Bioluminescence is the emission of light by animals through special cells called photophores. The bioluminescence is cause by a chemical reaction containing phosphorus molecules or symbiotic bacteria. It can be found in many organisms from diverse kingdoms including fish, algae, fungi and invertebrates. Pressures are immense in the deep-sea environment. This is due to the fact that 1 atmosphere of pressure is added for every 10 meters that one descends. Check the handout/chart of the depths of the deep-sea region and the pressures associated with them.

The main adaptation to the pressure is the reduction or loss of the swim bladder. Many fish have also developed swim bladders filled with lipids instead of gas. The lipids are relatively incompressible when compared to gases.

Feeding has also become highly specialized with adaptations such as large mouths, distendable stomachs, needlelike teeth and lures for attracting prey. These adaptations help the fish to take advantage of virtually any sized prey that may come along. This is important since feeding opportunities are scarce in the deep-sea.

Mating opportunities are scarce in the deep-sea. This is due to the low population densities that prevail in the deep-sea. With low frequencies of possible mating opportunities, fish need to take optimum advantage of the ones that do present themselves. As an example of one species solution to this problem, lets consider the deep-sea anglerfish. The male anglers are small and act as parasites. They attach themselves to the female and fuse into her circulatory system. She supports them both while the male is there solely to fertilize the femaleÕs eggs.

When examining these adaptations it is necessary to take into consideration the environmental factors that select for many of the unusual features observed in deep-sea animals. The ocean is divided into several regions including the Epipelagic, Mesopelagic, Bathypelagic, Abyssopelagic, and Hadalpelagic regions. The deep-sea begins in the Bathypelagic at 1000 meters continuing through the other regions. The Challenger Deep, in the Mariana Trench, reaches a maximum depth of 10,991 meters (36,061 feet). Please refer to the handout for the regions and the depths associated with them. Light penetrates only to the mesopelagic zone. As a result the deep sea is covered in darkness. Factors such as the absence of light, immense pressure, and an extremely limited amount of foraging opportunities are responsible for the adaptations observed in the deep-sea fishes

LIGHT ADAPTATIONS
The absence of light has directly influenced the development of bioluminescence. Bioluminescence serves a variety of functions. Bioluminescence serves to aid in prey attraction, communication and courtship (Castro, P. and Huber M 1997). While bioluminescence aids mesopelagic species in predator evasion by breaking up the silhouette of the fish, this does not apply to deep-sea species since there is no background lighting against which to throw a silhouette. This is because that the light below one thousand meters comes solely from bioluminescence (Randall D. and Farrell A. 1997). Some species utilize light created by symbiotic bacteria, while others have intrinsic tissues that emit light through a chemical reaction. The light of such fish is generated through photophores.

There are two general categories of bioluminescent light emitted from deep-sea fish including, blue and red. The first category of light is that which falls in the blue spectrum. The blue light is used primarily for long-range communication with conspecifics. While blue light is highly effective for communication purposes, it can also be used for predatory purposes by attracting prey items. An excellent example of this is the deep-sea anglerfishes. Females of these species use a special appendage projecting from the cranial region to attract smaller prey items. The prey organisms are attracted to the light emitting appendage and then consumed by the angler once it is within range. Female anglerfish will also use their Òfishing rodÓ to attract males for the purposes of mating. An important note here is that the flashing sequences emitted by deep-sea fish can be quite complex as there is evidence of species-specific flash sequences (Randall D. and Farrell A. 1997).

One downside of the relatively visible blue light is that larger predators easily intercept it. However, some evidence also supports the theory that bursts of the relatively bright blue light will help the emitting species to evade predation by momentarily blinding the invading predator. Evidence supporting this concept comes from research in which bursts of light from the caudal photophores of myctophids was immediately followed by a tail flick response (Mensinger A., Case J.), which is a conditional, involuntary predator invasion response.

The second category of bioluminescent light is orange or red light. The red light is used primarily as a prey detection device. This device is very effective to the predators because the prey is unable to detect the light, but the predator is able to locate its quarry. Since most of the deep-sea fish are black or reddish in color, studies have shown that between two and five percent of the blue wavelengths are reflected as opposed to the twenty percent of the red light reflected (Mesinger A., Case J. 1990) which demonstrates the advantage of red light in this scenario. These organs can be located either in the cheek area or sub-orbitally. Observations show that when located sub-orbitally, these organs do not directly illuminate the eye itself (Mensinger A., Case J. 1990) so the user is not blinded. This organ can rotate behind a black pigment or shine outwardly turning the light on and off acting much like a headlight. The prey is unaware of the light illuminated from the predator due to the fact that visual pigments in their eyes do not react to those particular light wavelengths (Paxton J., Eschmeyer W. 1998). This adaptation enables the fish to search more effectively.

