Tanaid

Image: Moorea Biocode

You know when you turn over a stone and find half an ant's nest in all its bustle, a small community of millipedes waking up from its slumber and a surprisingly large spider scurrying away into the undergrowth and you think "wow, who knew there would be so many legs under this one stone?"

That's pretty much how I felt when I found out about the Tanaids.

Image: Moorea Biocode

Tanaidacea is an order of crustaceans that almost no-one in the world ever talks about. It seems that at least 1,000 species have been named so far but it's thought there could easily be ten times that if only we could find them all.

Almost all Tanaids are marine and live on the sea floor, be it in the intertidal zone or thousands of metres deep in the abyssal plain. Some species live in brackish estuaries, a handful in freshwater environments, and several others in weird places like salt lakes, hydrothermal vents and underwater mud volcanoes.

Image: WoRMS for SMEBD
This one belongs to a genus called Whiteleggia, which is great

They clearly like to experience water in all its guises. Next time your cup of coffee gets cold, take a close look. There may have been just enough time for a few Tanaids to move in.

Possibly a lot more than a few. In some areas their population size has frequently been measured at more than 10,000 individuals per square metre and, on occasion, over 100,000! In the abyssal plain they are often the most abundant crustacean and their numbers almost rival that of polychaetes.

Image: Moorea Biocode

They are true deep sea dons. I imagine House Tanaidacea and House Polychaeta coming to an uneasy ceasefire after decades of war. Old worms wandering the abyssal plain, telling their great-great-great-great-etc. grandchildren about how all this used to be coral before the House Wars turned it all to mud.

Tanaids are a lot more famous down there than they are up here.

Image: Moorea Biocode

I suppose part of the problem is their tiny size. Most of them are not much more than 2 mm long, which is almost nothing at all and certainly not big enough to appreciate the details.

There are a few bigger ones though, especially two species within the aptly named genus Gigantapseudes. One of these deep sea fellows can reach 7 cm (3 in) in length, which is enormous compared to the others. Glad to see them get in on the abyssal giganticism that's going round!

Image: Blazewicz-Paszkowycz et al
A whole bunch of Tanaids. I don't know what's up with the spaghetti guy!

But take a closer look and you'll see they come in all shapes and... at least a few sizes. Some of them have stupendous claws about as long as the rest of their body. Most Tanaids feed on detritus but some are not averse to getting their gnathopods into some meaty prey. Whether those mighty claws are actually used for violence, I don't know. Some of them look too big to be used for anything at all.

Tanaids are split into three living groups:

Image: WoRMS for SMEBD

The Tanaidomorphs tend to live in tubes which they presumably construct by secreting some kind of... stuff. The tubes can be on the sea floor, algae or, in one species, manatees. I don't know whose idea that was but it sounds like a strange and novel relationship for both the manatee and the Tanaid. Someone needs to do the sitcom.

Tanaidomorphs are long and smooth, as befits a tube-dweller and it's thought that the female might spend just about all her life in her tube. It's probably up to the male to go out and find himself a lady friend when the time comes.

Image: WoRMS for SMEBD
Nototanais antarcticus, male and female

He might be armed with huge claws...

Image: WoRMS for SMEBD

Or utterly enormous ones. It's not known what they're used for. Maybe they just look sexy.

Image: Blazewicz-Paszkowycz et al

The second group, Neotanaidomorpha contains only a few dozen species (for now) almost all found in the deep sea. A few live closer to the surface in the frigid waters of the Southern Ocean.

Here too the male has enlarged claws.

Video: lscottrowe

Finally, there is Apseudomorpha, the most diverse of them all. These guys are free-living, frequently crawling about on the sand or burrowing beneath its surface. Others live in a whole host of environments, like one that lives inside sponges and another that is a parasite of sea cucumbers. And we all know how much sea cucumbers love their parasites!

But there's more! Check it out:

Image: Blazewicz-Paszkowycz et al

(a) An apseudomorphan with hairy claws for filter feeding, (b) a flat one that burrows under the sand, (c) this one has a twisted body so it can nestle deep within the spirals of abandoned snail shells, (d) another flat one, this time adapted to living on and between bits of coral, (e) another burrower.

It's fascinating stuff! When you take a close look, these tiny, barely-known shrimpy things reveal themselves to be minutely and expertly adapted to their peculiar lifestyles. And I hadn't heard of any of them!

Video: Phil Salmon
Looks don't get much closer than this

One other tiny detail. Tanaids belong to an enormous group of crustaceans called Peracarida, which includes amphipods (like my favourites, the Skeleton Shrimp), isopods (like everyone's favourite, the woodlice or pill bugs) and the almost completely unknown (nobody's favourite) hooded shrimp.

The most obvious thing that ties them all together is the fact that the females carry their eggs around in a pouch called a marsupium. In some peracarids these hatch into larvae that can drift around on the current before entering adulthood. In others, like Tanaids for example, they skip the larval period and hatch into something called a manca, which is basically a kind of adolescent. It can't swim too well so it ends up settling down quite near to its mother.

What I'm trying to say is, that manatee will have to get used to the idea of having entire generations of Tanaids living on his skin.