Family STYELIDAE

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Family STYELIDAE

Compiler and date details

P. Kott, Queensland Museum, Brisbane, Queensland, Australia

Introduction

The family Styelidae Sluiter, 1895 is diverse, containing solitary species (Styelinae) as well as colonial ones, some with cloacal systems (Botryllinae) and others without (Polyzoinae). Family characters, evident in the larger solitary species, are often reduced or absent in the smaller and more simplified zooids of colonial taxa. Basically the family is distinguished by having four pharyngeal folds on each side of the body, simple (unbranched) branchial tentacles, a thick external coat of circular muscles over a layer of longitudinal muscle bands, internal glandular folds in the stomach wall, usually more than one gonad on each side of the body, those on the left outside the gut loop, and the internal parietal body wall often raised into upright leaf-like or flat-topped bodies known as endocarps projecting into the atrial cavity. The function of the endocarps is not known, although it appears that they would interrupt the excurrent stream of water as it moves through the atrial cavity and in some cases (e.g. in Polycarpa) may impede the release of gametes from the atrial cavity.

The best known genera of the solitary Styelinae are Styela Fleming, 1822, with branching male follicles in the body wall around the outside of the long ovarian tubes that converge to the atrial apertures; Cnemidocarpa Huntsman, 1912, with similar ovarian tubes but with compact testis follicles closely applied to the sides or beneath the ovarian tube, the vasa efferentia joining the vas deferens along the mesial surface of the ovary, and Polycarpa Heller, 1877, with many short ovarian sacs (often in several rows, or scattered), their short ducts usually distant from the excurrent aperture. The testis follicles are beneath the ovaries. Asterocarpa Brewin, 1946 has branched gonads of the cnemidocarp type around the ventral margin of the body; and Monandrocarpa Michaelsen, 1904 has polycarp-type gonads but each gonad has only a single pair of testis follicles. In Styela and Cnemidocarpa the number of gonads on the left is often only one or two, although they are more numerous on the right.

Polycarpa is unusual in having some viviparous species in which the oviducts are directed ventrally or in which the ovaries with their short oviducts are present only ventrally, resulting in the retention of eggs in the atrial cavity where they are fertilised and the embryos are incubated. Berrill (1955) has proposed that viviparous species are adapted for life in the open sea in that gametes are less vulnerable to dispersal and populations are maintained more tightly. Polycarpa (like Molgula, in the Molgulidae) contains solitary species with a morphology amenable to adaptations for egg retention, and with test and body shape that can adapt to a free living existence on the open sea floor, e.g. with a strong feeding current from a large branchial sac rather than the small currents of colonial zooids; and with a rigid sand-encrusted test that protects the body from eroding sediments. The viviparous larvae in these genera also tend to lack the light sensitive ocellus–implying that light sensitivity is not advantageous where there are no shadows, such as on the open sea floor (see Berrill 1955).

Like Aplousobranchia (which are almost exclusively colonial), colonial subfamilies Polyzoinae and Botryllinae have viviparous larvae. These are convergent with aplousobranch larvae and it appears, therefore, that viviparity is associated with a colonial habit rather than an open sea habitat. Like aplousobranch larvae, the larvae of Botryllidae and Polyzoinae are light sensitive (negatively phototropic on settlement, being attracted into shaded habitats such as in crevices and on under-surfaces).

Polyzoinae, characterised by their separately opening atrial apertures, are a diverse subfamily with numerous genera, ranging from semi-independent zooids joined by basal stolons, to fully embedded ones. The subfamily well demonstrates the trend to simplification of morphology with replication and size reduction of zooids. The smaller zooids have flat branchial sacs with as few as three internal longitudinal vessels, and small and ephemeral gonads, while the larger forms (e.g. Polyandrocarpa) have all the characteristics of genera of the Styelinae—well-formed branchial folds and larger and persistent gonads. In the Botryllinae, the zooids form well-integrated cloacal systems comparable with those in the aplousobranch families Polyclinidae and Didemnidae and certain genera in the Holozoidae (Sycozoa, Distaplia, Hypsistozoa and Hypodistoma). Species have been assigned to one of two genera, Botryllus and Botrylloides, although a high degree of intraspecific variability in many of the taxa and a simple zooid morphology have resulted in difficulties in defining the parameters of both species and genera (see Sabbadin 1979; Monniot & Monniot 1987; Monniot 1988; Kott 2003).

The most diverse genera of the Styelinae in Australian waters are Cnemidocarpa (14 species) and Polycarpa (33 species). Both genera are well represented especially by indigenous species. The Indo-west Pacific tropical genus Polycarpa is known in Tasmania and New Zealand but not further south. Styela, never speciose, is represented by three, possibly introduced, opportunistic species, each with a cosmopolitan range. Monandrocarpa Michaelsen, 1904 and Asterocarpa Brewin, 1946 are each represented by a single species, and both genera are known only from the Southern Hemisphere.

Polyzoinae, are also well represented by the Polycarpa-like Polyandrocarpa Michaelsen, 1904 (10 species of which eight are indigenous), Oculinaria Gray, 1868 (monotypic) and Eusynstyela Michaelsen, 1904 (two species); one genus with reduced numbers of branchial folds, Stolonica Lacaze-Duthiers & Delage, 1892 (10 species); and those without branchial folds, Polyzoa Lesson, 1830 (one species), Metandrocarpa Michaelsen, 1904 (three species), Symplegma Herdman, 1886 (three species) and Chorizocarpa Michaelsen, 1904 (three species). In Botryllinae, four species each in both Botryllus Gaertner, 1774 and Botrylloides Milne-Edwards, 1841 are presently recorded from Australia.