Symbiosis in Marine Ecosystem By Yumna Mahmood

No system can work on its own. Every being and every object needs a partner. Let’s start with our bodies. Teeth and tongue together masticate the food we eat and form a bolus. People with no teeth can’t rely on their tongues alone to form a bolus. Similarly, clouds are gathered by the wind in the sky to cast shade. Clouds can’t move without wind.

All around the world, someone is either helping or being helped. High above in the sky and deep down in the oceans, buddies are helping their friends and mates around the clock.

The scientific terminology for “help” is symbiosis. Symbiosis can be defined as the persistent association among members of different species. This persistent relationship could be:

  • Beneficial to both species; called mutualism.
  • Beneficial to one species only while the other is neither benefitted nor harmed; called commensalism.
  • Beneficial to one species while causing harm to the other; is called parasitism.

3 of the 4 parts of the world are covered with oceans, forming the Marine Ecosystem which harbours millions of species in it which together play a huge role in maintaining the vitality of the ecosystem. Every organism is linked to the other via life-long biotic relationships of give and take, as described above.

Discussion about marine ecosystems would be incomplete if corals and coral reefs are excluded. The sedentary animals that stand like guards on the borders beneath the layers of the ocean, are homes to many animals, exhibiting the best example of symbiosis. The well-known pairing of clownfish and sea anemone (Coelentrate) is an example of type 1 symbiosis that is called mutualism, as explained above. The clownfishes are not harmed by the paralyzing tentacles of the sea anemone. Instead, the fishes are protected from their enemies which could be other carnivorous fishes while the anemones sting the fishes that try to feed on the clownfish residing in the vicinity of their stinging tentacles.

Though no one can think of pairing up with the largest and the most aggressive marine fish, the sharks. However, remoras have somehow managed to be listed in their good books by feeding on parasitic copepods and skin scrapings. While the sharks, as a token of their gratitude, let them ride for free on them and feed on their prey, passively though. This mutualistic symbiosis has been found among remoras and whale sharks.

Sponges that are sedentary animals, appear in symbiotic relationships with microbes in deep seas. In their publication related to Sponge-microbial symbiosis, Praise Tochukwu Nnaji et al discussed the symbiotic relationship between marine sponges and microorganisms. According to the publication, sponges being porous, harbour bacteria, providing them with a place to reside. In turn, the bacteria (Pseudomonas, Vibrio, Micrococcus, etc) help in providing nutrients to the sponge by digesting the particles it filters. This mutualistic symbiosis helps both of them to survive the harshest environment as bacteria can safely inhabit the body of the sponge while the latter can get ingested particles digested.

Hermit crabs that are known to inhabit empty shells of molluscs, pair up with sea anemones to protect themselves from their predators. The sea anemones are lifted by the hermit crabs and placed on their shells. When its predators attack its shell, the arsenal of stinging cnidocytes of anemone attack in defence, forcing the predator as big as an octopus to let go of the crab. In turn, the sea anemones visit different regions where they can feed.

Just like we have carwash in the human world, some marine species also indicate the availability of fish cleaning services. Sea anemone species Bartholomea annulata plays a visual role in indicating the presence of cleaner shrimps Ancylomenes pedersoni which attracts the fish towards the reef wanting to be cleaned. While researching this topic, Lindsey K. Huebner and her colleague found that even if the shrimp is not visible on the reef, the fish would swim toward the reef containing anemone spp.

“These visual cues facilitate fish cleaning interactions with shrimps, and provide a previously unknown symbiotic benefit to cleaner shrimp from association with sea anemones.”

Similarly, the cleaner fishes in the oceans are a major part of the marine ecosystem. They could be obligate cleaners, staying all the time with their partner species, or they could be facultative cleaners, which could be thought of as part-time cleaners of some fish. Their behavioural studies, as conducted by Sampio et al and discussed in their publication related to cryptobenthic fishes, suggest that the fish that desires to be cleaned, such as the bridled goby, moves its pectoral fins in such a way as if demonstrating the desire to be cleaned. As a response to behaviour, French angelfish swims to its lateral regions and starts pecking it. The fish stays still while being pecked, in a headstand pose. This exemplifies mutualistic symbiosis, as the French angelfish get its meal from the fish they clean such as Coryphopterus glaucofraenum- bridled goby, while the latter gets rid of the ever-unwanted ectoparasites.

