Sea squirts (class Ascidians, phylum Tunicates) are sessile invertebrates, tipically pear shaped, with 2 openings, oral (incurrent) and cloacal (excurrent) siphon. There are solitary ascidian sea squirts like Halocynthia papilosa (a key species for our research on the Costa Brava) and also two types of colonial ascidians: called social ascidians when the colony is made up of individuals connected with a basal stolon but each complete in themselves; and compound ascidians where individuals are deeply connected and share an exhaling siphon.
The Sea squirt body is covered with a tough membrane. Water is taken in via the oral siphon, filtered through the branchial sac and expelled via the cloacal siphon together with faeces and gametes. Water is moved through the body by the synchronous beat of cilia on the gills. Sea squirts are filter feeders, collecting small suspended particles from the water, actively filtering water at an impressive daily rate (173 l/day measured in Phallusia).
Most ascidians are contemporary hermaphrodites. Fertilisation is external with eggs giving birth to pelagic, tadpole-like swimming larvae. 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 is the origin of the backbone of vertebrates). For this reason, tunicates are classified in the superphylum chordates, together with vertebrates. Ascidians are our closest relatives amongst invertebrates.
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 due to their habit of shrinking when approached, they are amongst the dominating organisms along Costa Brava’s vertical walls often found under overhangs as they prefer low light conditions.
Compound ascidians can look very similar to sponges. Looking carefully at the shape of the smaller pores (oral siphons) only in ascidians you can see a typical star shape.