Pressure Adaptations
Next to the absence of light in the deep-sea, perhaps the next most limiting factor is the extreme pressures present. In the deepest benthic regions, pressures can reach up to one thousand atmospheres. This has necessitated the evolution of certain crucial strategies to make life possible in such extreme conditions. Most fish use swim bladders to maintain buoyancy. The swim bladders are inflated with gases to maintain buoyancy. Due to the extreme pressures of the deep, inflation with gases is not practical. Fish at these depths rely more upon the dense surrounding waters to maintain close to neutral buoyancy and fill their bladders with lipids and wax esters to add buoyancy. Studies have shown that these fish do not show degeneration of the gas cells and the rete mirabile, although there is reduction in swim bladder size and an overall thickening of the bladder wall (Randall D. and Farrell A. 1997).

Feeding Adaptation
In the deep-sea environment the feeding opportunities are few and far between. Due to the lack of forage the fish need to take advantage of every opportunity that comes. Thus, they have evolved large mouths, elastic jaws, aggressive dentition, distendable stomachs, and lures to attract prey. Saccopharyngoid gulper and swallower eels have elastic mouths that expand to more than ten times the size of the animalÕs entire body. This is the largest proportionate mouth-body volume of any known vertebrate (Helfman G., Collette B., and Facey D. 1997, pg 300). Along the same lines, the viperfish (Chauliodus sloani) increases its swallowing capabilities by having the ability to detach the pectoral girdle from the skull. These are just two of the adaptations fish use to maximize each feeding opportunity.

Aside from having large mouths, many deep-sea fishes have the ability to greatly distend their stomachs. The enlarging of the stomach allows for the consumption of items that can exceed their size by as much as two to three fold. An example of this is the black swallower whose stomach can extend out in front of its jaw after an exceptionally large meal. This trait allows them to forage on items of a much wider range of sizes than if their stomachs were not so elastic.

Characteristically, deep-sea predators have teeth that are typically long and needle like which heightens their likelihood of prey acquisition (Gordon B.L. 1977). As discussed earlier, bioluminescence is an effective means of food acquisition. Once again the classic example of this method of prey attraction is the deep-sea angler (family Diceratiidae). While these adaptations greatly benefit the predatory natures of deep-sea fish, scavenging constitutes a large part of their palate. One example of this is the deep-sea eels and the hagfish whose diet comes mainly from carrion (Merrett N.R., Saldanha L. 1985).

Reproduction
Due to the low concentrations of conspecifics, mating opportunities are scarce the deep-sea. As a result, deep-sea fish have developed means of greatly increasing their chances of reproducing. For instance, female anglers use their bioluminescence to attract males. In some species once the male arrives he engages in what authorities have termed Òmale parasitism.Ó This involves the male latching on to the femaleÕs body with his jaws. The maleÕs circulatory system then fuses with the femaleÕs and she provides him with nourishment. He has no need to feed or for locomotion, and thus becomes, in essence, a sack of sperm which is readily available to fertilize the femaleÕs eggs. This method virtually guarantees successful copulation. Aside from this some deep-sea species are hermaphroditic. This is extremely beneficial since there are so few mating opportunities. Many deep-sea fishes partake in a larval stage prior to full development. Stomiiforms, which include the viperfish (Chauliodus sloani), and numerous other deepwater fishes breed near their deep resting areas, but the eggs float to the surface where they hatch and become part of the plankton. As the larva develop into juveniles and go on to their adult stages they descend to the depths (Paxton J., Eschmeyer W., Kirshner D. 1998). This allows the developing young greater access to prey items in the more productive waters near the surface.

