Sea Squirts Excerpt

INTRODUCTION

By Daniel Knop

Sea squirts look almost like flowers growing in an undersea “meadow”, and create aesthetically enchanting, unusually attractive splashes of color on the coral reef. Their colorful zooids are resplendent in brilliant blue, bold yellow, and blood red, and sometimes they create glass-like filigrees that look immensely fragile. Their closely packed, rounded bodies cover the substrate with attractive cushions from which
they extend their siphons hungrily into the seawater.

To the unpracticed eye, a sea squirt, or tunicate, might look like a sponge-like creature, a primitive life form. But that is far from the mark: sea squirts are not lower animals and are totally different from sponges and jellyfishes. Beneath the tunic of the sea squirt beats a heart that supplies the tiny organism with hemolymph. In addition, at an early developmental stage the sea-squirt larva bears a close resemblance to a human embryo!
Some books state that sea squirts are vertebrates, but this is not quite right. They belong to the Chordata, a phylum of animals that possess a notochord. This huge group, which contains around 40,000 species, comprises not only the vertebrates (the most highly evolved chordate form, which includes humans), but also other life forms, one of which is the tunicates.

The characters shared by sea squirts and vertebrates are apparent at an early developmental stage and consist of, among other things, a precursor of the spinal column and the brain as well as a branchial gut which is supplied with hemolymph by a beating heart. However, during the subsequent period of further development these characters go their separate ways in order to achieve species-specific specialization for a particular way of life: whereas in humans and other vertebrates the precursor of the spinal column is replaced by a rigid dorsal skeleton, in the tunicates it disappears during metamorphosis of the larva. The same applies to the rudimentary brain and the adjoining spinal-cord-like structure: these tissues change to a ganglion, a simple aggregation of nerve cells that permits the recognition of only the most basic environmental stimuli and corresponding reactions, for example, when the outside of the tunic of the sea squirt is touched, or when foreign bodies enter the intake siphon. By contrast, in vertebrates these tissues develop into a highly differentiated nervous system that permits complex modes of behavior—running, climbing, swimming, scratching at parasites, brewing coffee, or designing a jet plane.

In tunicates the branchial gut develops into a basket-like organ that occupies the major portion of the body, forming the basis of the creature’s subsequent earning of its daily bread by enabling it to extract nutritious planktonic organisms from the seawater. In vertebrates, however, the branchial intestine develops into lungs that permit gas exchange with the air of the atmosphere. Each organism follows its own genetic programming, which prepares it for particular habitats and ecological niches. Some survive by developing stronger legs and running faster and climbing better than others, as well as developing a more complex nervous system. Others seek out the benefits of a more restricted way of life by giving up the ability to move and at the same time foregoing a nervous system with all the sensory perceptions that are needed to cope with mobility: if you don’t walk, you don’t need legs or a brain to tell you where to go.

Sea squirts are chordate animals that have evolved an unusual way of life. In the case of some genera, this also applies to their mode of reproduction: their larvae do not develop into sea squirts, but instead into so-called ozoids, whose function is limited to asexual reproduction. They are unable to develop gonads in order to reproduce sexually, but remain stuck in a larval developmental stage and regress again after they have produced buds, their purpose in life having been fulfilled. These buds eventually develop into blastozoids, which grow up to be normal sea squirts that produce reproductive cells so that the whole process can begin anew: they reproduce sexually, and in this way produce further ozoids. This mode of reproduction, alternating from generation to generation, is seen in the genera Botryllus and Clavelina, for example, and is termed metagenesis by scientists.
Nowadays we know much more about sea squirts than we once did, but many questions remain to be answered; for example, why and how does the sea squirt, the only animal known to have this ability, reverse the direction of flow of its hemolymph at set intervals? It will be a long time before we understand everything that lies hidden beneath those colorful tunics. But that is all part of what fascinates humans about these brightly colored creatures.