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.  

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).

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? 

[5] Collins, E. L. (2007) The effect of caffeine and ethanol on flatworm regeneration. Electronic Theses and Dissertations. [Accessed 02/06/2015].