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Student Project

Cilia of Turbellarians

Taylah Gerloff 2016


The order Prolecithophora are quite easily distinguished from other orders of Turbellaria due to features such as their unbranching sac like gut and number of eyes (Karling, 1993). This study species is a possibly undescribed prolecithophora species that was found off the east coast of Australia, in temperate waters. This is a common habitat type for prolecithophorans Hinde 2001). One interesting thing that was found within this species is what looks like a symbiotic relationship with algae in the gut of the species. Even when the individuals were not fed for a week, the dark algae could still be seen in their gut, which suggests that the algae is not only a food source, but possibly an important relationship. Turbellarian species have been known to have symbiotic relationships with other organisms such as crustaceans and bivalves, so with more research this could be a very interesting field to work in (Ax et al., 1989). These animals have a very basic bauplan, lacking any circulatory or respiratory organs, which limit their size and shape (Karling 1993). Their main anatomical features include a blind gut, central nervous system (controlled by a ‘brain’ or main ganglion), specialised pharynx, eyes, and cilia and muscles for movement (Karling 1993). Prolecithophorans are known to live all around the world, where their favoured conditions are met, however there are still many undescribed species that need to be researched (Noren & Jondelius, 2004). 

Physical Description

Generally speaking, all Turbellarians share the following physical features: bilateral symmetry, lack of a coelom, distinct dorsal and ventral sides, distinct ‘head’ and ‘tail’ ends, and triploblasty (they are composed of 3 cellular layers that are formed during development: mesoderm, endoderm and gastroderm) (Karling, 1978). Another feature that unifies all turbellarians is the possession of and use of their cilia (Karling, 1978). In fact, the name ‘Turbellaria’ refers to the ‘whirlpools’ that can be created by the moving cilia. This will be discussed in detail further on.

This species is from the subphyla Rhabditophora (Noren & Jondelius, 2002). Rhabditophora can be distinguished from other subphyla by differences in the morphology of the gut.  Triclad and Polyclad species have branching guts; three branches for Triclad and many branches for Polyclad (Noren & Jondelius, 2002). As seen in figure 1, this specimen has an unbranching gut, which is characteristic of Rhabditophora.

This particular specimen’s form can be described as a general elongated droplet of approximately 1-2mm in length. It has a distinct head end and tail end (shown in the figure below), and a blind gut. They are a translucent orange/brown in colour allowing visualisation of the gut without the need for sectioning. The head contains a ciliated groove which is used for feeding, as well as two sets of eyes.

Figure 1


Most species from the order prolecithophora are marine, however a select few are fresh water species (Worms, platyhelminthes, 2013). This specific species found is entirely marine. Different species within the order prolecithophorans have different feeding methods: carnivory, scavengery and symbiotic relationships (Worms, platyhelminthes, 2013). Within the order there are more than one type of pharynx, and the type of pharynx a species has can relate to the feeding habit. Unfortunately, the pharynx of this specimen could not be located and thus this could not be determined. However, as seen in figure 1, the gut of the specimen appears to have individual brown cells throughout, which could potentially suggest a symbiotic relationship between the species and algae. Even when the individuals were not fed for a week, the dark algae could still be seen in their gut, which suggests that the algae is not only a food source, but possibly an important relationship. It is known that some species do in fact have a symbiotic relationship with other organisms, so with further research, this relationship could be studied (Ax et al., 1989).

As prolecithophorans are quite versatile, their habitat ranges are quite broad. The study species was found on settlement plates, but they are free living organisms and thus can be found in many different habitats. They can be found in soft sediments, among biofouling communities (such as colonial corals or bryozoans) and low energy sub tidal environments.

Life History and Behaviour


Most flatworms, including prolecithophorans, possess the ability to regenerate and asexually reproduce via totipotent stem cells (Martín-Durán & Egger, 2012). This was not directly observed for the study species, so to determine if this is true for the study species, further research will need to be conducted. Prolecithophorans are all capable of sexual reproduction as well. Very little research has been conducted on the development patterns of prolecithophorans; in fact only two species have been the subject of such studies (Martín-Durán & Egger, 2012). It was found in both of these species that the cleavage pattern is unequal and without regular division patterns (Martín-Durán & Egger, 2012).  It was also found that they directly develop from juveniles into adults (Martín-Durán & Egger, 2012).


