Marine Biodiversity

An Essay By Hannah D. // 12/16/2015

Part of developmental biology studies how the environment affects the diversity and development of various organisms. Such factors can be abiotic, biotic, or symbiotic. Here, I focus on oceanic ecosystems and organisms to epitomize each type of factor and how they (in these cases) improve biodiversity.

Corals reefs are one of the most diverse ecosystems on the planet; nevertheless, their continued existence has long been a paradox of ecology (Rougerie, Fagerstrom, Andrie, 1992). Tropical reefs grow in warm, clear, shallow waters near the equator. This is one of the most nutrient-deficient habitats in the oceans. The solution? Corals have a symbiotic relationship with zooxanthellae, a type of plankton from which the coral receive many nutrients. The photosynthetic plankton also require the corals to stay near the sun, in the warm, shallow waters deprived of nutrients.

There are two types of natural vertical currents in the ocean. One flows upwards, and the other flows downwards. Guess which one exists in the tropical waters in which coral reefs live? Corals live where there is downwelling; in nuritious waters, upwelling occurs, in which cool, nutrient-rich waters from the ocean floor flow upwards to feed rich ecosystems such as California's kelp forests. The downwelling near coral reefs takes already nutrient-deprived waters and sends them down, blocking any influx of nutrition from the deep sea. Yet somehow, coral reefs flourish in biodiversity and maintain a nutrient flow that nets in output.

That is to say, they export more nutrients than they appear to import.

That's as odd as if America made trillions of dollars in profits (rather than trillions of dollars in debt) due to her spending more than she is selling. This strange feature of coral reefs has long been seen as an ecological enigma, which a group of researchers set out to investigate.

Many coral reefs are situated around volcanic islands (or around sunken volcanic islands). The researchers found that these volcanoes actually bring with them a source of upward water flow rich in nutrients called geothermal endo-upwelling (Rougerie, Fagerstrom, Andrie, 1992). Since these currents aren't part of the natural ocean current system, but are caused by heating from the volcano, they had previously gone by unnoticed. Now, however, we know that the abiotic factor of volcano-sourced nutrients sustains the biodiversity of many coral reefs in the oceans.

Another problem presented by the marine environment is that of invertebrate colonization. Many macroscopic invertebrates have microscopic larvae, and these larvae travel on the whims of ocean currents before they anchor somewhere and colonize a new habitat. The question is: how do they find a spot that is ideal? When researchers tested three invertebrate species (who are found in the same habitats together) in the larval stage, they found that each of the larvae species colonized when they recognized they were in the presence of a larvae of another of the three species tested (Verschut, Meineri, & Basset, 2015). Here, it is the biotic factor of sensing a community of familiar organisms that draws the larvae to colonize in a particular place. Somehow, the microscopic larvae know that when their companion species are around, it's a good idea to anchor down and grow into the macroscopic realm. This biotic factor has a major influence on the dispersion of the three species - including sea snails and echinoderms (like sea urchins and starfish).

Zooxanthellae are a very social creature, and are not friends with coral alone. They are also symbiotic with sea anemones, where they are transmitted vertically so that zooxanthellae exist within the zygotes. Vertical transmission is the process by which the sea anemones pass on their symbiotic relationships to their offspring; the zooplankton are actually present at the very first stages of sea anemone life, at the gamete and zygote stages. (The other form of passing on symbiotic relationships with microbes is horizontal transmission, which is more like catching and spreading a cold. But a good cold.)

Again, these zygotes are dispersed to new environments due to ocean currents, including areas where zooxanthellae don't normally live. By vertical transmission, however, symbiosis can be carried out just about anywhere the larvae settle down to and allow them to develop, grow, and flourish (Davy & Turner, 2003).

These three studies show how abiotic factors affect the biodiversity of corals, how biotic factors affect colonization behavior in some marine invertebrates, and how symbiotic factors affect the life cycle and healthy development of sea anemones. Environmental influences play a major role of development and biodiversity, and in the oceans no less than in any other habitat on earth.


Rougerie, F., Fagerstrom, J. A., & Andrie, C. (1992). Geothermal endo-upwelling: A solution to the reef nutrient paradox? Coral Reef Oceanography, Vol. 12(7). Retrieved December 15, 2015 from

Verschut, T. A., Meineri, E., & Basset, A. (2015). Biotic interactions affect the colonization behavior of aquatic detritivorous macroinvertebrates in a heterogenous environment. Estuarine, Coastal and Shelf Science, Vol. 157:120-128. Retrieved December 15, 2015 from

Davy, S. K. & Turner, J. R. (2003). Early development and acquisition of zooxanthellae in the temperate symbiotic sea anemone Anthopleura ballii (cocks). Biological Bulletin, Vol. 205(1). Retrieved December 15, 2015 from


Fascinating read!

Fascinating read! I've never heard of all those details on coral reefs. Amazing stuff.

The colonization of the micro larvae was especially interesting, as I've asked myself the same question: how do they know where to "anchor down"?

The essay itself was well written, as always. There was one sentence that appears to have an extra word, "take": "That is to say, they export more nutrients than they appear to take import."

Arthur | Fri, 12/18/2015

"My greatest wish for my writing is that it would point you to the Savior."

Thanks so much, Arthur! And I

Thanks so much, Arthur! And I just fixed that. : P

Hannah D. | Fri, 12/18/2015

"Reason itself is a matter of faith. It is an act of faith to assert that our thoughts have any relation to reality at all." - G. K. Chesterton


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