Death of Corals Linked to New Source

Coral reefs are a notoriously fragile indicator species. Yes, those brightly colored underwater structures you may have seen in tropical seas are not rocks or plants, but actually billions of tiny invertebrates called polyps who secrete a hard calcium carbonate test (or shell). As the organisms reproduce, their offspring build their home on the wall of their parents test, thus adding on to the structure over time. These little inverts are aptly called  ecosystem builders as the structures they create provide food and shelter for an array of colorful fish, turtles, crustacea, enchinodermata, and annelidea to name a few.

Biodiversity of a Caribbean coral reef. Credit

Yet, as beautiful and important as they are, corals have been experiencing die off (bleaching) in the last 40 years, to the point where some reefs have declined by over half their original size. Scientists argued over the cause of this bleaching phenomenon; many blame the warming waters due to Global Climate Change. Others point to acidification of the oceans, pollutants, disease, or physical destruction as the cause.

The polyps emerge from their CaCO3 shells in order to filterfeed. Credit

However, new research points to a different culprit altogether, and luckily, this cause is much easier to address than Global Warming or non-point pollution sources. A new report compiled by Global Coral Reef Monitoring Network, the International Union for Conservation of Nature, and the United Nations Environment Programme entitled the Status and Trends of Caribbean Coral Reefs: 1970-2012 has put forth that the real destroyer of coral reefs is overfishing. Specifically, the overfishing of urchins and parrot fish.

 

Bi-color Parrot Fish. Credit

Long Spined Sea Urchin. Credit

These creatures are herbivores that feed mainly on green algae, like the kind that grows on reefs in tropical seas. This algae is a real problem for the corals because it blocks the sun. Coral polyps themselves are not photosynthetic, but their tissues host a symbiont zooxanthellae that is. These tiny, unicellular algae produce sugars which they then share with their host in exchange for protection. Around 90% of the polyps nutrition comes from their symbionts, so it is easy to see how a thick covering of algae over the reef would hinder the photosynthetic process of the zooxanthellae and consequentally, starve and kill the polyp.

What a tragedy to lose this stunning ecosystem. Credit

 Because parrot fish and urchins are relentless grazers, they clear away the algae layer so that they entire system continues to run smoothly. However, removing these herbivores from the system (by fishing industry or for sport) is beginning an unfortunate trophic cascade in which algal layers smother coral reefs and polyps starve.

This sad story is a reality in the Caribbean Sea where overfishing threatens to destroy the $3 billion tourist industry. Local Caribbeaners have begun reef restorations in hopes of bringing in corals to fill in the gaps, and the US government has banned the fishing of parrot fish in the US Virgin Islands in an attempt to increase their numbers and stop the algal accumulation. However, stricter measures like enforcing marine protected areas may help in limiting the take of urchins and parrot fish, which would keep the corals clean and alive.