More about the deep sea fish you meet in the game:


Weird and wonderful deep sea fish

Deep sea fish generally have some common physical features: they have large mouths and teeth, large eyes, large stomachs (in proportion to their body size) and dark colouration. Not only are many deep sea fish dark in colour, but many have a special dark lining in their stomach that may serve to conceal light emitted from the light organs of engulfed prey – protecting them from attracting other, larger predators while they digest!

Bioluminescence

In addition, most are bioluminescent in some way. Some are able to produce their own light through chemical reactions, others have a symbiotic partnership with bioluminescent bacteria who manufacture the light. Most deep-sea bioluminescence is blue – blue-green, as light of these wavelengths penetrates the furthest through water, but some fish produce light of different colours including reds and yellows. Scientists are not certain why fish use bioluminescence. There are many suggestions, and probably bioluminescence serves more than one purpose for most species. The arrangement of the bioluminescent organs provides some clues. Some are located on the underside of the fish so as to provide counter shading when the fish rise in the water column to feed. Others are arranged in distinct patterns and may thus be used for some kind of signalling, for mating or competing, for example. Some fish are able to control their bioluminescent activity by actively blocking or shutting off light production. One interesting use of bioluminescence is as a lure for prey.

Fish fishing other fish

A number of species of deep water fish have evolved “fishing gear”. They sport “rods” or protruding structures that often have a bioluminescent lure at the tip. Some of these structures are prehensile allowing the fish to lure a prey item ever closer to its mouth. Others keep their mouth open, so that unsuspecting victims will follow the lure and swim in to be captured.

Amazing prehensile jaws
The Stoplight Loosejaw (Malacosteus niger) has an enormous lower jaw that can be extended far in front of the fish. The long, inward pointing teeth on the jaw act as hooks to grab a prey object while the jaw retracts quickly drawing the prey into the mouth. There is no skin connecting the lower jaw bones in order to reduce drag in the water when the jaw is retracted. This fish has another interesting feature. It has a large photophore at the front of its head. However, instead of releasing blue-green bioluminescence as is usual for deep-sea organisms, (and which most are able to perceive) it releases a flashing red light. This enables it to see prey without being seen itself!

Coping with an irregular diet
Creatures in the deep sea are often few and far between. Predators must therefore either be very efficient, or capable of lasting long periods without food. A number of deep water species, such as the Black Swallower (Chiasmodon niger) have extremely distensible stomachs capable of storing food. These stomachs are often larger than the fish itself.

From larvae to adult – two different species or only one?
Many fish species have distinctly different life phases. Often each phase occupies a completely different ecological niche (ie the smallest may live with the plankton in the surface waters, the juveniles might live near the sea floor etc). The over-simplified “snap-shot” picture that collecting gear yields of deep sea life makes it difficult to be certain which fish are simply different life phases of a single species and which are, in fact, different species. Genetics will be an invaluable tool in this area. Researchers then will be able to build a better picture of fish species’ life histories.

Many fish species have distinctly different life phases. Often each phase occupies a completely different ecological niche (ie the smallest may live with the plankton in the surface waters, the juveniles might live near the sea floor etc). The over-simplified “snap-shot” picture that collecting gear yields of deep sea life makes it difficult to be certain which fish are simply different life phases of a single species and which are, in fact, different species. Genetics will be an invaluable tool in this area. Researchers then will be able to build a better picture of fish species’ life histories.

United forever
Some species of deep sea angler fish, such as Krøyer’s deep sea angler fish (Ceratias holboelli) form a weird kind of permanent relationship.  Again responding to the problem of individuals often being spread very widely, these fish have adopted a strategy of sexual dimorphism whereby the females are very much larger than the males. The males, however, have specially adapted biting teeth and when they find a female they bite into her body forming what becomes a permanent attachment. The body of the attached male parasite modifies to the point of being practically non-functional, except in terms of sperm production.

Collecting specimens
The Mar-Eco participants have already assembled a document listing around 600 already described fish species that researchers might expect to find in the waters around the northern mid-Atlantic Ridge. The list is based on collecting work that has occurred in the waters around the Azores and Iceland, as well as some of the findings from a number of biological explorations, including Murray and Hjort’s cruise in 1910.

Other deep sea creatures
Although deep sea fish provide more than enough fodder for fascinating study, there are many other actors in deep sea food webs. Many of these are extremely difficult to catch. They often have fragile bodies that do not withstand capture in trawls or nets, or the extreme change in conditions that bringing to the surface necessarily entails. These other creatures include some cephalopods (squids and octopus) and gelatinous zooplankton (jellyfish).

(Source: The above text is an edited version of MAR-ECO story about weird_and_wonderful_deep_sea_fish)


Deep-sea fishes.

Deep-sea fish come in many shapes and sizes, and have fascinating life histories and ecological adaptations. However, it is a sad reality that many deep-sea fisheries resources soon become overexploited, even depleted, before science-based management measures are introduced.

Deep-sea fishes are widely distributed across the Atlantic, and many live for several decades. The classical highly priced deep-sea species, orange roughy, may become a hundred years old, perhaps even more than 150yrs. Most species mature at high ages and have low fecundities, hence their reproductive capacity is limited. Growth rates tend to be slow, and natural mortalities low. Such life history features are incompatible with high exploitation rates.

Although some characteristics of deep-sea fish and their habitats have been studied previously, the database available for assessing vulnerability to modern exploitation remains very incomplete. Much information is qualitative, based on limited observations or the use of unvalidated techniques. A lot of the work has not been properly evaluated and reviewed, and a major part of current data can only be found in the rather inaccessible “grey literature” (unpublished reports etc.).


The provision of new data is hampered by lack of international dedicated efforts and opportunities. Therefore, mobilisation and networking of biologists and ecologists with special competence in deep-sea fish studies is essential. MAR-ECO offers this opportunity, and in a special project lead by Dr Maurice Clarke of the Marine Institute in Ireland, a dedicated team of scientists will pursue the study of life history strategies of deep-sea fish. Currently this team will focus on reproductive biology and ecology, growth studies, and mortality estimation for a limited number of key target species of the fisheries, utilizing sampling opportunities on MAR-ECO cruises. Samples will not only be collected on the Mid-Atlantic Ridge, but also on adjacent island and continental slopes. The project will also focus stongly on development and evaluation of methodology such as e.g. age determination procedures.

Such a life history strategy project within MAR-ECO will make a significant contribution to the efforts required for a responsible assessment and management of deep-sea fisheries resources. In addition it will add valuable end exciting new information to science on species that are exotic and poorly known. Studying the life histories of deep-sea fish will advance basic biological and ecological science and will also contribute to the sustainable management of internationally controversial fisheries.

(Source: The above text is written by Elinor Bartle and is a shortened version of  her text: Deep-sea fishes -it may be that their life histories make them especially vulnerable to exploitation, MAR-ECO.)


Other MAR-ECO stories on deep sea fish:

MAR-ECO story “Atlantic Expeditions uncover secret sex life of deep-sea nomads”