Botryllinae Adams and Adams, 1858 (Botryllidae)

KOTT, PATRICIA, 2003, New syntheses and new species in the Australian Ascidiacea, Journal of Natural History 37 (13), pp. 1611-1653 : 1645

Subfamily Botryllinae Adams and Adams, 1858 (Botryllidae)

Monniot and Monniot (1987) have referred to the definitions of Botryllus Gaertner, 1774 and Botrylloides Milne-Edwards, 1841 as unsatisfactory. Certainly these were based to a large extent on differences (e.g. the relative position of the ovary, embryo and testis) between the respective type species, Botryllus schlosseri and Botrylloides leachii , which have been shown to be inconsistent. Nevertheless, these taxa are not congeneric, as Monniot and Monniot (1987) and Monniot, C. (1988) proposed. In Botryllus , yolky ovoviviparous embryos develop in the atrial cavity, sometimes attached to the internal pallial wall (see Berrill, 1950; Mukai, 1977; Mukai et al., 1987). In Botrylloides yolky ovoviviparous to non-yolky true viviparous larvae, receiving nutriment from the wall of the brood pouch (see Mukai et al., 1987) in the parietal atrial wall, project externally from it.

In both genera the larvae are released through the common cloacal cavity and its aperture; buds of three successive generations co-exist in a colony; germ cells are transferred through a number of generations until they mature. Generally, the number of generations associated with egg maturation is not established for all species, although it is known that eggs mature when parental zooids reach adulthood and begin feeding. The testes mature after embryos begin development and embryogenesis occurs only in fully functional feeding adults (see Mukai et al., 1987). Owing to variation in colour and colony form, assignation of specimens in this subfamily to genus are problematical if embryos are not present. However, there are characters that complement their reproductive pattern that distinguish these genera. Botrylloides leachii , B. anceps , B. perspicuum and B. violaceus (see Kott, 1985, 1998) all have large atrial apertures that expose the atrial cavity to the common cloaca. However, the viviparous and ovoviviparous larvae are well protected in brood pouches in the parietal body wall. The smaller atrial apertures of Botryllus schlosseri and B. stewartensis (see Kott, 1985) would protect and retain the ovoviviparous embryos developing in the atrial cavity. Within these genera, the form of the stomach and the pyloric caecum and (within certain limits) the form of the colony are useful characters for species identification.

Before the works of Mukai (1977) and later works by the same author and his colleagues, data on breeding habit (that would ensure accurate genus assignations) were not always recorded and in other cases species have been wrongly assigned. Also, significant characters for species determination are not yet resolved and the taxonomy of the subfamily is in need of revision.