lower levels during the day, so they are less visible to predators relying on vision. At night, zooplankton can sneak up to the surface and snack on phytoplankton relatively safely.

https://www.britannica.com/science/zooplankton
https://en.wikipedia.org/wiki/Zooplankton

1.8.5 About jellyfish: a description a description of their main characteristics.

Jellyfish are animals of the phylum Cnidaria. They are freely floating animals. The body od an adult jellyfish looks like a bell, umbrella or bubble and there are a lot of different shapes, colours and sizes of jellyfish. Jellyfish are also one of the most ancient organisms who lived on Earth for hundreds of millions of years.
Jellyfish are radially symmetric animals and quite transparent because 95% of their body is water and it makes them perfectly camouflaged. They are made of three layers: surface layer epidermis, inner layer gastrodermis and central layer mesoglea which separates the epidermis and gastrodermis. In the epidermis there are stinging cells (cnidocytes) which contain stinging organelles. Jellyfish have a painful sting that can sting or kill other animals. The majority of jellyfish use their stinging cells for catching prey and for defense. Jellyfish swim by pulsations of the bell. They are very slow swimmers but speed is not important for them because their food is plankton or small sea organisms that they catch with tentacles. These animals have a very simple nervous system and they don't have a brain so they can't do anything else than sting. When the jellyfish touch the prey with their tentacles, which surround the mouth and stimulate stinging cells, the jellyfish eject their stinging tentacles, release poison and kill the prey. One interesting phenomenon is called bloom or swarm, and it consists of hundreds or even thousands individual jellyfish. The jellyfish have separate sexes and fertilization is external. They also don't have breathing organs so they breathe by diffusion.
Although jellyfish are dangerous animals who can even kill people, they have an important role in the marine feed chain.

https://eu.oceana.org/en/feature-about-jellyfish
https://www.thoughtco.com/facts-about-jellyfish-4102061
https://animalcorner.co.uk/animals/jellyfish/

1.8.6 DrymonemaDalmatinum: the big pink is back in our sea.

Drymonema dalmatinum

Drymonema dalmatinum, also known as The Big Pink or Stinging cauliflower was recently recorded by an amateur diver at the northern coast of Italy. This jellyfish hasn't been seen since 1945. It was first observed in 1880 by a German naturalist, Ernst Haeckel along the Dalmatian coast.

Not much is known about this species due to its rare sightings. This species can be found at the central part of the Atlantic Ocean and along the Mediterranean Sea. Its flat umbrella, which can reach up to 1 meter in diameter, is pale rose to a golden brown color. Its numerous tentacles are translucent and the number of them increases with age. It also has long, thin oral arms which when extended can reach up to more than 1 meter. Scientists still haven't discovered whether its poison is harmful for humans or it isn't, even though it is considered poisonous. Experts say that some jellyfish have two life stages- an early stage when they live at the bottom of the ocean, and a late phase when they turn into a floating jellyfish. That can be the explanation to the question why we haven't seen this extraordinary animal for such a long time. The Big Pink feeds on other jellyfish such as Aurelia spp. (Moon jellyfish). In the time of increased population of its prey it forms huge swarms.

I think that the appearance of the jellyfish is a good thing because it enhances the biodiversity of the Adriatic Sea. It can also be a bad thing because the jellyfish can multiply too much and endanger the local marine animals.

https://en.wikipedia.org/wiki/Drymonema
https://www.express.co.uk/news/nature/497054/Drymonema-dalmatinum-jellyfish-seen-for-first-time-since-Second-World-War
http://www.oceanography.ucy.ac.cy/medusa/species/descriptions/drymonema.html

