PhD thesis

16 Results and discussion
Results and discussion
Larval development
Terebratalia transversa, a representative of Rhynchonelliformea
Larval development of Terebratalia transversa and regional specification during
embryogenesis has been described previously (Freeman 1993). My results
are congruent with these data. The oocyte (Fig. 1A) divides approximately 2
hours after fertilization (hpf) at a water temperature of 11.5 °C and two polar
bodies are formed (Fig. 1B). Cleavage is radial and the first two cleavages are
holoblastic (Fig. 1B, C). The early blastula is composed of rounded cells (Fig.
1D) and gastrulation occurs approximately at 19 hpf (Fig. 1E). In the gastrula,
the wall of the archenteron forms contact with the cells of the ectoderm, i.e.,
the blastocoel virtually disappears (Fig. 1F). Later in development the gastrula
elongates and the blastopore becomes slit-like elongated (Fig. 1G). The three
larval lobes start to form as the embryo elongates further and an apical tuft
appears, which is lost later in development (Fig 1H, I). At this stage the larvae
become positively phototactic and usually swim in the upper part of the water
column. At approximately 75 hpf the larvae are almost fully developed and the
apical, mantle, and pedicle lobe are formed. Only the setae continue to grow
at this point of development. The fully developed larvae eventually become
negatively phototactic. Then, they swim towards the bottom of the culture dish
and repeatedly touch the surface with their apical lobe, probably in order to test
if the substrate is suitable for metamorphosis. Larvae settle and metamorphose
between 120 and 300 hpf. The juveniles still retain the larval setae and the
lophophore starts to form after settlement (Fig. 1J). Metamorphosis appears to
be catastrophic since all tissues seem to be reformed during metamorphosis
(Stricker and Reed 1985a, 1985b).