Ascidians, Sea squirts
Scientific name : Ascidiacea
The phylum Tunicates is mainly represented, for what divers normally observe, by the sea squirts (class Ascidians).
These probably does not rank between the more well known animals for divers. In many environments they go unobserved, similar to plastic bags, or to vegetables.
North Sulawesi is extremely rich in ascidians, both in number of species and of individuals. Bunaken walls are often covered with the yellow Polycarpa aurata. At a closer look also the abundance and variety of Rhopalaea is revealed. To identify in details the smaller species it takes good observation, but it is definitely worth of. Sea squirts are sessile invertebrates, tipically pear shaped, with 2 openings, oral (incurrent) and cloacal (excurrent) siphon. This, at least, is how a solitary ascidian looks like. Also colonial ascidians exists, in 2 types: they are called social ascidians when the colony is made up by individuals connected with a basal stolon but each complete; they are called compound ascidians when individuals are deeply connected and share an exhaling siphon. The blue one in the middle of the picture is a solitary ascidian (Rhopalaea sp.). The lower left, black and yellow, is a social species (Clavelina robusta). The white-greenish on the right (Atriolum robustum) and the red one on the background (Didemnum sp.) are compound ascidians.
Sea squirt body is covered with a though tunic. Water intake is through the oral siphon, water is filtered trough the branchial sac and expelled through the cloacal siphon together with faeces and gametes. Water is moved by the synchronous beat of cilia on the gills. Most ascidians are contemporary hermaphrodites. Fecundation is external. The fecunded egg gives birth to a swimming, pelagic, tadpole like larva. After drifting for a period, the larva settles on the bottom, and becomes a small ascidian. The larva is provided with a dorsal chord (the same structure that originate our backbone). For this reason, tunicates are classified in the superphylum chordates, together with vertebrates. Ascidians are our closest relatives amongst invertebrates. Sea squirts are filter feeders, collecting small suspended particles from water. An ascidian can actively filter an impressive amount of water daily: in Phallusia a filtering rate of 173 l/day has been measured!
They can live in any marine environment, but the greater diversity is in the external reef, in the deep reef and in caves.Often unnoticed by divers, they are amongst the dominating organisms along Bunaken's vertical walls.
In shallow environments, like lagoon or the reef front, often they are found under corals or rocks. Few species can live where the light is very intense, usually species with symbiotic algae.
Compound ascidians in particular are sometimes very similar to sponges.
The more obvious difference: sponges are uncapable of movements. If moving a hand close you can see the siphons moving, this is an ascidian. Ascidians can feel a possible threat, and can close the siphons to protect the delicate internal organs.
Often, looking carefully at the shape of the smaller pores (oral siphons) only in ascidians you can see a typical star shape.
The phylum tunicates includes organisms that are part of Plankton, free fluctuating in the water, often in colonies.
They are called Salps and Pyrosomes (class Thaliacea).
Ascidian systematic is largely based upon internal charachters (the gills). Therefore it is reserved to specialists. The easiest way for the divers, to make order in this group is to divide it in solitary, social and compound species.