Last month, in the article about marine plastic pollution, I mentioned animals called salps. Not everyone will know what a salp is. They belong to the Tunicata, a division of the Phylum Chordata – animals with spinal cords, which includes all the vertebrates, and us.
The most familiar of the tunicates are the sea squirts, so called because they resemble bags of hard jelly which, if squeezed, squirt out water from their siphons. This bag – the tunic, hence the group’s name – is made of cellulose, the only example of the compound in the animal kingdom. Sea squirts can be either single, for example, Ciona intestinalis, which you might see on the lower shore, or colonial, like the beautiful Botryllus schlosseri, whose individuals resemble little flowers set in colourful gelatinous encrustations growing on rocks or sea weed. They are all filter feeders, sucking in water from one siphon, trapping food particles in their mucus-lined pharynx, and expelling waste through another siphon. They are eaten by humans in some countries – if you ever go to Japan, you might try sea squirt sashimi, said to taste like rubber dipped in ammonia.
Other tunicates, including the salps, are planktonic. Fishermen sometimes come across them unwittingly, because salps get caught around fishing gear, appearing as small strings of clear jelly. On closer examination, this jelly can be seen to consist of many identical parts, each like a transparent barrel with a small dark blob that is the creature’s digestive system. This string of jelly is a salp colony. The individuals are usually just a centimetre or so in length; a colony can be several metres long. Other planktonic tunicates are solitary and mostly very small, but one species, Thetys vagina, can reach 20 centimetres.
A particularly strange group of planktonic tunicates belong to the class Appendicularia, tiny creatures that look like tadpoles a few millimetres long, living in a sort of transparent house. Water is pumped into the house and food particles filtered out; when the house becomes clogged up with food, it is shed and a new one secreted. One appendicularian, called Oikopleura, occurs in large numbers in the plankton – I recorded 1,842 per litre in a collection made in 1984.
Why should we be interested in salps? My answer is, of course, that we should be interested in all living things – without knowledge of our fellow creatures, we can’t fully understand the world in which we live, so can have no comprehension of what we are doing to it, or how to repair the damage. Personally, I find animals, all animals, far more attractive and interesting than anything humans can create, and while the average sea squirt might not be much to look at, some colonial ones on the sea shore are very pretty, and the salps and their relatives, floating ghost-like in the oceans, are strangely beautiful.
Salps have an important role in the marine food chain. During plankton blooms they can reproduce asexually, at a faster rate than most other multicellular animals, and so are able to consume huge amounts of phytoplankton. Their resulting faecal pellets, and eventually their own dead bodies, form a large component of the carbon-rich detritus that has been raining down on the sea bed for hundreds of millions of years. But as I wrote last time, salps, like all marine creatures nowadays, ingest plastic particles. Recent research, by scientists in Cork, Galway and Villefranche-sur-mer in France has shown that salp faecal pellets containing plastic remain at the sea surface for longer than normal, so instead of being safely locked away on the sea floor, they have more time to break down and release yet more CO2 into the atmosphere. There are worse manmade sources of greenhouse gases contributing to climate change, but this nevertheless shows how our disregard for nature can have all manner of unforeseen consequences.
Planktonic tunicates provide homes for other creatures – small fish have been recorded taking shelter inside them, and certain species of pelagic amphipods (related to sandhoppers) use salps as homes, eating their insides and using the barrel-like body as a nest for their eggs and a sort of micro-submersible to travel in.
But what should really make us respect tunicates is that without them, we might not be here at all. Of course, that could be said of a great many organisms, but we are directly linked to these humble creatures. Adult sea squirts bear no resemblance to any vertebrate, but their larvae do.
The life cycle of the planktonic tunicates is complicated, often alternating between a solitary asexual and a colonial sexual stage, but reproduction in the sessile sea squirts is simple. They are all hermaphrodites, i.e. they have both male and female parts in the same body. Eggs and sperm are released into the sea, where fertilisation takes place. In all the sea squirts and many of the planktonic tunicates, the fertilised egg develops into a tadpole-like larva similar to the adult appendicularian. This stage is for distribution only and lasts just a couple of days, because their digestive systems are undeveloped. When they have found a suitable home, they settle down, head first, and stick themselves to the substrate. Their tails degenerate, their feeding apparatus develops, and they grow into the adult filter-feeding bag-like animal.
The interesting thing is that the larva not only resembles a tadpole, it has, running through its tail, a notochord – the precursor of our spinal column, and one of the characteristics of the Chordates.
The fossil record of the sea squirts is poor – jelly doesn’t fossilise well – but creatures resembling tunicates have been found in Cambrian rocks. One, named Shankouclava, was described from southern China in 2013. It resembled a modern sea squirt, but lived about 520 million years ago.
We will never know for certain where the first chordates came from, but one generally accepted theory is that the larva of a very ancient tunicate, instead of settling down on the sea bed and becoming a filter-feeding lump, remained free-swimming, and its reproductive and digestive systems developed so that the animal could live its whole life in the plankton. Put simply and unscientifically, it had become a very primitive little fish.
This strange development could have come about by a process called neoteny, which results in paedomorphism – an adult retaining juvenile characteristics. This has occurred many times in the animal kingdom: the appendicularians are paedomorphic; there are several species of paedomorphic insects; there are paedomorphic garfish too (more of them another time). But the best-known example is the axolotl, a salamander found in Mexico (now critically endangered in the wild). Like all amphibians, it starts out as an egg in the water, from which hatches a tadpole, with a tail and external gills. This tadpole grows into a salamander up to 30 centimetres in length and capable of reproduction, but instead of developing lungs and moving onto land, the adult axolotl retains its gills and stays in the water. Neoteny in axolotls is caused by a lack of thyroid-stimulating hormone, so the thyroid gland is unable to produce thyroxin, which controls metamorphosis. Axolotls in captivity can be induced to metamorphose into normal salamanders by injecting them with iodine.
So if you ever come across a sea squirt or a salp, stop to think – if those ancient ancestors had not become paedomorphic, but had changed as usual into sessile, filter-feeding bags, there might never have been any fish, and so no amphibians, no reptiles, no birds, no mammals – no us. The seas would instead be ruled by very intelligent octopuses, and the land by giant ants, and the planet would probably be in a far better state than it is now.