Energy Conservation
While the majority of the adaptations that we have discussed conserve energy there are a wide variety of morphological trends that free up energy to be used in other areas. Examples of this include underdeveloped muscles and reduced skeletal structures. By not developing these structures the conserved energy can be used for functions necessary for life in the deep-sea. References Castro, Peter Ph.D., and Huber, Michael E. Ph.D. Marine Biology. Boston: McGraw Hill, 1997. Gordon, Bernard Ludwig. The Secret Lives of Fishes. New York: Grosset and Dunlap, 1977. Helfman, Gene S., Collette, Bruce B., and Facey, Douglas E. The Diversity of Fishes. Berlin:Blackwell Science Inc, 1997. Mensinger, Allen F., and Case, James f. ÒLuminescent properties of deep sea fish.Ó Journal of Experimental Marine Biology 144 (1990): 1-15. Merrett, N.R., and Saldanha, L. ÒAspects of the morphology and ecology of some Unusual deep sea eels (Synaphobranchidae, Derichthyidae, and Nettastomatidae) From the eastern North Atlantic.Ó Journal of Fish Biology 27 (1985): 719-747. Myagkov, N.A. ÒUnusual Brain Structure of Luminous Shark, Isistius brasiliensis (Dalatiidae).Ó Journal of Ichthyology 24 (1984): 109-112. Ed. By Paxton, John R. Dr., and Eschmeyer, William N. Dr. Encyclopedia of Fishes. San Diego: Academic Press, 1998. Ed by Randall, David J., and Farrell, Anthony P. Deep Sea Fishes. San Diego: Academic Press, 1997. Thorne-Miller, Boyce, and Earle, Sylvia A. Ocean. San Francisco: Collins Publishers, 1993.

Engage: Get the students to share any knowledge they may have in regards to bioluminescence, pressure in the deep-sea, and importance of taking advantage of scarce breeding and feeding opportunities.

Preparation: Pictures of deep-sea fish displaying the adaptations should be up and ready. These pictures can be found in many of the references provided in the background information section listed under "references." Also, there are many web sites displaying such pictures. They are easily found by doing subject searches. The film should be prompted and ready to play. The pin-up activity should be set up. Materials for the design your own fish activity should be at the ready. The bioluminescent communication materials should be laid out and the area cleared and prepped. Also, the bag of colored marbles should be prepared for the "scarce feeding opportunities" activity.

Procedure: Activity 1:

1. Prepare the students for the glow stick activity. The classroom should be arranged so that there are not any obstacles in the center of the room.
2. Give each student a glow stick. It is necessary to bend them so that the chemical reaction is activated and the stick begins to glow.
3. Choose a couple of students who will get to be the predators the number of predators will depend on the size of the class. For a class of 25, there should be about 3-4 predators.
4. Divide the other students into groups of 4. Each group represents a different species of bioluminescent organisms.
5. The students can come up with secret flashes on their stick so that they know their group from another group. The goal is to not get eaten by the predator.
6. Send the predators into the hall.
7. The groups should be broken up. The groups should try and find each other so that they can reproduce. The students should try and find the other members of their group based on their secret flashing code.
8. Turn all the light off and let the predators into the room. The predators are going to tag anyone they can. If a student gets tagged, then they must they are out and must wait until the next game starts.
9. If two members of the same group find each other, then they can go release another member of their group that has been tagged.
10. The game will end if the predator tags everybody.
11. This can be done a number of different times and the different people can play the predators.

Activity 2:

1. This will first involve asking for a few volunteers.
2. This activity will demonstrate the difficulty of finding a mate in the deep-sea.
3. One at a time, the students will be blind folded and spun around.
4. They will then be asked to pin the male anglerfish on the female.
5. Relate the difficulty that they had to the actual difficulty of the mating in the depths.

Activity 3:

1. Fill and opaque bag with 50 blue marbles and 5 red marbles.
2. Have each student reach into the bag, select one marble, write down the color of the marble and replace the selected marble back into the bag.
3. Once each student has selected a marble inform them that if they chose a red marble they got to eat that week.
4. Then discuss the scarcity of finding prey with the students.

Activity 4:

1. Have each student fill a two-liter soda bottle with water.
2. Have the students fill their test tubes half way with water and quickly invert into the two liter bottle so that the air rises to the back of the test tube.
3. Cap the bottle tightly and then have the students squeeze the bottle and watch the test tube sink.
4. Explain that this is the same principle that does not allow gas bladders to work at high pressures. Thus creating the need for lipid filled bladders.

Activity 5:

1. Pass out the necessary materials for the students to design their own bioluminescent organism. Glow in the dark paint and glue is available. After the pictures are drawn and cut out, glue (or tape) a Popsicle stick on the end so that they can be held like a lollipop.
2. Show and tell each studentÕs animal. 3) End the lesson with a worksheet on bioluminescence, or a small quiz to check and see what they have learned.