Many abyssal animal species grow to a much larger size than their shallower-water congeners. An example is the giant squid, up to 13 m long, surpassed by the even bigger colossal squid, or by the herring king, a sort of eel over 11 m long.
Another example is the isopod crustacean Bathynomus giganteus which can reach a size of 50 cm for a weight of 2 kg while the coastal members of this group do not exceed one or a few cm in length. It is unknown whether the tendency to increase in size is an adaptation to face the scarce food resources of the deep sea, or the strong pressure of the deep sea, or is due to quite other reasons.
David Attenborough, in his television series on the abyss Blue Planet, said that the large size of an abyssal animal would result in less heat loss and a decrease in the need for constant activity, characteristic of small organisms.
What causes Deep-sea gigantism?
One of the abysmal organisms for which there is a justification for the large size is the giant tubular worm: these creatures, in fact, live near hydrothermal springs, which supply them with large quantities of energy in the form of bacteria.
Giant oarfish, for example, has an elongated and ribbon-like body, rather fragile, silver in color and streaked with dark oblique bars, with the skin dotted with small tubercles. The short head is bluish. Juvenile individuals have a set of very fine teeth per jaw which disappears in adults.
It is characterized by a bright red dorsal fin that runs along its entire body, by the presence of a crest of rays placed above the head and by two long pelvic fins similar to oars. The main peculiarity of the herring king is undoubtedly its length: it can reach a length of 11 meters although it does not normally exceed 3 and, consequently, the weight can reach several quintals.
For this reason, it is considered by many to be the longest bony fish in the world. The size makes it almost immune to predators and are probably the origin of the legends that are told about sea monsters. In February 2010, in the Gulf of Mexico, at a depth of about 1500 metres, a specimen was sighted by a bathyscaphe whose length was roughly estimated at over 17 metres.