Scanning electron microscopy of lattice organs in cyprids of the Rhizocephala Akentrogonida (Crustacea: Cirripedia)

Abstract

The morphology and disposition of lattice organs, putative sensory structures in the carapace of cyprids, are described with scanning electron microscopy from six rhizocephalan species representing four of the five families in the suborder Akentrogonida: Clistosaccus paguri and Sylon hippolytes (Clistosaccidae), Arcturosaccus kussakini (Duplorbidae), Mycetomorpha vancouverensis (Mycetomorphidae), and Diplothylacus sinensis and Thylacoplethus reinhardi (Thompsoniidae). The disposition of pores and setae on the cyprid carapace is also surveyed. In T. reinhardi, poor quality of the specimens allowed only confirmation that lattice organs are present. Cyprids of all other species except M. vancouverensis have two anterior and three posterior pairs of lattice organs on the carapace, just as in almost all other Cirripedia. Cyprids of C. paguri and S. hippolytes are nearly identical in both general shape, carapace setation, and morphology of the lattice organs. In both species the lattice organs are oval–circular pore fields, except the second pair which is very elongate and match-shaped, a morphology unique to the Clistosaccidae. The similarities in cyprid morphology are interpreted as apomorphies supporting the monophyly of the recently rediagnosed Clistosaccidae. Both thompsoniid species have oval–circular lattice organs, indicating a possible sister-group relationship between the Clistosaccidae and the Thompsoniidae. None of the lattice organs of A. kussakini are oval or match-shaped. In M. vancouverensis there is only a single anterior pair of lattice organs, a probable autapomorphy. But the morphology of the M. vancouverensis cyprid is otherwise very reminiscent of those in the Rhizocephala suborder Kentrogonida, such as in being ornamented with numerous (> 100) carapace setae in no fixed pattern compared with the few (< 80) setae in a fixed disposition seen in the other akentrogonids. In cyprids of all Akentrogonida examined, the lattice organs lack a large terminal pore. By out-group comparison with all other cirripedes and with the Ascothoracida, this is regarded as an apomorphy that supports the monophyly of the Akentrogonida.