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A Simple Explanation of the Symbiotic Relationships in the Ocean

Symbiotic Relationships in the Ocean Explained
Symbiotic relationships are very common in oceans that boast of a highly diverse ecosystem. Here is ScienceStruck's compilation of different types of marine symbionts and their relations.
Sonia Nair
Last Updated: Mar 26, 2018
The pearlfish shares a bizarre relation with the sea cucumber. The translucent, eel-like fish lives in the anal cavity of the sea cucumber so as to escape predators. Though the relation is commensal, as the host remains unaffected; sometimes, the fish may turn parasitic and feed on the gonads of the sea cucumber.
Two-thirds of the Earth's surface is covered with marine waters. Marine ecosystems are home to thousands of plant and animal species. There are more than 17,000 species that inhabit the deep seas where sunlight does not reach. In short, marine ecosystems exhibit a tremendous biodiversity. As in case of any other ecosystem, the organisms found in oceans interact with one another in different ways.

The term symbiosis is defined as the close and long-term interaction between two living species that live together. The two species that share a symbiotic relationship are called symbionts. Such a relation may be beneficial for both species that share the relation, or for any one of them. Sometimes, the relation may prove harmful for any one of the partners. If both species benefit, the relation is termed mutualism. If only one symbiont benefits and the other suffers, the relation is called parasitism. If one partner benefits without hurting the other, who does not gain anything from the relation, it is called commensalism. If members of same or different species compete with one another for resources, it is called competition. Predation is another type of symbiotic relationship, wherein the stronger predator species feed on the weaker prey species. Marine ecosystems witness some of the most amazing, bizarre, as well as interesting symbiotic relationships.
Burrow Buddies
Steinitz shrimpgoby
Blind shrimp and goby fish
Some types of goby fish and shrimp share a mutualistic relationship. Usually, a goby-shrimp pair lives together in a burrow made by the latter. The partially blind shrimp is protected by the sharp-eyed goby. In return, the goby gets a shelter for resting and laying eggs by sharing the shrimp's burrow. Though blind, the shrimp can sense the sudden retreat of the goby into the burrow, and it follows the fish. The shrimp uses its antennae to keep in touch with the goby, and the goby uses its tail to signal danger.
Cleaning Freaks and Clients
Morey eel
Cleaner shrimp working on a Mediterranean moray eel
Cleaner shrimp engage in cleaning other organisms by removing parasites and dead cells from the latters' bodies. These shrimp are often found in large groups near cleaning stations, where large fish, turtles, etc., visit for getting themselves cleaned. The shrimp cleans the skin as well as the mouth and gills of its client. While the shrimp derives nutrients by ingesting the parasites and dead cells, the client fish benefits by getting itself cleaned. Though client fish are strong predators, they spare these cleaner shrimp. The image shows a Mediterranean moray (Muraena helena) with a cleaner shrimp (Lysmata seticaudata).
Scary Hosts and Glowy Pals
Long-lure Frogfish
Female anglerfish has a filament with a bulbous tip to lure prey.
Many species of deep-sea anglerfish share a mutualistic relation with luminous bacteria. The female anglerfish have at least one filament on its head. The tip of the filament has a bulbous structure which houses the bacteria. The fish moves its filament with the luminous tip, so as to attract prey as well as potential mates. While the bacteria emits light, the fish offers protection and nutrients in return. It has also been suggested that the bacteria are dependent on the host for synthesizing chemicals required for luminescence.
Green turtle from down with two remoras
Remoras attached to a turtle
Most of the hitchhikers of the ocean share a commensal relationship with their hosts. One classic example is the remora or suckerfish. These fish attach themselves to sharks, manta rays, and turtles so that they can travel long distances without losing energy. The remora has a sucker-like organ on the top of its head; and it sticks to the body of the host, using this organ. So, remoras benefit from the host, while the latter remains unaffected. It has also been suggested that apart from enjoying the free rides, remoras clean the skin of their hosts. So, the relation can be mutualistic too. Another hitchhiker is the emperor shrimp, which rides on sea cucumbers and large nudibranchs. It has been found that the shrimp feeds from the bottom of the ocean, as its host moves slowly. It may hide under the body of its host, to escape predators.
Emperor Shrimp rides nudibranch
Emperor shrimp rides on a nudibranch
Spiny Protectors
Cardinal fishes
Cardinalfish among the spines of sea urchins
Certain species of cardinalfish exhibit a commensal relation with sea urchins. The image shows the Banggai cardinalfish (Pterapogon kauderni) and a species of long-spined sea urchin (Diadema setosum). The fish with black stripes can easily hide in between the black spines of the sea urchin. Even if the predators locate the fish, they spare the latter because of the venomous spines of the urchin. Apart from that, these cardinalfish are immune to the venomous spines of urchins. It has also been noted that these fish do not trigger the feeding reaction of their hosts.
Tongue Eaters
Fish Tick
A parasitic isopod settled on a fish tongue
The tongue-eating parasite (Cymothoa exigua) is an isopod that feeds on the tongue of the host fish. The male isopod attach itself to the gills of the fish, while the female settles on the tongue. It has been suggested that these isopods are born as males, and the one that enters the mouth (through the gills) of the host and settles there, turns female. It feeds on the tongue and attaches its body to the tongue stub. The host fish uses the parasite as its tongue. The parasite continues to feed on the blood and mucus of the host.
Parasitic isopod on hawkfish
An isopod attached to the gills of a hawkfish
When it comes to competition, anglerfish compete with each other for food, as the sources are scarce in the deep sea. Likewise, porcelain crabs may compete with one another for territory (sea anemone). When it comes to predators, marine ecosystems are home to deadly predators like sharks and killer whales. In short, some of the most amazing symbiotic relationships are seen in marine ecosystems. This is only a brief overview about symbiosis in the oceans. You may conduct an in-depth study to understand more about such relationships in marine waters.