Doliolida

The Doliolida are an order of small marine animals of the subphylum Tunicata. They are in the class Thaliacea, which also includes the salps and pyrosomes. The doliolid body is small, typically 1–2 mm long, and barrel-shaped; it features two wide siphons, one at the front and the other at the back end, and eight or nine circular muscle strands reminiscent of barrel bands. Like all tunicates, they are filter feeders. Unlike the related class Ascidiacea, which are sessile, but like the class Appendicularia, they are free-swimming plankton; cilia pump water through the body which drives them forward. As the water passes through, small particles and plankton on which the animal feeds are strained from the water by the gill slits. Doliolids can also move by contracting the muscular bands around the body creating a temporary water jet that thrusts them forward or backward quite quickly. The Doliolida have a complicated life cycle that includes sexual and asexual generations. They are nearly exclusively tropical animals, although a few species do occur as far to the north as northern California. Doliolids alternate through sexual and asexual generations. The sexual generation consists of individuals featuring eight muscle bands, each having male or female gonads. These individuals are called gonozooids. Fertilized eggs produce slightly different individuals, featuring nine muscle bands, no gonads, and two stalks growing from each individual's body: the shorter one at the ventral side, and the longer one growing from the dorsal edge of the posterior siphon. These asexual individuals are informally called 'nurses', and each one produces an astonishing number of mature progeny asexually; such progeny include both sexual and asexual zooids in three sequential 'generations'. The nurse produces buds (which grow into new zooids) in its ventral stalk, but the buds grow and mature on its dorsal stalk. Each bud is an aggregate of a few dozen cells, and the way it gets to its final place is the first peculiarity of doliolid reproduction. Buds are immobile, but are actively carried by special mobile cells, called phorocytes, which literally means 'carrier cells', shaped like amoebae. Each bud is transported by several phorocytes, which follow a clearly defined path across the nurse's body: up the ventral stalk, in a spiral along the left side of the 'barrel', and finally onto and along the dorsal stalk.