|
The Regeneration of the Benthic Ctenophore, Coeloplana agniae.
|
|
Morgan Purdy 2015
|
|
|
|
Introduction | |
Ctenophora is an
exclusively marine metazoan phylum, playing an important role in pelagic
ecosystems as planktonic predators [1,2]. Like Cnidarians, previously combined
with Ctenophores in a now defunct taxon Coelenterata,
the ctenophore body plan is arranged radially around an oral-aboral axis and
the mesoglea is thick and buoyant [1]. Ctenophores have a bi-radial symmetry
about two imaginary planes. The oral end of the ctenophore body bears the mouth,
while a brain-like organ is located at the aboral region [1]. One of the main
differences, separating Ctenophores from Cnidaria, is the presence of eight
rows of cilia, between the two body poles, that can move the relatively large
body of a ctenophore at speeds of up to 50mm/sec [1, 2]. The cilia are congregated
together in paddles known as ctenes (or combs) which are aligned single-file in
each of the eight comb rows [1, 2]. These comb rows originate at the aboral
pole and end before reaching the mouth on the oral end [1]. A rudimentary
brain, the apical (or aboral) organ, is a combination receptor-effector that
controls the cilia comb rows [1. 2]. Ctenophores also bear two long tentacles,
one on each side of the body, that originate on the aboral hemisphere [1, 2, 3].
The tentacles, which are highly extensible and contractile, emerge from deep,
ciliated pouches (tentacle sheaths) [1. 3]. Both tentacles bear a lateral row
of tentilla (singular: tentillum), which are threadlike filaments [1, 3]. Numerous
specialised cells, collocytes, are found embedded within the epidermis of each
tentacle and tentillum [1, 2, 3]. These specialised cells release a sticky
substance that adheres to prey [1, 2, 3]. Skeletal mesoglea, an elastic gel
containing collagen and cells of various types and functions, occupies most of
the body volume [1].
Platyctenida, an
order of Ctenophora, are considered
the “odd ducks” among ctenophores. Unlike other ctenophores, the flattened, benthic
platyctenids resemble sea slugs and flatworms. The characteristic comb rows of
ctenophores are absent in the adult form [1].
As a result, locomotion is achieved by muscular undulations of the body
(c.f. flatworms) [1]. Adults still possess two well-developed, tentilla-bearing
tentacles that are used as the primary means of prey capture [1, 3]. The larvae
of platyctenids bear six comb rows, however, upon metamorphosis the comb rows
are lost, the proportion of mesoglea to other tissues decreases, the body
flattens in the oral-aboral axis, and coelenteric canals (part of the
coelenteron; the central gastric cavity) branch to form a complex, whole-body
network [1]. Platyctenids are able to reproduce asexually, excising fragments
from the margin of their flattened bodies [1]. These fragments undergo
differentiation and grow into new individuals [1]. A study conducted by Freeman
(1967)[4] shows that asexual reproduction defines the capacity for regeneration
in the benthic ctenophore, Vallicula
multiformis.
In Coeloplana, a genus
of benthic Platctenida, comb rows are
replaced by simple papillae containing a blind meridional-canal outgrowth [1]. Most
Coeloplana species are ectosymbionts
associated with Echinodermata or Cnidaria [3]. Many benthic ctenophore
species, including those of Coeloplana,
use pigmentation and colour patterns to camouflage themselves [1]. Coeloplana agniae, a species of benthic
ctenophore, can measure up to 25mm and assumes a round-to-oval shape. C. agniae is transparent which is
typical of planktonic ctenophores. This species has been shown to follow the
same asexual reproduction as was observed in V. multiformis [4].
Regeneration of Planarian flatworms is widely published in
the scientific literature, however, little research has been spent on the
regeneration capacity of ctenophores. Information on the mechanisms underlying
the regeneration process can be obtained by investigating the regeneration of
missing body parts in model organisms.
|
.jpg) |
Figure 1 |
|
.jpg) |
Figure 2 |
|
|
|
Materials and Methods | |
The Coeloplana agniae
specimens were harvested from Heron Island on rock plates. They were
transported to St. Lucia campus, University of Queensland, and maintained in a marine
aquarium located in the Goddard Building. Five individuals were then collected
and transported to the Goddard laboratory. The ctenophores were cut [see below]
with a sterile scalpel, before the fragments were placed into separate petri
dishes containing approximately 5mL of filtered sea water (enough to cover the fragments).
