Invertebrate Herbivory
November 6, 2010 8:16 PM   Subscribe

How do invertebrates digest grasses?

Today I heard a talk about how the Spartina marsh grasses are being munched on by the square-back marsh crab on Cape Cod. Now, digesting grasses is a big challenge for mammals, typically requiring two or three stomachs, cooperative bacteria, and quite a bit of rumination. How do the crabs manage it?

I understand that other invertebrates happily munch on plants, too. Some of them suck out the juices, and plenty munch on leaves. And termites have some bacteria/gut fermentation thing going on. But is there some generalized strategy for digesting a cellulose-heavy diet that matches the mammalian solution?
posted by Jasper Fnorde to Science & Nature (8 answers total) 4 users marked this as a favorite
The guts of earthworms contain the enzyme cellulase, which breaks down cellulose. I believe that cows also have enzymes that break down cellulose, too. Humans do not have this enzyme, which is why it looks like we poop whole kernels of corn.
posted by honeybee413 at 8:27 PM on November 6, 2010

Good question. From the Wikipedia article on termites:

"All termites eat cellulose in its various forms as plant fibre. Cellulose is a rich energy source (as demonstrated by the amount of energy released when wood is burned), but remains difficult to digest. Termites rely primarily upon symbiotic protozoa (metamonads) such as Trichonympha, and other microbes in their gut to digest the cellulose for them and absorb the end products for their own use. Gut protozoa, such as Trichonympha, in turn rely on symbiotic bacteria embedded on their surfaces to produce some of the necessary digestive enzymes. This relationship is one of the finest examples of mutualism among animals. Most so called "higher termites", especially in the Family Termitidae, can produce their own cellulase enzymes. However, they still retain a rich gut fauna and primarily rely upon the bacteria. Due to closely related bacterial species, it is strongly presumed that the termites' gut flora are descended from the gut flora of the ancestral wood-eating cockroaches, like those of the genus Cryptocercus."
posted by dephlogisticated at 9:12 PM on November 6, 2010 [1 favorite]

Ah, I see you already mentioned termites. As far as I know, very few creatures other than bacteria have managed to produce cellulases, and thus the vast majority of plant-eating animals rely on some form of symbiotic gut flora.
posted by dephlogisticated at 9:14 PM on November 6, 2010

Leaf-cutter ants take their pieces of leaves back to their nest and mulch it up. This is used to grow a symbiotic fungus. The ants eat the fungus.

Cattle (and other ruminants) do not directly produce enzymes which can break down cellulose. They rely on symbiotic bacteria in their rumens to do that.
posted by Chocolate Pickle at 10:40 PM on November 6, 2010

Well, I thought I was going to get to put my specialist hat on, but it's already been answered. Pretty much any animal (vert or invert) which extracts nutrition from consuming cellulose does so through symbiotic gut bacteria.

Also, you've commented that mammals which eat grass tend to have complex guts relative to mammals with other diets. That's also true in invertebrates. In fact, in a single organism (e.g., certain dung beetles) which digests cellulose in one life stage and has a different diet in a different life stage, the relative length and complexity of the gut will change appropriately.

Take home: the invertebrate solution is essentially the same as the mammalian solution -- long, ramified guts packed with symbiotes.
posted by endless_forms at 5:12 AM on November 7, 2010

Actually, I'll amplify a bit.

Grossness warning: the following icks a lot of people out.

Take the above-mentioned dung beetles; consider Onthophagus gazella, a species specialized on cattle dung. The larvae consume the cellulose which remains in the dung after the cow is through with it. The adults consume microorganisms which are living in the dung.

But lets go back to those larvae for a second. They depend on their symbiotic gut bacteria to process the cellulose, but their guts just aren't long enough. So, they digest a glop of dung, excrete it, let the symbiotic bacteria do their work on it outside the body, and then reingest it. It's called an external rumen, and is known in other inverts (such as ostracods) dependent on a cellulose diet.

So it's not just forming a longer and more ramified gut; there's also the external rumen, which is a sort of virtual longer gut which exists outside the organism.

(By the by, the dependence of dung beetles on symbiotic bacteria is one of many reasons the use of antibiotics in factory farming is problematic. The residual antibiotics in the dung means that there will be no natural help in burying it. There's a whole literature stream on that subject, though it's a bit out of my own balliwick.)
posted by endless_forms at 5:22 AM on November 7, 2010 [4 favorites]

Synthesizing from previous answers: Google "crab cellulase" suggests to my non-biologist's eye that crab digestive juices really contain cellulase. I like that the article titled "The last piece in the cellulase puzzle" is from September 2010, so apparently you're wondering about a hot issue in crab enterology.
posted by themel at 10:27 AM on November 7, 2010

Thanks, all. I'll chase down themel's references, as well as Herbivory in Crabs: Adaptations and Ecological Considerations, which a Facebook friend passed my way. Hot issues in crab enterology! Also, YouTube has videos of crabs in the act of nocturnal herbivory. This internet thing sure is great!
posted by Jasper Fnorde at 5:24 PM on November 7, 2010

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