Saturday, April 21, 2012

Relaxing in the Rectum

Many organisms living in the sea benefit from a symbiotic relationship with other organisms around them. One example of this is the relationship between the sea cucumber and the pearl fish. Sea cucumbers are echinoderms that have a shape very similar to soft cucumbers (1). They have “leathery skin” and their body is essentially one long digestive tube. These invertebrates crawl along the floor of the ocean and can reside in both shallow and deep areas of the ocean. They are omnivorous and usually eat small particles and detritus they find on the ocean floor. The sea cucumber can grow between 2 to 200 cm and can live to be between 5 and ten years old (2). Spawning for the sea cucumber usually occurs between June and August. The sea cucumbers release their sperm and eggs in the water and allow fertilization to take place. These fertilized eggs continue to develop into larvae, which become suspended in the water column for up to 70 days. Eventually, these planktonic forms settle on the ocean floor, where they change into tiny little juvenile sea cucumbers. Growth is slow for these invertebrates, and it can take up to 4 years for the cucumber to reach adult size (3).

The symbiont of the sea cucumber is the pearl fish. Pearl fish are “eel-shaped” fish that are found worldwide, but primarily in tropical, shallow areas around coral reefs. They have long, slender bodies that lack scales and they usually have transparent skin. The tail of the pearl fish is long and pointed, and the anus of this fish is located close to its neck. Pearl fish typically grow to be about 15 cm long (4). The female pearl fish releases clumps of eggs late in the summer to begin the life cycle. These eggs rise to the surface and hatch, turning into a specific type of larvae called vexillifers. These larvae live among the plankton until reaching a length of about 7 to 8 cm. At this point, the larvae develop into tenuis. These forms descend to the ocean floor and begin their search for food and a host (5). The pearl fish will constantly be on the lookout for sea cucumbers to create a symbiotic relationship.

Description of Relationship:
Once a pearl fish (Onuxodon or Carapus) finds a sea cucumber (Holothuroidea), it immediately begins to smell around to distinguish between the head and the anus of the cucumber (6). Once it finds the anus, the pearl fish works its way into the rectum of the sea cucumber, eventually being completely engulfed in the digestive canal of its host. There it will spend the day inside, using its host as a form of protection. At night, the pearl fish comes out to feed on small crustaceans, but it doesn’t go too far from its host (7). After feeding, the pearl fish returns to its host and waits for the sea cucumber to take a breath. When the anus opens for respiration, the pearl fish simply swims back inside, seeking shelter in the rectum of its host (8). The pearl fish and the sea cucumber have evolved a symbiotic relationship know as commensalism. In this relationship, the pearl fish benefits because it gains a place to live that is cozy and protected from predators as well as any nutrients that can be absorbed as they flow out of the cucumber’s anus. Meanwhile, the sea cucumber appears to be unaffected by this relationship. It doesn’t even seem to notice the pearl fish entering its anus. As far as we know, the pearl fish is not taking anything from the sea cucumber. The reproductive success of the cucumber remains the same. Therefore, since the pearl fish benefits and the sea cucumber is neither helped nor harmed, one can argue that this relationship is one of commensalism. Many other organisms have benefitted from relationships similar to that of the sea cucumber and the pearl fish. The pearl fish have also learned to penetrate the bodies of other invertebrates such a starfish, sea squirts, and clams. A number of crabs and polychaete worms have also evolved to live inside sea cucumbers and have become specialized for gaining protection from the cloaca of that host (9).

Cost/Benefit Analysis:
The relationship between the pearl fish and the sea cucumber is not obligatory, but the pearl fish benefits from its symbiosis with the sea cucumber. The costs for the pearl fish in this relationship are very small. They must expend energy searching for a sea cucumber and for wiggling their way into the anus of the host. On the other hand, the benefits for the pearl fish are immense. The fish is provided protection from predators when it resides within the rectum of the cucumber. They also can take up any nutrients found in the waste products being excreted from the rectum of the sea cucumber. Another benefit for certain species of pearl fish is that they can use the rectum of the sea cucumber as a place to develop into their adult forms and complete their life cycle (9). While the pearl fish is provided with shelter, the sea cucumber has nothing to gain from this relationship. Nor does it have anything to lose. Both the cost and benefit for the sea cucumber is nothing. The reproductive success of the sea cucumber is not affected and the pearl fish benefits, it is likely that this relationship will persist as commensalism. Eventually, there may even be an adaptation that would benefit the sea cucumber, creating a relationship that is more like mutualism.

8. echinoderms/sea_cucumbers

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