Tunicates are small invertebrates to be found in numerous seas and oceans of the world, many of them living not only on rocks, but also on the underside of ships and boats. Also called sea skirts, these tiny invertebrates are very much studied by marine scientists to find out more about their strangely close genetic kinship to vertebrates – according to some specialists, they are even more closely related to vertebrate species than humans.
These special creatures were named after the way they look: they have a barrel-shaped body covered in a flexible, but very firm tunic. Their body shape makes them incredibly resilient – sea skirts have been found unharmed even after the wildest storms, even after tsunamis. They fix themselves to the substrate or to the rocks and ships with the lower part of their tunic and they have two tube-like organs called siphons protruding from the tunic, too. The animal uses one of the siphons for filtering the water in order to extract nutrients, while the other siphon is used for expelling the filtered water. Many tunicate species have almost completely translucent or whitish bodies, while others are colorful and can be brown, red or yellow.
Sea skirts have clear blood and a heart that behaves strangely, too. The circulatory system of these small animals has also surprised marine biologists: the blood circulates in one direction in the body for 100 beats of the heart, then the heart stops, starts again and pumps the blood in the opposite direction. The reason for the phenomenon is unknown.
There are more than 2,100 species of sea skirts known to scientists today, most of them living in shallow waters, usually not deeper than 200 meters. Most of them are solitary creatures, living sessile lives, permanently attached to the substrate or object they choose, but some are known to be colonial or pelagic.
Most sea skirts are suspension feeders, meaning that they capture food particles such as microscopic planktons by filtering the water around. However, some species are detrivores, obtaining the nutrients they need from the decomposing particles of plants, animals and feces they find in the water, and others are known to be predators, sitting around and waiting for tiny crustaceans to swim by.
Most tunicate species are hermaphrodites, having a single testis and an ovary. They employ two methods to avoid self-fertilization: they either have the sperm and the eggs inside their body mature at different times or they use chemicals to make the egg and the sperm reject each other. In solitary species, reproduction occurs through external fertilization – the eggs and the sperm are released into the water where they combine, after which larvae emerge and swim freely. Other species, especially colonial ones, reproduce by sperm being released into the water and drawn by others into their bodies, where the fertilization takes place.
There is one species of sea skirts, the Oikopleura dioica, that reproduces only once during its life. These tunicates are also the most studied members of the whole group, considered by many experts to be a model species, its genetic material bearing a striking resemblance to the genetic material of vertebrates.
Golden Sea Squirt
The golden sea squirt is a sac-shaped marine animal belonging to the class of ascidians and found in the tropical waters of the Indo-Pacific region, at depths ranging from 5 m to 50 m. Also called the gold-mouth sea squirt or the ink-spot sea squirt, these tunicates are sessile creatures. They live attached to the substrate, and therefore they prefer areas were water currents are moderate. They can be found living alone or in larger colonies.
These tunicates are quite small, usually measuring around 15 cm when fully developed. The tube-like body of ink-spot squirts is covered in a colorful, tunic-like layer of cellulose that protects the inner organs inside the central cavity. Ink-spot tunicates are usually white, with purple and bright yellow patches, a bright blue or purple marble pattern on the outside, and orange or greenish-golden on the inside (color provided by the microscopic algae that live in symbiosis with the squirt and visible around the upper edge of the siphons). The sac-like body is attached to the substrate on one end, through a muscular foot, and features two orifices on the upper side – these are the two siphons used by the animal to get food and to eliminate waste products. These tunicates also have a brain, nervous and digestive systems, and a circulatory system with a heart that alternates the direction of the blood and blood vessels in the tunic.
Like all sea squirts, this species also combines filter-feeding with consuming the nutrients provided by the symbiotic algae inside their body. Golden squirts pump water into their body through their branchial siphon, they filter the water to extract food particles, valuable bacteria and plankton and they expel the filtered water through their atrial siphon found on the side of the body.
Like all other ascidians, these sea squirts are able to reproduce sexually, but some of them are hermaphroditic, engaging in cross-fertilization and self-fertilization as well. Fertilization takes place externally in both cases, but the eggs are kept inside the body of the animal. Very soon after being released, the eggs hatch into tadpole-like larvae that have a nerve cord and in most cases gills as well. The cord disappears when the young golden sea squirt finds a place to settle and attach to.
Blue Lollipop Tunicate
The blue lollipop tunicate goes by many names, all of them referring to the creature’s appearance. Often called blue cauliflower corals or lollipop corals (even though they are not corals), these beautiful creatures live in colonies in the shallow waters of the Indo-Pacific region, especially around Indonesia. They prefer waters that are rich in nutrients such as planktons and bacteria, where they can satisfy their enormous appetite.
The body structure of blue lollipops is similar to the structure of other tunicates – they have thick stalks that are attached to the substrate and support the head of the tunicate. The head is actually not a homogenous part – it is composed of many small polyps called zooids. The name tunicate derives from the presence of a thick, hard and leather-like skin that looks like a tunic worn by the animal. Blue tunicates live in colonies – in many cases, as many as several thousand zooids join to form smaller or larger mounds.
These bright-blue colored critters belong to a class of tunicates called ascidians – bottom-dwelling tunicates that are firmly attached to the substrate and are unable to move. They are filter feeders, which means that they feed on the nutrients they filter out from the water column. They take in water through a tube-like organ on the upper part of the body called the oral siphon, then the water passes through the animal’s mouth and gill slits where it gets filtered by the sticky mucus inside that traps small food particles and then it is eliminated through the atrial siphon, also found on the upper body.
Blue tunicates, like most other ascidians, are hermaphrodites, meaning that each individual has both male and female reproductive organs. Individual tunicates release their reproductive fluids into the water column, where fertilization takes place. Blue tunicate larvae float freely in the water until they find a suitable spot of substrate where they can settle. In the early stages of development, the larvae resemble tiny tadpoles and they have a rod-like notochord – feature that accounts for the blue tunicate’s inclusion into the phylum of chordates. However, the cord disappears in later stages of development, after the larva has found a place to attach itself to and the adult blue lollipop tunicate has no features that would make it look anything like an animal that once had a nerve cord.