Larvacea

Image: screen

Uh oh! It's that time again. That time when we, the mighty vertebrates, must visit the modern representatives of a cadet branch that stretches back to time immemorial. It's full of spineless malcontents, many of whom have descended into a life of crime and manual labour but, in our beneficence, we shall oblige them with a visitation.

Don't worry, I'll bring some thick gloves and gas masks along.

Image: screen
We''ll need the gloves when we get to all the mucus

We're looking at tunicates again! You might remember that the phylum chordata contains all the animals that have a notochord at some point in their embryonic development. It's a long, flexible rod that runs all the way down the body and in most vertebrates becomes bony and develops into the spine. Some peculiar fish never do that, they retain the notochord their whole life. Tunicates don't even do that, they actually lose the notochord!

Without the stabilizing influence of the notochord tunicates are free to become as weird as they please. We've seen Salps who look like a tube of jelly, Pyrosomes who look like a windsock and Sea Squirts who look like a jug.

Image: screen
Larvacean body. The rest is a tail

The cool thing about Sea Squirts is they have a larval stage which bears a striking resemblance to a frog tadpole, with a brain and a notochord and everything. Their paths soon part when the Sea Squirt gives it all up for a life attached to the bottom of the sea.

Larval Larvaceans also look like Sea Squirt larvae but their metamorphosis is so moderate that even as adults, they still look a lot like Sea Squirt larvae! Most of them will be less than 1 cm (0.4 in) long with a blob for a body a long tail trailing behind.

Image: screen
It's eating. YUCK!

The notochord is in this tail. It's quite strange because our notochord becomes a spine and then all our organs are packed around it. Larvaceans have their notochord almost naked and alone. Their organs are all packed into the little body and it contains almost nothing but gonads and a digestive system, because Larvaceans know what life is all about.

The first thing to do is eat, and this turns out to be a remarkably extravagant affair for Larvacea.

Video: Plankton Chronicles

They secrete a whole load of mucus to form a giant net that completely surrounds them. It's known as a "house" and for at least one Arctic species it can occasionally reach 2 metres (6.5 feet) across! They live in a mucus palace!

By wriggling their tail they can set up a current of water which passes through the house and the corridors within. The house has two sets of filters, one for blocking particles which are too big to be eaten and another for trapping the actual food.

Image: NOAA/OER
Larvacean house.
Judging from fig. 1.3 in this pdf I think the red things are the outer filters covered in muck and the white stuff are the food nets

The food nets are so fine that they trap plankton too small for almost any other filter feeder to catch. The Larvacean can now use a kind of treadmill of mucus to take food from the nets and send it right into its stomach.

Image: Neptune Canada
Larvacea houses are very fragile and almost invisible!

This only lasts for a few hours before the house gets so clogged up that the Larvacean can't use it any more. With a wriggle of the tail they escape their house and begin construction on a whole new one, or inflate one they prepared earlier. Their old house begins to sink with the weight of food particles and slowly drifts to the bottom of the sea.

Assuming it doesn't get snapped up by a hungry fish on the way down, these houses are a significant constituent of the marine snow that feeds the abyssal depths of the ocean. It becomes the muck that all those Sea Cucumbers wallow in!

Video: Casey Dunn

The thing about Larvaceans is not just that they leave behind a healthy alternative to the Gingerbread House in their wake, it's also the sheer number of them! With sufficient food they can form dense clouds that rival copepod swarms! Most Larvaceans are also hermaphrodite, though they can't self-fertilize because they release their sperm before their own eggs are developed. The eggs are released by tearing a hole in the body such that the adult dies.

In one species the entire lifespan is known to be about a week or two depending on water temperature. In that time they eat and make merry, build numerous homes for themselves and donate huge amounts of food to the poor and needy in the deep sea. A life well lived!