Appendicularia are small, free swimming pelagic tunicates with a trunk and a permanent chordate tail. The trunk epithelium secretes a gelatinous "house" that encloses the animal. Appendicularians have features that resemble those of the "tadpole" larval stages of the sessile Class Ascidiacea — therefore they have been called Larvacea before. Most species of appendicularians are warm water forms, found in all oceans. Also, most of them live in the epipelagic, but increasingly new species are discovered in the meso- and bathypelagic. In general, appendicularians are a few millimetres long (including the tail but without the house), some are much larger, up to 90 mm.
Except in Kowalevskia (the single genus in the family Kowalevskaiidae — not occurring in our area), in all Appendicularia a short ventral endostyle is present and posteriorly of that, also ventrally, a simple heart. A pair of branchial openings (gill slits) lead from the floor of the pharynx to the exterior. There is no peribranchial cavity, nor a cloacal siphon. The digestive track consists of a mouth, pharynx, oesophagus, buccal glands, a one- or two-lobed stomach, and a rectum that opens on the ventral side. The posterior part of the trunk is occupied by the relatively large gonads. All appendicularians are protandric hermaphrodites, except for Oikopleura dioica — the only species with separated sexes. There is no alternation of generations as is the case in the other class of pelagic tunicates, the Class Thaliacea.
The tail is broad and long, attached to the ventral side of the trunk, between rectum and gonads. It is twisted and recurved, the free end directed anteriorly.
The epidermis of the anterior part of the trunk is provided with specialised glands, the oikoplasts, that secrete the material forming the complex feeding house. There are two types: the Fol's oikoplasts and the Eisen's oikoplasts. The first are the largest, more anteriorly located and usually elongated. The second are centrally located, smaller and more or less square or round. Their precise position and shape are of taxonomic value, but they can be extremely difficult to be seen, especially in badly preserved specimens.
The food of an appendicularian consists of micro-organisms, especially small and smooth unicellular algae and protozoans. The house, together with the pharynx and the tail, is essential in the complex feeding mechanism that allows the animals to feed on a highly concentrated suspension of particles extracted from the water that is pumped through the house by the beating tail. Generally, the house is spheroid and possesses two inlet openings covered with filtration grids (mesh size ranges at least from 13 to 300 µm; see Flood and Deibel, 1998). The opening give entry to funnels, leading to the midline trunk and the tail chamber. The animal itself is suspended in the tail chamber; undulatory movements of the tail produce a water current in the tail chamber, drawing water in through the funnels and out through the exhalent opening. Underway, the water passes a very fine filter (mesh size range at least 0.15-0.24 µm; see Flood and Deibel, 1998) in the house where the food is concentrated. Next, the captured food particles are trapped in the mucal network produced by the endostyle. By ciliate activity the food is transported to the mouth and from there, further handled in the digestive track. The water movement can be reversed to eject unwanted particles or to clean the filters. If the filters are clogged and the feeding mechanism gets choked, or in case of emergency, the animal can escape from its house by an emergency exit. Following such an event, the animal secretes a new house. The house renewal rate can be from about 2 to 5 new houses per day (Flood and Deibel, 1998). The house is very delicate and the animal itself can escape if necessary; therefore it is very rare to find a complete specimen, that is, the appendicularian inside the complete house.
By the beating tail, water is drawn in through the filter openings in the house and out through the exhalent opening. This water current is essential in the process of food filtering, but it also provokes a certain locomotion through the sea, albeit slowly and not really swimming. The movement is logically in the direction of the inhalant openings, and opposite from the exhalant opening in the house. However, the animal inside is reversed compared to the direction of movement because the anterior of the animal points backward and vice versa.
The male and female gametes are released in the sea; the fertilised eggs (0.1 mm ø) develop into a larva with a trunk an a tale in one line, resembling the "tadpole" larva of the sessile Ascidiacea. At a certain stage, the tail twists and shifts and next, the first house is build within the first day. Further development concerns mainly the volume increase of the somatic cells, the development of the gametes and the releasing of these. There is no alternation of generations. After spawning, the animal dies.
Appendicularia are cosmopolitans, occurring from the poles to the equator. Species diversity is higher in the warmer waters. The number of species and abundance decrease towards higher latitudes and colder waters. Most of the appendicularians live in the epipelagic and are rare below 50 m.
Indigenous North Sea appendicularian species are Oikopleura dioica and Fritillaria borealis. All other species mentioned here are (potential) occasional visitors.
Appendicularians are an important item in the marine ecosystems. Because they are small and move slowly, they are an easy prey for carnivorous zooplankton, including larval and postlarval fish.
There are three families in the Class Appendicularia: the Oikopleuridae with 11 genera and 37 species, the Fritillariidae with three genera and 30 species and the Kowalevskaiidae with one genus and two species.
The key to the appendicularians of the North Sea starts at Page 455: Appendicularia. The following species are included: