Muscular systems in gymnolaemate bryozoan larvae (Bryozoa: Gymnolaemata)

Abstract

Gymnolaemate bryozoan species exhibit larval forms that can either be planktotrophic cyphonautes or one of the several types of lecithotrophic larvae. Due to the lack of both a phylogenetic system of the Bryozoa and detailed information about larval morphology for many species, it is diYcult to elaborate hypotheses about the evolution of larval forms in this group at the moment. In this study, phalloidin staining and confocal laser scanning microscopy are used to provide detailed morphological descriptions of the three-dimensional structure of muscular systems in the larvae of Flustrellidra hispida, Alcyonidium gelatinosum, Membranipora membranacea, Bugula fulva and Bowerbankia gracilis. A comparison to the available data from the literature is added. The results show muscular systems to be much more complex than previously recognized. In all but one species, an anterior median muscle inserts with one end at the basal side of the apical organ and with its other end at the papilla of the pyriform organ. This muscle can be paired or unpaired and was referred to as "neuromuscular strand" in earlier descriptions. All species except Bugula fulva exhibit a coronal ring muscle running concentric to the larva's main locomotory ciliary band. Lateral muscle strands that connect the pallial epithelium to either the corona or oral epithelium are found mainly in the cyphonautes and pseudocyphonautes larvae and, in modiWed shape though, also in some coronate larvae. Transversal or shell adductor muscles can be shown to co-occur with lateral muscles and possibly originate from the same source.The Wndings are in accordance with the idea of a multiple origin of lecithotrophic forms within gymnolaemate Bryozoa.

Ultrastructure of mesoderm formation and development in Membranipora membranacea (Bryozoa: Gymnolaemata)

“…The musculature of the cyphonautes larva shows distinct functional groups (Gruhl 2008), which already diVerentiate early in development, as shown in the present study. Apart from muscle cells, the most abundant cell type is mesenchyme.…”

Serotonergic and FMRFamidergic nervous systems in gymnolaemate bryozoan larvae

“…1. For further descriptions of larval morphology in the following species; see Gruhl (2008) and references therein.…”

“…Besides the planktotrophic cyphonautes, shelled lecithotrophic larvae and at least Wve diVerent types of coronate larvae are known within the subtaxon Gymnolaemata (Zimmer and Woollacott 1977b). Taxonomic distribution as well as signiWcant morphological diVerences argue for a convergent evolution of the various coronate larvae (Zimmer and Woollacott 1977b;Gruhl 2008). It can, therefore, be expected, that their independent evolution has also resulted in apomorphic characters in their nervous systems.…”

Development, organization, and remodeling of phoronid muscles from embryo to metamorphosis (Lophotrochozoa: Phoronida)

“…Pelagic larvae of marine invertebrates usually have a well-developed apical organ, which can be withdrawn into the larval body via contraction of the apical muscle. This muscle, which extends between apical plate and body wall or wall of the digestive tract, occurs in tornaria ([32]: Figure 4), bryozoan larvae [46,52], entoproct larvae [53], and nemertean larvae [44]. If the larva is threatened, the contraction of this muscle pulls the apical organ into the larval body.…”