Tuesday, June 11, 2013

The Importance of Sea Cucumber Poop!

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Awesome pic by Mevallee
Let's just admit it. Poop is awesome. Its natural, all animals do it and its frakkin' hilarious. PLUS its biologically important to everyone so we can't just stop talking about it..

I have written about the importance of the poop of other echinoderms in prior posts-particularly this one about green sea urchins (Strongylocentrotus droebchiensis)

Recent research has brought a powerful spotlight on not just the ecological, but the overall, importance of sea cucumbers to the environment.  Sea cucumbers occur all over the world and at all depths. Often, when present, they are abundant or at least a significant part of the fauna present.

But the key dynamic present to their importance is that they cycle or process what they eat and what they defecate contributes to the health of the habitat they inhabit.

1. Sea Cucumber poop buffers against ocean acidification on coral reefs
2008-11-26 SAPONA WRECK - Sea cucumber poop
Image by scuba.linda
A paper by Kenneth Schneider (Stanford University) et al., including echinoderm researcher Maria Byrne's  was published in the Journal of Geophysical Research (2011. 116: G04032)  received a lot of press, such as this account (in Australian Geographic) and this one (which has an interview with Byrne) about how sea cucumber "poop" is important to geochemical processes on a coral reef.

I realize that articles about "coral reefs saved by sea cucumber poop" sound kind of silly on the surface, but read and understand below.... (note also the Journal of Geophysical Research? Important stuff gets put in there.)

Coral has to develop or accumulate calcium carbonate, which is the mineral used to compose coral skeletons, at an equal or better than the rate at which the coral loses calcium carbonate via erosion, natural dissolution, etc.

A survey of the sea cucumbers Stichopus herrmanni and Holothuria lecuospilota in One Tree Reef, Australia showed that the sea cucumbers could digest and dissolve so much of the adjoining sediment and rubble (ie the sand) that they actually contributed up to 50% or MORE of the total amount calcium carbonate dissolved over a night time. Presumably this was made available for coral to use for reef development.

Chemically, calcium carbonate is very alkaline or basic. So, sort of like an antacid. What do you do when you have stomach acids that are misbehaving? Drop some of those tablets to "cancel" out the acidity.

So, sea cucumbers contribute calcium carbonate to the coral reef's "chemical budget". They act like a natural antacid to neutralize other acidic environmental sources. Under normal conditions, there's an equilibirum. The abundance or number of sea cucumbers can affect this.

Thus, in theory,  MORE sea cucumbers might produce so MUCH alkalinity (or "basic" poop to the water) that conceivably they could function as a control or at least a buffer against increases in more acidic sea water.  This obviously is important when you consider ocean acidification resulting from global warming.  Sea cucumber poop is an important part of helping to keep the geochemical balance of a coral reef in equilibrium.

2. Sea Cucumbers EAT tasty bottom poop and clean it up!  
Poop is processed into useful nutrients! Over abundance of nutrients (i.e eutrophication) is broken up by sea cucumber feeding!

A recent paper in PLOS one from Thomas MacTavish and colleagues in New Zealand studied a local sea cucumber Australostichopus mollis and how its presence affects the nutrient cycling in its surroundings.

MacTavish and his colleagues studied a nutrient-rich environment covered by algae, mussel feces and other nutirent-rich goodies. Under normal circumstances, these would build up bacteria, ammonia and other factors creating conditions that contribute to the growth of  algae, which ultimately chokes everything else out (aka eutrophication).

But you put a sea cucumber into these settings? They LOVE it! They eat and all sorts of good things happen:
  • Bacterial abundance increases
  • Organic material (i.e., the goo) begins to decompose more quickly
  • Organic materials are redistributed from the marine sediments into the water
Sea cucumbers help to break down organic material and redistribute the nutrients! The poop is an important part of that process.

This has implications....

3. Eating good poop cleans up aquaculture environments
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Image by Jeremy and Christine
Papers such as the one above, this one focusing on the tropical Stichopus japonicus and this one on Parastichopus californicus in cold-temperate waters all show that many people have picked up on the fact that sea cucumbers are useful animals for breaking up environments that suffer from being choked in nutrients.

Eutrophication-the overabundance of nutrients resulting in undesirable growth of algae and hypoxia-is a common problem in aquacutlure ponds.
Sea Cucumber aquaculture
Image by Smartfish-ioc
But putting a sea cucumber into the mix? A critter that LOVES organic nutrients and gooey stuff like that?  It would go to town! Cleaning up the bottom and cycling those bottom nutrients...  Seems like a win-win solution for cleaning up the bottom of say a fish or mussel farm where feces from the animals accumulate in huge amounts.

So yes. Sometimes sea cucumbers eat poop. And then poop poop, which is probably "cleaner" than what went in the first place...

4. Sea Cucumber poop is good for plants (mangroves, seagrass, etc.), which are part of a healthy ecosystem
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Image by Eunice Khoo- "Mermate"
So, by this I don't just mean ONLY the poop-but the animal digesting and then processing the sediment.  This follows everything from the above-they break down organic detritus and make the nutrients available to the water column preventing hypoxia and other bad things going down in the sediment...
(its a great video, but I didn't enter the description!)

The nutrient cycling role of sea cucumbers has been observed as an important part of ecology. One post I put up awhile back shows that the presence of sea cucumbers leads to more productive sea grass!  and thus a more diverse and healthy tropical ecosystem.

Think of them as earthworms! go through the bottom sediments, eat all the organics and leave the sediment.. that's sea cucumber poop!

5. Deep-Sea Cukes have pretty diverse microbial faunas that live in their guts! (and thus their poop!)

Deep-sea sea cucumbers perform very much the same kind of function as the shallow water ones. They live in much finer mud and are often rained upon by nutrients from the surface. Many of these critters, such as Molpadia (shown here) live buried in the mud.

Most of their overall morphology seems devoted to processing mud..in one end and out the other...
We add to that another spin!  There are whole microbial faunas that live INSIDE their guts! Go to these past posts to read more about them..

Remember just how abundant these can be in the deep sea. Some occur at a density of 220 individuals per square meter!

How much of this fauna comes out in their poop?  How does it contribute to the local environment?

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