The specimens were kept in the petri dishes, in the laboratory, for seven days.
On the seventh day, the fragmented ctenophores were examined and any visual
signs of regeneration were recorded.
Cutting of the
ctenophores:
For operating, an organism was placed in a petri dish of sea
water, allowed to assume an extended/relaxed state and cut. One specimen was
sliced directly through the middle of the oral-aboral axis of the organism
(i.e. 50%) therefore each fragment contained one complete tentacle (and sheath)
and a proportion of the apical organ (Figure 3.A)). The second organism was cut
so that one fragment was approximately 75% of the original ctenophore and
contained the apical organ, one complete tentacle (and sheath), approximately
half of the second tentacle sheath, while the second fragment was approximately
25% of the original ctenophore and contained the half the tentacle sheath
(terminal end) (Figure 3.B)). A third ctenophore was sliced so that one
fragment was approximately 90% of the original ctenophore and contained the
apical organ, one complete tentacle (and sheath), almost the complete second
tentacle sheath (minus the terminal end), while the second fragment was
approximately 10% of the original ctenophore and contained the terminal end of
a tentacle sheath (Figure 3.C)). Two more ctenophores were cut so that there
were two replicates for each fragment percentage (i.e. one ctenophore was cut
into 75 and 25 percent fragments and another was cut into 90 and 10 per cent
fragments).
|
 |
Figure 3 |
|
|
|
Results | |
Regeneration was not able to be assessed on the seventh day
due to the nature of the ctenophore fragments. The
fragments had shrivelled and an accurate assessment was not possible.
|
|
|
Discussion | |
A technique that allows for the assessment of regeneration needs to be developed in order to complete the experiment.
A study conducted by Freeman (1967) [4] showed that the capacity
for regeneration in the benthic ctenophore, Vallicula
multiformis was defined by the asexual reproduction of the organism. This previous study showed that within 15 days after the incision, both fragments of V. multiformis ctenophores (one fragment with one set of tentacles and the apical organ; and one fragment with one set of tentacles) were able to regenerate whole animals [4].
Perhaps the current study did not allow sufficient time for the regeneration of the fragments to occur. Ideally, this investigation would be conducted again, over a larger period of time.
A study investigating the effect of caffeine and ethanol on flatworm regeneration showed that caffeine had no significant effect on the regeneration process, however, ethanol had an initial delayed effect on the
regeneration of the flatworms [5].
Further research on the effects of different substances, such as ethanol and caffeine, on regeneration, using ctenophores as a model organism, should be conducted.
Understanding how ethanol and caffeine affect regeneration can provide insight into the
understanding of how these drugs could potentially affect stem cells, and in turn, fetal development. Both ethanol and caffeine are commonly used substances and could be used by women who are carrying offspring, leading to adverse effects to the fetus.
Further investigations into the biology and ecology of these benthic ctenophores should also be conducted. Ctenophores play an important role in pelagic ecosystems as planktonic predators, yet little is known about the class that makes up the benthic ctenophores.
Potential investigation: At night,
ctenophores are noted for their bioluminescence, which emanates from
cells bellow the ciliated bands [1]; do benthic ctenophores share this characteristic with their sister ctenophores?
|
|
|
References | |
[1] Ruppert, E., Fox, R., & Barnes, R. (2004) Invertebrate
Zoology: A functional evolutionary approach (7th edition). Canada:
Brooks/Cole.
[2] Simiona, P., Bekkouchea, N., Jagera, M., Quéinneca, E.,
& Manuela, M. (2015) Exploring the potential of small RNA subunit and ITS
sequences for resolving phylogenetic relationships within the phylum Ctenophora.
Zoology 113: 102–114.
[3] Eeckhaut, I., Flammang, P., Lo Bue, C., & Jangoux,
M. (1997) Functional morphology of the tentacles and tentilla of Coeloplana
bannworthi (Ctenophora, Platyctenida), an ectosymbiont of Diadema setosum
(Echinodermata, Echinoida). Zoomorphology 117:165–174.
[4] Freeman, G. (1967) Studies on regeneration in creeping
ctenophore Vallicula multiformis. Journal of Morphology 123: 71-83
[5] Collins, E. L. (2007) The
effect of caffeine and ethanol on flatworm regeneration. Electronic Theses and
Dissertations. [Accessed 02/06/2015].
|
|
|
|
|