The Malacostracans gastric mill and other reasons to not dispair.

photo: James Reardon

photo: James Reardon


The closer you look at nature, the more marvellous and intriguing it becomes. Our species has doubled in my lifetime; there are now more than 7 billion of us, all clutching to, or aspiring to clutch to a smart phone.  Phytoplankton can reach that sort of population figure in a few square meters of well-lit ocean surface. They’re autotrophs, obliged by their biology to absorb soluble carbon dioxide and excrete organic compounds that underpin ‘the life aquatic’ and a great deal more, all via the engine of photosynthesis.  Being more of a zoologist than a botanist, that’s all well and good but where do the animals come into it? Well, feeding on this phytoplankton and the organic matter synthesised by it are the zooplankton. Excepting marine mammals, reptiles and sharks just about every other order of life represented in the oceans will have a few species or life history stages that constitute zooplankton. Some are single cell protozoa and a few are the tiny larvae of future giants but most are copepods, tiny crustaceans. These tiny beasts might seem incredibly vulnerable in the open ocean but it’s all a matter of scale. Most copepods are only 1-2mm (~0.05″) in length and at that size the world behaves quite differently. If you’re that small water is less of a liquid and more of a thick soup.

Copepod, one of the mirriad tiny ocean organisms that make our planet work.

Copepod, one of the mirriad tiny ocean organisms that make our planet work.

Usually with the second set of cephalic appendages (read: legs growing out of head), copepods swim through the marine soup doing all the same things we do (minus the smart phone thing), enjoying their youth, looking for food, finding a mate etc. Because of the very different world they live in their behaviour can appear strange to our eyes. Watching copepods in a small aquarium makes them look more like little rocket ships than tiny crustacea to the anthropocentric eye. They seem to be able to change the trajectory of their travel without need of a turning curve, acceleration and deceleration appear instant. They are also semi transparent, allowing the observer to look at the internal workings of a set of organs pretty much as complex as our own.

Of course larger crustaceans often provide the same opportunity. The New Zealand shore shrimp is much like hundreds of other shrimp species around the globe and uses transparency to aid its camouflage against detection by predators. If you can’t mimic the environment perfectly you might as well let it shine right through you! Shrimps like Palaemon affinus are detritivores, they eat anything organic, usually the dead or the dying that they can find or capture as it floats past. In a dynamic environment like the ocean, food is washing about everywhere and these shrimps manage to make a modest living from the simple act of keeping themselves clean. If transparency is your cloak against predators you’d better stay clean and smart and so these shrimps are permanently attending to their ablutions. And what they collect becomes a tasty treat.

Palaemon affinis, New Zealand shore shrimp, pulping detritus and the primary producers and consumers of the ocean. Photo James Reardon

Palaemon affinis, New Zealand shore shrimp, pulping detritus and the primary producers and consumers of the ocean. Photo James Reardon

Shrimps and other crustaceans don’t have grinding teeth in a mouth as us vertebrates would understand it. Rather they have a section of foregut that is known as the gastric mill. This muscular tube is filled with ossicles or little hard calcified plates that grind against each other and reduce whatever is ingested into a paste ready for digestion. Literally peristalsis with teeth. I wish I had one, then I could enjoy a movie whilst eating without the annoying interuption of the sounds my jaw and teeth make grinding food in my head.

So why bother putting this in a website post? Because sometimes it’s good to re-engage with the spectacular wonder of the microscopic world. No matter what sort of a mess we can make of things, these tiny organisms are going to find a place to survive. Sure, ocean acidification is decimating zooplankton and shrimps will have problems of their own no doubt, but in  general their world continues on as strange and wonderful, even to a zoologist, as life from another universe.



~ by motolorax on July 8, 2014.

One Response to “The Malacostracans gastric mill and other reasons to not dispair.”

  1. […] The Malacostracans gastric mill and other reasons to not dispaire. […]

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