The functional organization of the sclerome of five species of Acanthogorgia (Octocorallia, Holaxonia)

Abstract

Specimens of the genus Acanthogorgia (Acanthogorgiidae, Octocorallia) show different sclerite types in the colonie parts like axis coenenchym, polyps and tentacles. Applying high resolution images and three dimensional µCT-scans we compared five Acanthogorgia species (A. aspera, A. armata, A. granulata, A. hirta, A. muricata) from the North Atlantic Ocean. µCT-applications allow to investigate the mechanical set-up in the complex bond of sclerites and tissues. We studied the functional organisation of the sclerome to understand how the sclerites interact for mobilisation of the colony and the polyps.
The results show differences in forms, sizes and arrangements of the sclerites between the species of Acanthogorgia. The general pattern of the building structure closely relates to the function of the body parts and is the same in all investigated taxa. As a basis, the rigid axial skeleton allows gentle elastic swaying of the colonies to prevent breakage in strong currents. The tissue reinforced by skeleton structures enables the upright movement of tentacles as well as the expansion and contraction of the polyps. The basal parts of the polyps are anchored in the axial coenenchym, but the polyps of this genus cannot retract in the coenenchym. The study provides a schematic model of various dynamic interactions within the organisms construction to give a first view in the biomechanic of sclerite structures.

Full text

The functional organization of the
sclerome of five species of
Acanthogorgia (Octocorallia, Holaxonia)
Ulrike Buschewski1,2, Götz-Bodo Reinicke1,
Peter Michalik 2, André Freiwald3
Introduction
Specimens of the genus Acanthogorgia (Acanthogorgiidae,
Octocorallia) show different sclerite types in colony parts such as
axis coenenchym, polyps and tentacles. In order to understand
how these sclerites interact for mobilization of the colony and the
polyps, we investigated the spatial arrangement and interaction
of sclerites and tissues. We compared the functional
organization of the sclerome of five Acanthogorgia species from
the North Atlantic Ocean.
Material & Methods
•24 samples from different expeditions in the North Atlantic
•3D macro photographs
•micro-ct scans
1| (A) Acanthogorgia polyp with „en chevron“ pattern; (B) sclerites of Acanthogorgia: tentacle
sclerite (t), spindles (sp), crown spicula (cr), irregular radiates and thorn scales (c); (C) 3D
reconstruction of a polyp. En chevron row is marked in margenta, coenenchym sclerites in green.
Contact: ulrike.buschewski@meeresmuseum.de
1Deutsches Meeresmuseum, Katharinenberg 14-20, 18439 Stralsund, Germany
2Universität Greifswald, Zoologisches Institut und Museum, Loitzer Straße 26, 17489 Greifswald, Germany
3Senckenberg am Meer, Abteilung Meeresforschung, Südstrand 40, 26382 Wilhelmshaven, Germany
Conclusions
The functional colony structure
of skeletal elements and tissues
is constructed by different
modular units (like the polyp
anchoring, the anthocodial and
tentacles armature). These units
vary in numbers, sizes and
forms of their sclerites. Mobility
of tissues decreases with larger
and increases with smaller
sclerite sizes. They connect with
the tissues and play an
antagonistic role towards the
hydraulic erection of the polyps.
Limits in sclerites forms and
sizes variability result from a
trade-off in rigid stiffness vs. the
hydraulic antagonism and
necessary mobility as well as
the predator pressure.
3| The en chevron pattern allows
the orderly movement of
contraction and expansion.
A.
armata
A.
aspera
A.
granulata
A.
hirsuta
A.
muricata
Polyp
Length
4
-8 mm
1
-3 mm
1
-5 mm
< 1,5 mm
<5 mm
Surface
smooth
spiny
smooth
spiny
smooth
Sclerite
organisation (fig. 2)
both
both
simple
simple
both
Crown
spicula:
number
& arrangement
n=12
-16
longest
in the
centre
n=10
-13
longest
in the centre
n=9
-12
moderately
long
n=7
-10
smaller
to the oral
1
very long distal
5
-7 small
Coenenchym
Thickness
in sclerite layers
1
-2
1
-2
2
-3
1
-2
2
-3
Surface
smooth
rough
rough
rough
smooth
Sclerite
composition
spindles
, radiates
spindles
, radiates,
thorn
scales
radiates
, spindles
radiates
, thorn scales,
spindles
spindles
, radiates
4| The crown spicula are projecting
radially and the still retracted
tentacles are armoured by short
flattened sclerites.
Results
The five Acanthogorgia species show differences in the length and the surface of the polyps.
Long polyps have a smooth surface. There is a variability in sclerite organization shown in fig.
2. Often the smaller polyps have a simpler structure. The numbers of the crown spicula vary
from 6 to 16 sclerites, and they are differently arranged depending on the species. The major
difference in the coenenchym structure is sclerite composition. No differences could be
observed in terms of tentacle sclerites between species.
2| The variability in sclerite
organizations in the en chevron
pattern in the polyps.
A. armata A. aspera A. granulata A. hirsuta A. muricata
ExpansionContraction
Sclerite Package
Simple
ABC
t
c
cr
sp
coenenchym at axis
anthocodium
crown
spicula
tentacle