This week we are continuing our voyage to the bottom of the deep sea. While last week we focused on the adaptations that fishes have suffered, this week we are focussing on the biodiversity. In concrete, we are explaining crustaceans, squids, cnidarians (corals, jellyfishes and anemones), fishes and worms.
In 1840, the scientist Edward Forbes concluded that there wasn’t life under 550 meters depth. Nowadays, it is known that this is not true because recently it has been found a fish at 8,100 meters. It has been determined that the relative abundance of animals depends on depth. In fact, in general terms, the abundance decreases with depth, but this don’t exclude that there are a lot of species.
Amphipods are by far the most abundant crustaceans in the deep sea. They are small animals with the body compressed laterally and without a carapace, which feeds on carrion and live inside cavities made by themselves in the sea floor. These small animals are transparent, except for them eyes, which are red due to a pigment in the retina.
Deep sea amphipod. They are characterized by the presence of a transparent body with red eyes. (Picture from http://www.astronoo.com/es/articulos/bioluminiscencia.html)
Other deep sea crustaceans are stone crabs, with a carapace of 7.5 cm length and legs of about 15 cm; the armoured shrimp, one of the species that lives at 6,000 meters and has a length of 7 to 10 cm; and more.
In spite of the general thinking that deep sea squids are all large, like the giant squid, which can achieve a length of 18 meters; the truth is that this is an exemption because there are some spices of just 4 cm. They hunt with the suckers in the tentacles and driving the prey to the mouth. Most of these squids are bioluminescent and can regulate the colour, the intensity and the angular distribution of the light.
The Humboldt or jumbo squid (Dosidicus gigas) lives in the western coasts of Central and South Amercia and can achieve a length of 4 meters, which feeds on fishes and practise cannibalism.
CNIDARIANS: CORALS, JELLYFISHES AND SEA ANEMONES
Differences between shallower cnidarians and deep ones are due to differences in the food distribution. In the deep sea, anemones and corals don’t have directly phytoplankton and zooplankton, and they depend on the nutrient rain from the shallower waters of the ocean. On the other hand, jellyfishes have a slow metabolism to survive in hard conditions. It supposes slower growth, but a longer life.
To give an example, this crown jellyfish inhabits between 200 and 2000 meters depth and can measure until 15 cm. It feeds on small crustaceans and organic matter. Its red colour let them be camouflaged in the environment. In addition, they are bioluminescent animals.
Deep-sea jellyfishes are voracious predators, but also can be a prey for some fishes. They produce light discharges to attract small animals. To dissuade predators, they expel a brilliant particles stream.
An habitual feature of deep-sea jellyfishes, but also present in other groups, is gigantism. It means they are bigger than their equivalents in the shallow ocean. The possible explanation to this could be that bigger animals are more efficient than smaller to get food when the environmental conditions are almost constant during long periods of time.
Gonostomatidae fishes are the most abundant vertebrates in the Earth and live in the mesopelagic zone. Together with the lantern fishes, they represent a 90% of the captures in the pelagic trawling fishery. Deep-sea fishes usually have a length between 2,5 – 10 cm and a thin and soft body, but there are exceptions.
There are some examples here:
- Anglerfish: These fishes inhabit in the deepest parts of the oceans and present the optimal colouration to absorb the few light that arrive and, in this way, to be camouflaged. They present a light in the end of the antenna, which let them to capture preys.
- Spiny lantern fish: Because of its silvery body, this fish is not much vulnerable since its contour can’t be seen clearly. In addition, spiny lantern fish presents a bag in the eye with bioluminescent bacteria.
- Pelican eel: This animal can measure 2 meters long. Its enormous mouth are connected directly to the stomach.
- Tripodfish: Tripodfish has long prolongations in its pelvic and caudal fins, which let them put on the sea floor, while it is waiting for its prey.
- Black swallower: This small fish has the ability to dilate a lot its stomach and, in this way, it can swallow preys bigger than itself.
Deep-sea worms can be from microscopic to measure 2 meters long and are one of the most abundant and different invertebrates. They can be of different groups: polychaetes, tubular worms, sipunculids and equiurids. They live partly or totally buried in the sediments.
Tubular worms usually live in big groups near to thermal springs and present red bright gills as a consequence of a high level in hemoglobin to absorb oxygen. In addition, they can retain sulfurs, which will be used for symbiotic bacteria.
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