Didemnum molle

Difficulty: Meerwasser-Lexikon rates it 'not suitable for aquarium keeping', and Quality Marine notes that sea squirt care is generally considered difficult. Quality Marine notes that photosynthetic 'green barrel' tunicates seem to survive better than others.

Acclimation: invertebrates cope very poorly with changes in water chemistry, even minor ones; acclimate it slowly, by floating or drip, without rushing. None of the sources consulted gives a protocol specific to sea squirts.

Placement: give it a well-lit spot, since its Prochloron cyanobacteria need light; in the wild it lives on lit hard bottoms. It can move slowly: do not be surprised to find it elsewhere, even on the glass.

Water and filtration: sea squirts tolerate minor drops in water quality; Quality Marine suggests parameters similar to those for mushroom polyps and zoanthids. Because feeding has to be heavy, plan fairly strong filtration, good flow and skimming balanced to remove nutrients and toxins while leaving enough plankton: some sea squirt species secrete toxins.

Tankmates: these attached filter feeders do not bother corals, but avoid aggressive tankmates. None of the sources consulted describes species-specific signs of good health. Avoid copper, which harms invertebrates.

It is partly photosynthetic: its Prochloron cyanobacteria supply nutrients, and a 1986 study (Olson, Marine Biology) showed that its growth increases with light. But it is still a filter feeder on phytoplankton, zooplankton and fine organic particles: light alone is not enough.

According to Quality Marine, food demands are high: direct daily feeding is a must, with brine shrimp nauplii, live rotifers and a splash of mollusc juice. To deliver it, use a baster or syringe, or invert a cut plastic bottle over the colony, squirt the food underneath and remove the bottle after about 5 minutes. That Pet Place advises zooplankton or phytoplankton several times a day: they need an almost constant food supply to avoid starving.