These flat worms are free living flatworms, meaning that they can move about freely. With their photoreceptors (eyes) they are able to move about determining the difference between light and dark; giving them quite directional movement (Karling 1993). They move by using the cilia located on their ventral side. In figure 2, some cilia can be visualised around the outside of the animal. While observing the study species, a ‘whirlpool’ of water and particles in the water was seen, being moved by the cilia.

Anatomy and Physiology


Prolecithophorans are bilateral acoelomates. They lack any circulatory or respiratory organs, which limits their size and shape due to gas exchange (Hinde, 2001). This is why many of the species are extremely small, or flat – to facilitate gas exchange. Respiration has to occur through the whole body surface, which makes them vulnerable to dehydration (Hinde, 2001). This is why most species are found in aquatic environments (Hinde, 2001).
As seen in figure 2 below, the gut of the study species is blind (having no anus for excretion). This is common among turbellarians. Figure 2 also shows the enveloped ‘brain’ of the species. This ‘brain’ is usually just the main ganglion controlling the central nervous system of the animal (Littlewood & Bray 2001).Prolecithophorans do in fact have a nervous system broken into two parts: central nervous system and peripheral nervous system (Littlewood & Bray 2001). Most prolecithophorans have two pairs of pigmented eyes on the head end near this ‘brain’, and as seen in both figure 1 and 2, this study species does as well. 

Figure 2


Cilia is present all throughout the body form of prolecithophorans. They can easily be seen on the ventral surface (as seen in figure 2), this is usually used for movement along with muscular contraction. However, cilia can also be found in the pharynx, genitals and even photoreceptive cells of individuals. A study by Piskurek et al. (1998) found a new type of ciliated photoreceptive cell in a species that belongs to the order prolecithophora. The use of cilia in these species is obviously very varied, and an area that could be studied more in the future. This species, along with some other prolecithophorans, also have a ciliated groove visible on the head (Worms, Platyhelminthes, 2013). Having this groove on the head could help move food particles directly into the mouth.

Biogeographic Distribution

Prolecithophora have been labelled as having a cosmopolitan distribution. This means that they can be found anywhere around the world where their favoured conditions are met. These conditions are generally thought of as being temperate waters (Noren & Jondelius 2004). Little is known about species that live in other areas (tropical waters, deep ocean or freshwater), however there are species that persist in these conditions. This particular species was found in Moreton Bay, Queensland, Australia. Moreton Bay is often referred to as a merging zone of sub-tropical and temperate conditions and organisms.

Evolution and Systematics


It was first believed that turbellaria were the most basal bilaterians, and could help with research into the evolution of bilateral symmetry (Karling, 1993). However, more recently it has been believed that this could belong to Xenacoelomorpha: a new phylum that used to be a part of Platyhelminthes. While they may not be able to explain how bilateral symmetry arose anymore, they are still an important stepping stone in understanding the diversification of bilateria


The position of Platyhelminthes on the phylogenetic tree has been debated upon since the phyla was first described. Within the phyla there has also been a lot of issues to do with classification. For example, what was first thought of as a class of Platyhelminthes - acoelomorphs - have now been moved into a new phylum of their own with Xenaturbellaria. Despite this fluctuation, the classification of Prolecithophora seems to be settled upon (Laumer, 2015; Noren & Jondelius, 2002; Noren & Jondelius, 1999).

Kingdom: Animalia
Phyla: Platyhelminthes
  Subphyla: Rhabditophora
Class: Turbellaria
Order: Prolecithophora 

Conservation and Threats

This species, along with many other turbellarians is not a described species. In a number of studies, undescribed turbellarian species were found (Noren & Jondelius, 2004; Christensen, 1981). This suggests that turbellarians as a whole are quite common, and therefore do not need any conservation efforts. More research must be conducted into describing these species before determining if this particular order is under threat or not. 

The main threat to prolectihophorans is climate change. As said before prolecithophora are commonly found in temperate waters, and with increasing temperature they will have to disperse to new areas will provide them with the conditions they need. However, little is known about other species (ie deep sea species) that may be more affected by climate change. There are also the endemic freshwater species that will be heavily affected by climate change.


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