Think of a huge baleen whale with barnacles on its elongated snout, tail fin, and head. Barnacles are enjoying a free ride over the world’s largest mammal while the whale itself appears to have no problem with their colonies over it. Different species of barnacles are commensal over different species of whales. Coronula diaderma commensals humpback whales only while on grey whales Cryptolepas spp. is found.

Other marine commensals are sea cucumbers and pearlfish. The pearlfish inhabit the intestine of the sea cucumber the whole day and come out to feed by night time only. The sea cucumber provides it protection from its predators as well as from being digested in its gut. Sea cucumber doesn’t appear to be benefited much though, but at least it’s not consumed or parasitized by the fish.

Marine animals exhibit parasitic symbiosis as well. Ciliates which are microscopic unicellular animalcules are found to be in parasitic symbiosis with clams, and two-shelled animals or bivalve molluscs. Folliculinids cause skeletal deformities in corals.

Even huge marine mammals are not safe from parasites. The macroparasites that parasitize mammals are endoparasitic worms. Sirenians have digenean as the dominant endoparasites. Cetaceans harbour Cestodes and Nematodes along with Digeneans.

As for vertebrates such as eels, Gadus spp. is parasitized by Vibrio anguillarum. Carl J. Sindermann in his publication related to marine parasitism writes that Vibriosis is hemorrhagic septicemia. The bacterium causes anaemia, resulting in the pale colouration of the gills, skin lesions also appear that penetrate deep into the muscles resulting in muscle necrosis. Moreover, the fish’s spleens get enlarged and the liver and kidneys are also adversely affected, leading to death, ultimately.

Just like humans, fishes have lice as their ectoparasites. These ectoparasites are brachiurans such as Argulus or copepods such as Lernae. Argulus spp. is an oval, dorsoventrally flattened parasite with a stylet that acts as a piercing mouthpart. As they grow into adults, they develop hooks on their appendages and suckers on their oral region for firm adherence on their host as it swims in the ocean. The fishes appear irritated, flashing, and trying to get rid of the pinching parasites by rubbing themselves on rough surfaces. If the infestation is heavy, the fish may lose scales, appear lethargic, and avoid swimming deeper in the water.

The above discussion and examples of the symbiotic relationship provide a clear view of what is happening between the fishes and other marine animals which appear to crawl or snuggle to us, while they work out to maintain the balance in the ocean. However, eye-opening research by T.L.F. Leung and R. Poulin advocates seeing these relationships as elastic ones rather than strictly as defined in textbooks. According to their publication, the relationship depends on the circumstances in which it takes place and should be viewed keeping them in view.

They support their argument as, “However, in practice commensals are indistinguishable from parasites that cause their host comparatively low levels of pathology, and mutualists that exert such a heavy price for their service that is barely compensated by the benefits of the association.”

They further state, “While according to our labels, these organisms (parasites and mutualists) are from opposite ends of the spectrum, in reality the lines are blurred and the position they occupy along the theoretical continuum may be highly variable and circumstantial.”

The supportive example of their argument is the life cycle of digenean trematode which requires coral polyps as its intermediate host and butterfly fish as its definitive host. The trematode Podocotyloides stenometra infects coral polyps and causes them to appear plumpy, pink, and swollen, making them recognizable to the butterfly fish Chaetodon multicinctus which feeds on infected corals, ingesting the trematode as well, thus completing its lifecycle. This cycle can be viewed as parasitic symbiosis if the relationship between the coral and trematode is considered but appears mutualistic if the relationship between the butterfly fish and trematode is observed. Hence, it can be said that mutualism and parasitism are two faces of a coin.

Given the above-stated example, it's an appeal to take the examples quoted in this essay as a compilation of the phenomena occurring in the deep waters rather than fixed parameters to assess symbiosis between marine animals. We rely on hardworking researchers and scholars to update our knowledge and concepts regarding the interactions taking place in the wild, be it open air, wide lands, or deep blue waters.

by Yumna Mahmood