The appendicularian house

Abstract

Abstract The ‘house’ building capacity was early considered an important feature of the appendicularians, and the name ‘Oikopleura’ was coined by Mertens (1830, p. 210) soon after their original description by Chamisso and Eysenhardt (1821). OiKoc; (Oikos) is Greek for ‘house’ and 1rX.eupa (pleura) is Greek for ‘rib’ or the side of an animal, but came early to be applied to the lining membrane of the chest wall (Skinner, 1961). Thus, Mertens’ term ‘Oikopleura’ most likely points to the respiratory function which he erroneously assigned to the houses of these animals. However, it took almost 30 years before Mertens’ (1830) observation of a house surrounding an appendicularian was verified by Allman (1859). He suggested the house to be a ‘definitely shaped secretion, destined to act as a nidamental covering for the ova’. Moss (1870) gave some structural notes on inflating houses and presented interesting chemical data on the house material of an unidentified oikopleurid species, but did not comment on its function. Fol (1872) gave the first detailed description of the external houses in all three main families of appendicularians. His drawings still represent the most detailed published accounts of those of the Fritillariidae (26 species) and Kowalewskiidae (two species). A single photograph of a Kowalewskia house (Alldredge, 1976c) and a behavioural study of Fritillaria pellucida (Bone et al., 1979) add little to this knowledge. For the Oikopleuridae (about 29 species) on the other hand, Fol’s (1872) description of the house of Oikopleura albicans (at that time named O.cophocerca) was soon supplemented by the study by Eisen (1874), probably on O.dioica. Whereas Fol (1872, p. 495) suggested that the house served some protective function against preda-tors, Eisen (1874, p. 6) was the first to vaguely suggest it served ‘till at infora naringsamnen till munoppningen’ (to introduce nutritional substances to the mouth opening). However, undoubtedly the detailed structural studies of Lohmann (1898, 1899c, 1909a, 1933, 1934), mostly on O.albicans, were needed before any precise ideas could be established on the functional significance of the house with respect to feeding ecology. The great complexity and extreme fragility of appendicularian houses has been a challenge for numerous subsequent investigators and improved understanding of the precise structure and mode of operation of the house has only slowly emerged.