by Matthew Norton
The sea is full of weird and wonderful creatures with appearances and ways of living that you do not see on land. Larvaceans, a group of transparent, swimming invertebrates are a shining example of this point with a weird tadpole-like appearance which actually resembles the larvae of its close relatives. They make a living by using their long tails (relative to their body size) to draw in water and then filter out small food particles.
Filter feeding is a very common practice among marine animals, but larvaceans are particularly effective at it and able to catch tiny particles, even down to small bacteria. They manage this by producing a ‘house’ of mucus which surrounds the animal and can be much larger than the animal itself. These house contains a complex arrangement of funnels and finely meshed, and sticky, filters which concentrate and catch the microscopic food particles.
Unfortunately, these filters are easily clogged with these particles which forces larvaceans to abandon their house and build a new one every few hours. This may sound like a lot of hard work, but with their rapid growth rate and short lifespan, only a few days in some species, larvaceans get a lot out of each house they build.
Once abandoned by their original owners larvacean houses are by no means useless. In fact they are a valuable food source because of the food still clogged in the filters and other material they accumulate as they sink, such as phytoplankton (microscopic plants), bacteria and faecal pellets (poo). Many animals take advantage of this sinking food source, such as copepods (small swimming crustaceans) and some species of eels who will actively seek out larvacean houses. Finally, these abandoned houses are especially valuable to animals on deep seafloor, who almost completely rely on food raining down from above, except around hydrothermal vents.
Larvaceans are a shining example of the ingenuity that marine animals can show so that they may survive, thrive and eventually reproduce. They are not the only group that build structures to this end, a crab’s shell is virtually separate from the animal and most of a jellyfish’s body is just jelly. However, larvacean houses are so well designed for their purpose that they take on a life of their own after being abandoned by their builder, showing that little goes to waste in the ocean.
From a human perspective
Larvaceans have proved to be a difficult group to associate with humans. They are an obscure group and there is no way (that I know of) that we directly exploit them for our benefit. Nonetheless, there are some aspects of their evolution and lifestyle which, with some work and imagination, can have future applications for humanity.
For starters larvaceans are more closely related to us than most other invertebrate life because we all belong to the phylum Chordata, which is separated into tunicates (e.g. larvaceans, sea squirts and salps), cephalochordates (e.g. lancelets) and vertebrates (all animals with a backbone). All of the above share a number of distinctive features, although in many species such features will only appear at certain times in their lives. The most notable being a notochord, a flexible rod that stretches through the length of the animal and provides them with structural support. In vertebrate animals this notochord develops into a spine made out of bone early in their development.
Because they share such features larvaceans can be used to better understand how certain features in vertebrate chordates evolved and their development at the molecular and genetic basis. Some have already studied the development of larvacean species, especially Oikopleura dioica, for this purpose. This species uses a number of genes to control the development of its own central nervous system with complicated names, such as hox1, pax6, pax2/5/8b. The important point here is that many of these genes are similar to genes found in vertebrate development and produces similar structures to the spine and parts of the vertebrate brain in larvaceans.
These similarities can be very useful for us as we can, in theory, perform experiments on larvaceans that we could not do with the embryos of vertebrate animals. In particular Oikopleura dioica are quick and easy to keep and breed in the laboratory with a lifespan of around 4 days and produce large masses of eggs. They also have a very small genomes (the complete set of genes in their cells) which makes them easier to work with.
The lives of larvaceans are also relevant, to some extent, with global environmental issues. For example the organic material (i.e. material containing carbon) contained within the sinking houses may play a role in locking up carbon dioxide away from the atmosphere. Indeed accounting for their contribution has increased the accuracy the models designed to explain how carbon is transported and consumed in the oceans.
Recently (2017), there has also been interest in how they can filter out and microplastic particles. This plastic could be locked away in houses that are buried in the deep seafloor. However, it is also likely that many of these houses, and the microplastics loaded on them, will be consumed by other sea creatures. The exact role of larvacean houses in microplastic transport will require, like many parts of their lives, further study.
I will be the first to admit that these links I have suggested between ourselves and our very, very distant cousins are limited, but there is potential and it can be explored in the years to come. Not every use we have for the sea, and the creatures that live in it, has been immediately obvious to us and even less so for species that are not well known to us. Nonetheless, it is in our nature to keep exploring the world around us and pushing to find ways to use nature to meet the challenges that nature throws at us.
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All other images are public domain and do not require attribution