Sea cucumbers play key ecological roles in the marine environment
- Category: News
- Published: Monday, 20 March 2017 08:31
- Written by Allain Bourgoin and Peltin Olter Pelep
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Allain Bourgoin and Peltin Olter Pelep
College of Micronesia, Marine Science Program, P.O. Box 159, Kolonia, Pohnpei FM 96941
Sea cucumbers are a vital part of the Pohnpei marine ecosystem. They offer a free cleaning service for our sea floors, keep ocean acidity levels in check, and share a symbiotic relationship with dozens of other sea creatures. However, due to their stationary lives in shallow waters, sea cucumbers can be easily collected for commercial profit. If harvesting strategies of sea cucumbers are not managed properly, they can be rapidly overfished, leading to “boom and bust fishing.” In fact, a single “harvest impulse” can impact the resource to the extent that there are not enough adults (brood stocks) left in the wild to be able to rebuild their natural populations. If this occurs, it can take years, if not decades before sea cucumber harvesting activities can resume. In some cases, restocking the harvested grounds will only be possible through reseeding with aquaculture-raised individuals: a very difficult and expensive task. Although impulse harvesting may generate short-term revenues for the local economy, it will likely jeopardize any hope for long-term returns and put at risk the health of sea cucumber populations and the reefs and waters they inhabit.
Other than the harvesting impacts, removing too many sea cucumber individuals from their natural habitats, notably the soft-bottom and reef ecosystems, can lead to much degradation of the ecosystem in which they live. A large number of exploited sea cucumber species are deposit-feeders, gathering organic detritus and sediments from the bottom substrate. Like earthworms, sea cucumbers swallow the sediment and extract the organic matter as the sediment travels through the gut. This process contributes to the marine environment by “cleaning” the sediment of various debris and microorganisms (bacteria, microalgae, etc.). Deposit-feeding sea cucumbers play the role of a person who regularly sweeps and mops a floor to keep it clean. If this activity ceases, the floor will rapidly become sullied and uninhabitable. The volume of sediment transiting through the guts of sea cucumbers is remarkable. It is estimated a single sea cucumber can process 4 to 37 pounds of sediment per year. Imagine the tonnage of sediment that is being processed on a yearly basis!
Sea cucumbers are also known as major “bioturbators”. Bioturbation refers to the process of overturning and reworking sediment layers by organisms. This activity mixes the surface and sub-surface sediment layers, which increases the permeability, as the sediment becomes more porous and less compact. This enhances the exchange at the water-sediment interface leading to higher oxygen supply in deeper sediment layers. In turn, this has beneficial effects on the infauna (organisms living within the sediment) community.
Another advantage of maintaining the presence of high densities of sea cucumber populations in their natural environment is their potential to modify the seawater chemistry. At a local scale, through their feces, sea cucumbers can increase the seawater alkalinity, which means that the seawater can become less acid. The oceans are slowly acidifying through global climate change. Acidification of the seawater will potentially have drastic long term effects on the degradation of the coral reef structure since acid dissolves the coral rock. By helping to prevent seawater from becoming more acid, sea cucumbers contribute to protect the reef structure itself.
Sea cucumbers are also the host to many small species of various taxonomic groups who either live on the body wall of sea cucumbers or inside the individuals. Without the sea cucumbers as a host, many of these creatures would simply be absent from the ecosystem. In turn, this variety and range of species forming symbiotic relations with sea cucumbers increase the total ecosystem biodiversity.
Finally, sea cucumbers serve as food supply to a number of predator species, thereby acting as a link between the microalgae and organic detritus (the sea cucumber diet) to the consumers of the higher trophic levels. Many of these predators are themselves fished by humans. In this way, sea cucumbers form an indispensable link in the marine food chain and their absence could have a cascading effect on other fisheries. In other words, without sea cucumbers we may have fewer fish to eat!
In summary, sea cucumbers offer many services to their ecosystem, notably: improving sediment quality, modifying the water chemistry, increasing biodiversity, and offering a link in the transfer of energy to higher trophic levels in the marine food webs. It is likely that large-scale depletions of sea cucumbers through excessive fishing will affect the productivity and diversity of the soft-bottom and coral reef habitats. Fisheries managers should consider these ecological benefits when developing and implementing policies relative to sea cucumber harvest. It could even be debated that the economic returns gained by protecting the sea cucumber stocks would largely outweigh the profits obtained when exploited unsustainably.
For more information relative to this subject refer the following scientific paper:
Purcell S.W., Conand C., Uthicke S., Byrne M. (2016). Ecological roles of exploited sea cucumbers. Oceanography and Marine Biology: An Annual Review 54: 367- 386.