Larval Development in Peltogasterella Studied by Scanning Electron Microscopy (Crustacea: Cirripedia: Rhizocephala)

Abstract

SEM and light microscopic studies on the larvae of the rhizocephalan barnacles Peltogasterella gracilis and P. sulcata reared in the laboratory revealed the presence of 5 naupliar instars, the newly found stage being instar 3. It resembles instar 2 but is larger and has a body slightly tapering towards the posterior end, a reduced subapical (preaxial) seta on the antennule, and both transverse grooves and a specific denticle ornamentation on the hind body of female larvae. In cultures, the larvae of both species develop into cyprids within 4–5 days after release. Peltogasterella nauplii have a size comparable to that seen in Sacculina and Peltogaster, but they are much smaller than the larvae of the cold-water Briarosaccus. The nauplii of Peltogasterella are lecitotrophic and have limb setation and other feeding structures highly reduced. Peltogasterella nauplii have a cuticular flotation collar encircling the body, but it is very small, delicate, and almost smooth. It therefore differs from the homologous, but much larger, and heavily ornamented flotation collars seen in nauplii of Peltogaster and Briarosaccus. A characteristic feature in Peltogasterella nauplii is the single subterminal seta at the distal end of each frontolateral horn, whereas nauplii of Briarosaccus and Peltogaster have two such setae. The antennulary basipod carries a short additional seta, which represents the only remaining rudiment of gnathobases. Nauplii of Briarosaccus and Peltogaster have a homologous, but significantly longer seta, whereas Sacculina nauplii lack this seta altogether. The nauplii of P. gracilis and P. sulcata are very similar in size and morphology, but those of P. sulcata differ by somewhat longer frontolateral horns and furcal spines. Another distinctive feature of P. gracilis nauplii is the ventral ornamentation of the hind body that shows different pattern at different stages of development and reflects an internal development of segments. This ornamentation is only weakly pronounced in P. sulcata nauplii. Comparison with larval development in Briarosaccus tenellus leads to the conclusion that the rhizocephalan ground pattern comprises six naupliar stages just as in other cirripedes (e.g., Thoracica). Comparison of lattice organ morphology indicates a sister group relationship between the families Peltogastridae and Lernaeodiscidae. Our study highlights the value of SEM revealed larval characters and of characters other than appendage setation as being important in rhizocephalan phylogeny.