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Visiting av vertebrate curator


Byrkjedal here shows a preserved Pelican eel
Ingvar Byrkjedal, a vertebrate curator at Bergen Museum, shared some fascinating deep sea specimens, some dating from nearly 100 years ago! 

Deep sea fish, explained Byrkjedal, 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!

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.

Icthyological disco


A couple of bristlefish (Gonostoma elongatum). Note the line of photophore (light organ) buttons along their undersides.

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.

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.

The Stoplight Loosejaw 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.

Byrkjedal said that the museum has had this problem with a collection of eels. The larvae are very thin and almost transparent, while the adults are much more substantial and black in colouration.

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.

Byrkjedal said that the museum has had this problem with a collection of eels. The larvae are very thin and almost transparent, while the adults are much more substantial and black in colouration.

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.


The drawing is from P.J.P. Whitehead et al. (1986):Fishes of the North-eastern Atlantic and the Mediterranean; vol. 3 (Unesco, Paris). Krøyer's deep sea angler fish (Ceratias holboelli), without a male attached.Byrkjedal hopes that they will find a joined sample during the MAR-ECO expeditions.  

In the sketch, the male is the small body to the back and base of the fish’s body. In the specimen to the right, there is no attached male. If you look very closely, you can see the lure hanging in front of the fish’s mouth. The spine holding the lure is retractable, enticing prey ever closer to the fish’s mouth.

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.

The picture is of a group of Russian scientists sorting samples aboard the Russian RV Smolensk cruise to the northern most MAR-ECO area, May-June 2003.
 
Some of the specimens Byrkjedal showed actually date from the 1910 expedition. When specimens are collected on research cruises, there is a protocol for preserving them, which, in theory says Byrkjedal, preserves them forever. Although much of this protocol was established over 100 years ago, there are some new additions today in order to facilitate modern molecular biological testing, and genetic testing, in particular.

First, says Byrkjedal, the researcher extracts a small tissue plug that is placed in nearly pure alcohol. This material will be used for DNA and other molecular tests. Then the specimen is placed in a formalin / alcohol bath for 24 hours to “fix” the tissues (this stops the process of decay). After which, the sample is stored in 70% alcohol. Alcohol is believed to be the best preservative for biological specimens. For the most part even a specimen’s colours are preserved, says Byrkjedal, although the reds tend to dissolve with time. All preserved samples are labelled with a species name (if possible), date, location, and other collecting details.

Other deep sea creatures


A large gelatinous zooplankton (jellyfish) collected May-June 2003 during the Russian RV Smolensk cruise to the northern most MAR-ECO area (see the net marks on the body)

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).

 

 

 


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