The jelly-like blobs above would hardly be considered to be evolutionarily advanced creatures at first glance. They are unable to move, passively filter feeding on plankton that drifts by, have no brains or other complex nervous systems and even lack a skeleton. However, unlike nearly all other spineless animals these are members of our own phylum. Like mammals, birds and reptiles, these sea squirts are chordates.
Normally when we think of an animal being a chordate we assume it has an internal skeleton, complex organ systems, a brain and a spinal column. Sea squirts, also known as ascidians, have none of these features. We’ll see how they qualify to be lumped into our phylum in a minute, but first let’s take a look at the form these animals take.
The adult sea squirts are sessile, meaning they are permanently fixed to a solid object. Like most sessile marine animals they get their nourishment by filtering out tiny planktonic plants and animals from the surrounding water. You can see below that there are two opening at the top, one is an incurrent siphon, which sucks the food-laden water in, and the other is an excurrent siphon, which ejects the filtered water and wastes. Within the body is a structure called a pharyngeal basket. These are gills which have been modified to also trap the food in its mucous coating. I described this feeding method a while back in Filter Feeders, Part II.
The larger opening on the "sea vase"
is the incurrent siphon.
The skin of a sea squirt is very thin, in fact this surface of the body is only one cell layer thick. This skin is supported by an outer covering known as a tunic (and is the source of another common name for these animals: Tunicates). This tunic varies greatly from species to species. In the sea vase above it is soft and translucent, enabling us to see the intestines and other internal structures within. Other species have a tunic that is leathery and nearly impenetrable to most predators. Many species, especially those found in the tropics, are quite colorful and this color comes from the tunic, not the animal inside. In addition to protecting the sea squirt, the tunic also helps to support its body and maintain its shape underwater. Removed from the water, gravity will collapse the tunic and render the once beautiful animal into a shapeless mass.
Sea squirts attach to the substrate using filaments which extend from the tunic. These are known as stolons and cling to the rock like plant roots. These filaments increase in number as the animal grows but cannot reattach if they become dislodged. A surprising component of the tunic matrix, normally associated only with plants, is cellulose.
Styela is an ascidian with a tough
leathery tunic.
So what makes these animals members of the chordate phylum? To answer this we need to look at the young. Sea squirts are hermaphrodites, each one producing both male and female gametes. These are shed into the water and fertilization takes place by chance. The egg develops into a free-swimming tadpole larva, so named because it resembles the young of frogs and toads. The tadpole has three main characteristics in common with other chordates. Within the tail is a primative spinal cord called a notachord, a dorsal neural tube and pharyngeal gills, all of which are present in the embryonic stage of higher vertebrates.
This larval stage lasts a very short time, mainly because the animal doesn’t have a mouth or functioning siphons so it is unable to eat. When it has found a suitable substrate to grow on it will cling to this object and undergo a rapid metamorphosis into the adult form. The tail is absorbed (losing both the notochord and the nerve tube), the siphons develop and the pharyngeal gills expand to form the plankton-trapping basket.
Although the examples given so far include solitary sea squirts, known as simple ascidians, there are also many types of colonial species as well. These types of complex ascidians are made up of numerous individuals that are all connected. Although the colony starts as a single tadpole larva, a colonial tunicate increases its size by asexual reproduction, budding additional members until it reaches its full size, which may be up to several feet across in some types.
The animal above is called a star tunicate. Each individual within the colony has its own incurrent siphon and captures its own food. The colony shares multiple excurrent siphons, as well as a common tunic that covers the entire group. In the closeup below each "star" is a mini-colony within the colony and each tiny "ray" on the star is an individual tunicate. The center of each star is the common excurrent siphon.
Other diaries in this series can be found here.