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 tenellus Boschma, 1970, revealed five naupliar instars, one more than previously 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 and one additional antennulary seta. Differences in outline, shape of antennulae, and hind-body denticulation, offer the potential of individual discrimination. All the nauplii possess a transparent, hollow cuticular ring around their body. This flotation collar represents the bulged margin of the larval head shield and is attached to the body along a delicate ridge. Three pairs of tiny pores in contact with the ridge possibly regulate inflation of the ring, but details of this mechanisms remain unknown. Due to total lecithotrophy, the nauplii of B. tenellus have limbs setation reduced to that needed for swimming only, and other feeding structures such as the labrum are also highly reduced. In the antennulae, the morphological changes in form and setation were followed from nauplius to cypris and shown to largely resemble events in ontogeny of the thoracican barnacle Semibalanus balanoides. On this basis we establish a homology scheme between antennulary setae in these two species. In both B. tenellus and S. balanoides, a naupliar seta, apically on the fourth antennulary segment develops into a conspicuous aesthetasc while one (B. tenellus) or two (S. balanoides) subapical 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 misleading or even erroneous and improvements are suggested. Notably replacing cypris carapace with the ontogenetically and phylogenetically more informative term head shield.