1.8.7 About squids: a description of their main characteristics.

About squids: a description of their main characteristics
Squids are one of the most interesting sea life animals in the world due to their characteristics and their behavior.
The squids have a wide range of characteristics and properties which vary from one species to another. Their sizes, colors and behavior are variable, depending on the species. However, what is really clear, is that they are harmless animals. Although they have large tentacles, they do not represent any risk for humans.
There are three hundred species of squids that have been already identified and classified. However the studies think there are many more undiscovered.
Squid have gills used for breathing (which pull oxygen from the water) and are highly adapted to their oceanic environment which makes them excellent swimmers.
At the front of the squids mantle is a directionally controllable siphon which the squid forces water out of that allows it to navigate and move through the ocean.
The skin is covered in chromatophores which enable the squid to change colour to suit its surroundings, making it effectively camouflaged.
These creatures represent a very important source of food both for their predators and for humans.
The squids reproduce in big numbers due to their fast growth. As a result, there are immense families of these species distributed in all oceans, in spite of their short lives, and their continuous hunt by humans.
https://www.whalefacts.org/squid-facts/
https://study.com/academy/lesson/squid-characteristics-classification.html
http://ourmarinespecies.com/c-squid/squids/

1.8.8 Benthos species: Pinna Nobilis (description and characteristics).

Pinna nobilis is a large species of a marine bivalve mollusc. It's length can reach up to 1.2 m. Pinna nobilis is endemic to the Mediterranean Sea where it lives at depths between 0.5 and 60 meters, but it can also be found buried underneath a small layer of sand.
Pinna nobilis filtrates the water and feeds with sea plankton. They breath with branchia. One of the most important characteristics is it's self defence. When something sharp enters the shell while filtrating the water, the shell wants to defend itself so it makes pearls. Pearls have smooth surface and it can't damage the soft inside of the shell. Pinna nobilis is a source of sea silk. In recent years, pinna nobilis has become threatened with extiction. Pinna nobilis is now strictly protected. Human pollution and fishing are the two most important reasons why Pinna nobilis is endangered. Pinna nobilis reproduces by external fertilization. After fertilization ensues larva. Larva floats and it's part of zooplankton. After some time it attaches itself to the sea bottom and adult bivalve evolves. Pinna nobilis's both adult and larval body are bilteral symmetric. It has open blood system and heart. Pinna nobilis is not a hermaphrodite. Pinna nobilis can often be found in symbiosis with shrimp that lives inside her shell. Their relationship is mutualistic, because shrimp warns the host when sees threat and Pinna nobilis provides shrimp a secure shelter and food, because they have similiar diet.
Pinna nobilis is very important species for our Mediterranean Sea. Poachers like to fish pinna nobilis, because it makes beautiful pearls and it's very big, so it has big shell and a lot of meat. We need to protect it from extinction.
Sources:
https://www.researchgate.net/publication/227163414_Population_structure_mortality_and_growth_of_Pinna_nobilis_Linnaeus_1758_Mollusca_Bivalvia_at_different_depths_in_Moraira_bay_Alicante_Western_Mediterranean

1.8.9 Benthos species: anemones of the Mediterranean Sea.

Benthos species: anemones of the Mediterranean Sea
Anemones are sea animals that look like flowers. This species have similarities with jellyfish and corals. They are also significant for their tentacles that give them a specific look. In the Mediterranean Sea live Snakelocks Anemones. Snakelocks Anemones are also spread in the shallow parts of the Adriatic Sea.
Snakelocks Anemones live attached to the rocks. Snakelocks Anemones usually appear in grey or light-green color. Their tentacles are their most special feature. They are wavy and snake-like so this species are called Snakelocks Anemones. Tentacles have purple tips. Snakelocks Anemones live in the sunniest places on the shore because they use sunlight as an energy source. Their tentacles are used for capturing prey which is usually a small fish or sea snail, which means they are some kind of predator. Snakelocks Anemones are reproducing like jellyfish, which means they have a polyp stage. The polyp produces sperm and eggs and fertilized eggs develop into the planula that later becomes an adult anemone. This species of anemones can live in symbiosis with algae. Algae provide them with oxygen and sugar and Snakelocks Anemones protect them. The interesting fact about this anemone is that their cells have protein that makes them glow fluorescent. Snakelocks Anemones can burn human skin with their tentacles so humans have to be very careful on the shores of the Adriatic Sea. This anemone is also a delightful food.
In conclusion, anemones are beautiful and special animals that make the seas even more various with creatures that live in them.

https://www.wildlifetrusts.org/wildlife-explorer/marine/anemones-corals-and-jellyfish/snakelocks-anemone
https://www.oceanografic.org/en/especie/mediterranean-snakelocks-sea-anemone/