Larval development of Myzostoma cirriferum (Myzostomida)

Abstract

The larval development of Myzostoma cirriferum is described by means of SEM, TEM, and cLSM. It is similar to that of other myzostomids and includes three stages: the protrochophore, the trochophore, and the metatrochophore. The protrochophore is a ball-shaped larva present in culture from 18-48 h after egg laying. It has no internal organs and its body is made of three cell types: covering cells and ciliated cells that are external and surrounded by a cuticle, and resting cells that fill the blastocoel. The trochophore is a pear-shaped larva that develops 20-72 h after egg laying; the body includes the same three cell types as the previous stage. The metatrochophore is a pear-shaped larva that develops between 40 h and 14 days and is characterized by the presence of two bundles of four chaetae. When fully developed, the metatrochophore has a digestive system (made of a pharynx, an esophagus, and a blind digestive pouch), two pairs of protonephridia, and a nervous system composed of a supraesophageal ganglion, circumesophageal connectives, and dorsal and ventral nerves. Metamorphosis generally occurs 7 days after egg laying. At that time, the metatrochophore loses its chaetae and becomes pleated ventrally. This ultrastructural analysis suggests that chaetae and the five ventral longitudinal nerve cords of M. cirriferum metatrochophores are homologous structures to those observed in some polychaete trochophores. Coupled with recent phylogenetic analyses, where the Myzostomida are placed outside the Annelida, homologies between myzostomid and polychaete larvae support the view that a trochophore appeared early during the spiralian evolution.

Nervous and muscle system development in Phascolion strombus (Sipuncula)

“…However, several recent independent molecular phylogenetic studies based on mitochondrial DNA sequence data indicate that Sipuncula may cluster with Annelida rather than with Mollusca (Boore and Staton 2002;Staton 2003;Jennings and Halanych 2005). Although our data do not provide any evidence for ancestral segmentation in Sipuncula, the existence of a distinct ventral median nerve is shared with dinophilid and dorvilleid polychaetes as well as with myzostomids and hirudineans (Payton 1981;Westheide 2000, 2002;Eeckhaut et al 2003). If the assumption that such a median nerve cord represents an evolutionary conserved character proves to be correct (see Westheide 1997, 2002), the hypothesis of Sipuncula being closely related to Annelida would gain additional morphological support.…”

“…A median nerve that is positioned between the two ventral nerve cords, as observed in juvenile Phascolion, has also been found in certain polychaetes and in the myzostomid Myzostomum cirriferum Westheide 2000, 2002;Eeckhaut et al 2003), although the expression of neurotransmitters in this nerve cord varies. While Myzostomum and the dinophilid and dorvilleid polychaetes investigated so far show positive immunoreactivity against α-tubulin, only the dinophilid Dinophilus gardineri, but not Trilobodrilus axi, contains both serotonin and FMRFamide.…”

The simplicity of males: Dwarf males of four species of Osedax (Siboglinidae; Annelida) investigated by confocal laser scanning microscopy

“…However, the crossing over does show some resemblance to the additional ventral fibers crossing in the middle part of the subesophageal ganglion of Trilobodrilus hermaphroditus (Mü ller and , which, while not suggesting they are homologous, does provide some support for our interpretation of an elongate first segment in Osedax males. The presence of a brain and two cerebral commissures in Osedax males, as also seen in the metatrochophore larvae of e.g., M. cirriferum (Eeckhaut et al, 2003), further supports their resemblance to the late metatrochophore or newly settled annelid larval stages, rather than earlier larval stages. The presence of prostomial anterior ''double'' bottleshaped sensory cells may be a reminiscence of the apical organ, observed in early trochophore stages by Rouse et al (2009).…”

“…The five ventral longitudinal nerves resemble the five ventral longitudinal nerves that have been found to eventually fuse into the ventral nerve cord in several other annelid larvae (e.g., Mü ller and Orrhage and Mü ller, 2005). Overall, the Osedax male nervous system is quite similar to the larval nervous system of Myzostoma cirriferum, with median, main, accessory, and (dorsal) lateral nerves (Eeckhaut et al, 2003). The separated longitudinal nerves seen in Osedax males also resemble the situation seen in the larvae of the dorvilleid Ophryotrocha gracilis (Mü ller and and the possibly progenetic Dinophilidae (Mü ller and Westheide, 2002).…”

Locomotion and fine structure of parapodia in Myzostoma cirriferum (Myzostomida)

“…Chaetae described in juvenile octopods, a certain placophoran, brachiopods and bryozoans are probably not homologous structures (Hausen 2005). The ultrastructure of myzostomid chaetae has not yet been analysed in details except for their larvae (Eeckhaut et al 2003). This paper aims at analysing the ultrastructure of the parapodia of the European myzostomid Myzostoma cirriferum Leuckart, 1836, especially the chaetae and to compare them with those of the other taxa.…”