Order APLOUSOBRANCHIA

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Order APLOUSOBRANCHIA

Compiler and date details

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

Introduction

The suborder Aplousobranchia (order Enterogona) is characterised by the division of the body into thorax, abdomen and sometimes posterior abdomen. Gonads are unpaired and are in the gut loop (in the abdomen), or posterior to it. Paired epicardial sacs (embryonic endodermal outgrowths of the posterior end of the pharynx) persist in adult zooids, although (with the exception of the Cionidae) their connection with the pharynx is lost and often they are fused. The endodermal tissue of the epicardial sacs is the regenerative tissue involved in repair and replication in all but the family Clavelinidae, in which the regenerative tissue is mesodermal (Berrill 1950). Most aplousobranch families are primarily colonial; although the monotypic Cionidae (genus Ciona) always is solitary and some exceptional solitary species occur also in Diazonidae, Clavelinidae and Euherdmaniidae.

The overall tendency in this suborder to form colonies through replication, interrupts the growth of individual zooids resulting in their progressive size reduction and simplification, and subsequently to the evolution of integrated colonies with zooids arranged in complex cloacal or other systems that enhance colony interactions with the environment (Kott 1989). Internal longitudinal branchial vessels (present in Ciona and Diazonidae) are usually absent although their remains (in the form of papillae) persist in some Protopolyclinidae, Ritterellidae and Polyclinidae. Eggs are fertilised externally only in Cionidae and Diazonidae. Associated with reduction in zooid size and increase in the numbers of replicates of a single genotype, gonads are relatively small in zooids of colonial taxa, the relatively few eggs produced are fertilised internally, and embryos, brooded either in the zooids or in the colonial test, are liberated as tailed larvae.

Australian Aplousobranchia have been revised by Kott (1990, 1992a,b, 2001, 2002, 2003, 2004a-c, 2005). Family level taxa are characterised by their colonial organisation; position, size and form of the gonads; site of fertilisation and brooding of embryos; size of the thorax and condition of the branchial sac including the number of stigmata and transverse branchial vessels; length of the oesophagus and shape of the stomach; and differences in larval adhesive organs and accessory adhesive apparatus.{0244}