PhD thesis

56 Chapter III
Altenburger and Wanninger
Brachiopod neuromuscular development 19
Fig. 2. Semischematic representation of
the larval musculature of craniiform and
rhynchonelliform brachiopods. (A) Musculature
of Novocrania anomala with
setae pouch muscles (red circles), medioventral
longitudinal muscles (white), and
transversal muscles (yellow-grey). Size of
the specimen is approximately 150 mm.
(B) Musculature of Argyrotheca cordata
based on Altenburger and Wanninger
(2009) with pedicle muscles (beige), longitudinal
muscles (orange), central mantle
muscles (brown), U-shaped muscle
(green), setae pouch muscles (red circles),
circular mantle muscle (light blue), serial
mantle muscles (dark orange), setae muscles
(purple), apical longitudinal muscles
(dark blue), and apical transversal muscle
(yellow). Size of the specimen is approximately
280 mm.
juvenile musculature comprises mantle margin muscles,
oblique muscles, as well as anterior and posterior adductor
muscles (Fig. 1F).
Neurogenesis
The first signals of serotonin-like immunoreactivity appear in
fully developed, metamorphic competent, bilobed larvae at
approximately 86 hpf (Table 1). At this stage, four flaskshaped
cells are present in the anterior-most part of the apical
lobe (Fig. 3, A–D). They are oriented in different directions
with only one pointing toward the apical pole of the larva.
The flask-shaped cells are connected to two ventral neurites
that extend posteriorly (Fig. 3, A–D). The flask-shaped cells
are lost during metamorphosis, and early juveniles have two
ventral neurites that project from the anterior lobe into the
posterior lobe (Fig. 3E). During subsequent development, the
ventral neurites become interconnected by a median commissure
in the mid-part of the juvenile (Fig. 3F).
The axonal marker a-tubulin is first expressed in juveniles 5
days after metamorphosis (Fig. 4A). Two solid neurite bundles
develop ventrolaterally in the anterior lobe of the juvenile and
subsequently grow in posterior direction into the posterior lobe
(Fig. 4B). Later in development, these neurite bundles close by
an anterior and a posterior commissure, and the median commissure
is established (Fig. 4, C–F). Serially arranged mantle
neurites extend from the anterior part of the ventral neurite
bundles in a lateral direction toward the mantle margin of
the juvenile (Fig. 4, B–F). Comparison of the position of the
a-tubulin signal in the juvenile and the serotonin-like signal in
the larva suggests that the larval ventral neurites are the earliest
neurites of the future ventral neurite bundles of the juvenile.
Table 1. Landmarks of Novocrania anomala development at 141C
Age (hours post
fertilization)
Gross morphology
Myoanatomy as inferred by
F-actin staining
3–4 First cleavage No signal No signal
26–30 Swimming, spherical gastrula No signal No signal
42–49 Swimming, elongated gastrula No signal No signal
65–73 Swimming, bilobed larva with
setae starting to develop
First signals of actin in setae
pouches (Fig. 1A)
No signal
86–96 Fully established, swimming,
bilobed larva with long setae
168 Settled juvenile after metamorphosis
Fully developed larval musculature
with setae pouch muscles, longitudinal
muscles, and transversal muscles
(Fig. 1D)
Juvenile with mantle margin muscles,
anterior adductors, and posterior
adductors (Fig. 1F)
Neuroanatomy as inferred by
antibody staining
Larval nervous system with four
flask-shaped cells in the apical organ
and two ventral neurites (Fig.
3, B–D)
Juvenile with two ventral neurite
bundles, commissures, and serially
arranged neurites (Figs. 3, E and
F and 4, A–F)