LARVAL DEVELOPMENT OF THE RHIZOCEPHALAN CIRRIPEDE BRIAROSACCUS-TENELLUS (MAXILLOPODA, THECOSTRACA) REARED IN THE LABORATORY - A SCANNING ELECTRON-MICROSCOPY STUDY

SEM investigations of laboratory-reared larvae of Briarosaccus tenellu s Boschma, 1970, revealed five naupliar instars, one more than previou sly described for the Rhizocephala. The external features of these and the cypris larva are described in detail. The youngest instar (NO) is characterised by a better developed furca than in subsequent stages a nd one additional antennulary seta. Differences in outline, shape of a ntennulae, and hind-body denticulation, offer the potential of individ ual discrimination. All the nauplii possess a transparent, hollow cuti cular ring around their body. This 'flotation collar' represents the b ulged margin of the larval head shield and is attached to the body alo ng a delicate ridge. Three pairs of tiny pores in contact with the rid ge possibly regulate inflation of the ring, but details of this mechan isms remain unknown. Due to total lecithotrophy, the nauplii of B. ten ellus have limbs setation reduced to that needed for swimming only, an d other feeding structures such as the labrum are also highly reduced. In the antennulae, the morphological changes in form and setation wer e followed from nauplius to cypris and shown to largely resemble event s in ontogeny of the thoracican barnacle Semibalanus balanoides. On th is basis we establish a homology scheme between antennulary setae in t hese two species. In both B. tenellus and S. balanoides, a naupliar se ta, apically on the fourth antennulary segment develops into a conspic uous aesthetasc while one (B. tenellus) or two (S. balanoides) subapic al and postaxially sited setae on the same segment develops into into four short setae in the cypris. Our study reveals that the terminology used in describing cirripede nauplius and cypris larvae is often misl eading or even erroneous and improvements are suggested. Notably repla cing 'cypris carapace' with the ontogenetically and phylogenetically m ore informative term 'head shield'.