Symbiotic Relationships as a Valuable Adaptation to Survive in Extreme Environments

In the deep sea, organisms must be adapted to survive stresses absent in surface waters such as extreme pressure, lack of light, food scarceness, toxic chemical flows and other factors that make it difficult for life to thrive. Symbiotic relationships – the interaction between two different organisms that share a close association with one another - allow for organisms to better survive predation, starvation and habitat variability. In cases involving mutualism, both party’s benefit from the association in a variety of ways. By studying these diverse species interactions, it can be revealed how fit organisms are to evolve and acclimatize in this extreme environment in order to persist here, by co-depending on or simply exploiting some of each other’s features.

In well-established mutualistic relationships, species can shape their morphology to support their interactions. Such can be seen in the case of the hydrothermal vent shrimp Rimicaris exoculata. The crustacean has a dense setae coverage in its mouthparts which houses the bacterial epibionts – Epsilonproteobacteria and Gammaproterobacteria - it needs to oxidise sulphur and iron compounds expelled from vent fluids. To support even more of the ectosymbionts possible, the species has developed a large cephalothorax, the inner side of which supports the setae. R. exoculuta obtains a rich source of nutrition from these chemoautotrophic symbiotic bacteria, supposedly through trans-epidermal mechanisms. In turn, the ectosymbionts live in an optimal microenvironment exposed to a consistent supply of electron donors and acceptors from the vent expulsions, as the shrimp migrate to richer waters where vent currents are stronger to improve the bacteria’s productivity (Jillian Peterson). GURI studied the ectosymbiont structure throughout the various life stages in the vent shrimp. It was discovered that the shrimp interact with the bacteria from the beginning of the lifecycle, as Gammaproteobacteria in the mucus surrounding the eggs may serve as a protective feature allowing for detoxication and defense against pathogens.

Peterson study of these epibionts revealed there was variance in the dominance of the bacteria with changing geo-chemical locations of the shrimp between oxidised seawater and the chemical streams, suggesting that by comparing the distribution patterns of these symbiotic bacteria will help understand the evolutionary processes of migration, and symbiont-host associations. From these studies it is seen that the two organisms have developed a stable mutualistic relationship that enables them both to survive in this harsh environment. Allantactis parasitica and various deep-sea gastropods along the Eastern coast of Canada, at depths as far as 1100m, have established a facultative mutualism resulting from predation pressures on both species. A. parasitica select specific positions on the shells depending on their own body size. Primarily, the gastropod species’ deter predators such as Leptasterias polaris with the presence of one or more anemones present on its whorl. The anemone almost always has its tentacles protracted, creating a shielding effect that makes the gastropod even further invisible to such predators.

Mericer Hamel 2008 discovered that in 100% of cases studied, predators made no obvious effort to approach the gastropods whatsoever with anemones present. In turn, A. parasitica are protected from Crossaster papposus as the gastropods disturbed movements make it more difficult for the starfish to capture the anemone. Although the two species are not co-dependent on each other, studies have shown that throughout A. parisitica’s life cycle, even from juvenile stages, the species will chose to move to a living surface than a soft-sediment one. Other benefits of this symbiotic relationship is that the life of the anemone associated with a gastropod results greater in reproductive and growth success (reprod+settle). The researchers discovered that A. parasitica developed twice as fast as juveniles as a result of a greater supply of food made available by the upwelling of nutrients as the gastropod counterpart was moving. The species also reached its maximum adult after 6-7 years, compared to asymbiotic relatives, who reached this level of growth after at least 11 years. It has also been observed that larger anemones position themselves on the last whorl of the shell so that they have optimal access to food made available by the burrowing actions of the basibiont. Smaller anemones, including juveniles, are situated further from the ground as they aren’t as likely to be pushed off the shell or suffocated by the amounts of particles disturbed by gastropods movements.

It was observed during studies on Rhinoclavis articulate that 4-6 anemones could even be found in the shells groves as the siphonal canal could protect the epibionts from scraping as the gastropod travelled, A. parasitica was also seen to synchronise spawning periods to that of gastropods in 52% of cases studied by Reproduction People. The congregation of species such as Neptunea despecta and Colus stimpsoni to reproduce resulted in anemone species being in close association while positioned on the whorls. By spawning at this time also, fertilization rates were maximized. The anemone has also been affected by its basibiont’s diet as Feeding People discovered that asymbiotic counterparts had a far less diverse diet, with fewer bathhhyl organisms present in their gastrovacular cavity. This suggests that the snails sediment foraging makes different food particles available to A. parasitica, more often throughout the day. With predators deterred, gastropods can feed for longer, uninterrupted periods, and anemones can exploit a constant supply of rich food sources.

There have now been 17 evolutionary accounts of bioluminescence in ray-finned fishes in association with symbiotic bacteria, however, it is still unknown if the participants have coevolved together over time. The relations between host and bacteria are quite fluid, with the fish organs holding various, not just one, bacterial population and the microbial symbionts being found in contact with many other species. Although there is little to no host specificity between the bioluminescent bacteria and their hosts, this relationship is clearly of importance to the two parties as it has continued to develop. The fish, who obtain the bacteria from their local environment, utilise the light produced to attract prey, camouflage, defend themselves and communicate. The bacteria, who decompose luciferin compounds using luciferase in the presence of oxygen to generate light, are well housed by their hosts, who have developed many structures to concentrate, control and display the bioluminescence. Here the microbes are supplied with nutrients and a safe environment.

As intra-species encounters in the vast deep-sea are unpredictable, organisms must maximise their ability to successfully reproduce and produce viable offspring. Sex-specific bioluminescent displays have been established by female ceratioid anglerfishes as population densities are so low, males are not interacting to compete giving the female a wide selection, she must lure a mate to her in the vast surrounding waters. The females’ esca is vital in attracting prey so that she may grow into a large and comparatively immobile form. The bioluminescent lure can then motivate males to locate her, instead of using incredible amounts of energy to find them herself, with no mutualistic bioluminescent displays of their own.

Herring also reviewed that other anglerfish species, such as female Chaenophryne draco, can show variations in caudal/orbital photophores and/or the presence of extra ventral photophores to make identification easier for potential mates. This mutualistic relationship between the anglerfish and the light-producing bacteria allows species to meet at great distances from each other and have positive effects on the reproductive abilities. It has now been discovered that some of the bacteria housed in anglerfish light organs have undergone great genomic reductions compared to free-living relatives. They have lost genes in their sequence that would allow them to independently produce their own amino acids and break-down certain nutrients. The mutually beneficial relationship has clearly become stable and dependable between the two organisms. Typically, there are pores present around the light organ in which the microbes are found, allowing free-flowing movement between the tissues and the outer waters. However, species such as Candidatus photodesmus dependant solely on Anomalopidae for survival and growth. Perhaps the metabolic capabilities of such gammaproteobacteria have reduced as the anglerfish host provides all the nutrients it needs to thrive.

Symbiotic relationships are a valuable adaptation that increase survival success in the deep-sea environment. While resources and information required to better understand the mechanisms behind these organism interactions are still quite limited, the benefits and losses experienced by the species involved has become easier to conclude from studies carried out across the region, including at hydrothermal vents and seeps.