The oyster leech, Imogine mcgrathi, is platyhelminth parasite, unlike what the common name suggests, to species of rock oysters, specifically observed within oyster leases and aquaculture. The number of I. mcgrathi in the wild and its role within the ecosystem of rocky coastlines is still unknown. This soft-bodied organism does not have any noticeable outstanding characteristics to the human eye but upon closer inspection it is extraordinary. I. mcgrathi is a hermaphroditic polyclad characterised by a mottled brown appearance and rows of ocelli. Indirect development and the presence of a passively feedings Göette’s larval form sets it apart from other species of Platyhelminth and opens many doors for future study. 

Imogine mcgrathi is a thick, oval shaped soft bodied flatworm with minimal marginal ruffling and colourless, transparent nuchal tentacles. Ranging from immature sizes of 10 mm x 6 mm to a mature size of 65mm x 45 mm (Jennings & Newman, 1996), this organism is a simple mottled warm brown colour with some dark brown microdots along the dorsal side while the ventral side is a cream colour with no distinctive markings (Figure 1). Common within the species, individuals can be infected with a haplosporidian parasite, Urosporidium cannoni, that can be physically identified through random large black spots along both the dorsaland ventral sides of the body (Figure 2A)(Jennings & Newman, 1996).

Characteristic of the platyhelminth phylum, I. mcgrathi has a primitive, radial body plan with very little distinguishable features, outside of the colouration, that can be observed by the human eye. Up close, there are many interesting physical features. Along the slightly ruffled margin of the body, I. mcgrathi has many marginal eyes that are densely packed in rows of three to four at the anterior end, while the posterior eyes are more scattered in rows of one to two (Figure 2B) (Jennings & Newman, 1996). There is also the presence of multiple cerebral eyes within the epidermis layer, found between and posterior the nuchal tentacles, in addition to the thirty tentacular eyes located within each nuchal tentacle (Jennings & Newman, 1996). The nuchal tentacles are, on average, 0.5 mm wide and 1.9 mm apart and can be observed as short and cone shaped (Jennings & Newman, 1996). On the ventral side of the body, the large pharynx can be seen around the middle axis while the gonophores are separate and posterior to the feeding apparatus (Figure2C) (Jennings & Newman, 1996).

I. mcgrathi is a type of parasitic platyhelminth known as an ‘oyster leech’ or ‘wafer’ (Jennings & Newman, 1996). These organisms’ prey upon commercial bivalves like the pteriid Pinctada imbricata (O'Connor & Newman, 2001), Saccostrea glomerata and Crassostrea gigas (Cox, Kosmeyer, O'Connor, Dove, & Johnstone, 2012).While they are confirmed as a predator to commercial bivalves, how I. mcgrathi enters the shell of the oyster is still unknown. I. mcgrathi is only observed to predate at night, generally spending the day hid beneath or within the oyster it is preying upon (O'Connor & Newman, 2001). Once inside, at night, I. mcgrathi has been found to eat the entirety of the oyster muscle, with the shell being left closed and connected at the hinge (O'Connor & Newman, 2001). Upon opening an oyster shell, I. mcgrathi has been sighted to be engorged and rounded, having consumed all the oyster meat available (Figure 3). There have been attempts to understand the feeding behaviour of the oyster leech, with the notable observations being: artificially shading the oyster and platyhelminth parasite does nothing to increase the predation during daylight hours (O'Connor & Newman, 2001).  Additionally, assuming to be the result of physical limitations, I. mcgrathi do have a size dependent predation differences; smaller I. mcgrathi were observed to only predate on smaller sized oyster and vice versa (O'Connor & Newman, 2001). Predation rates in smaller I. mcgrathi are higher than that of larger flatworms which is thought to be a method of compensating for lower flesh weights among smaller oysters; on average, a large-bodied I. mcgrathi would consume an a large oyster at a rate of 0.0035 oysters per day while small-bodied I.mcgrathi would consume small oysters at a rate of 0.057 (Figure 4) (O'Connor & Newman, 2001).

All platyhelminth species have a close relationship to their surrounding environment. I. mcgrathi, and all other platyhelminth species, are considered primitive organisms due to their simple body plan. Platyhelminth do not have a respiratory system like higher up animals (Nielsen, 2011). Because of this, they rely heavily on the process of diffusion for the uptake and release of gases through the body –this process is inefficient and is the reason why platyhelminth generally do not do well in anoxic environments (Nielsen, 2011). To maintain osmotic pressure, and help removes waste from the body, I. mcgrathi utilises the protonephridial system (Hertel, 1993). Hertel describes the protonephridia of most invertebrates as being tubules that open at the body surface area and end in one or more terminal structure, of which there are three types (Hertel, 1993). The flat shape of Platyhelminthes like I. mcgrathi would increase the surface area, allowing for more efficient gas exchange and waste disposal from the protonephridia (Urry, et al., 2018).

The behaviour of I. mcgrathi has never been observed in the wild and only observed in a laboratory setting a few times. The laboratory observations of this organism are also limited as they are only active during predation which occurs at night and generally spend the daylight hours avoiding light by laying within the oyster they are predating on or wrapped around the oyster hinge (O'Connor & Newman, 2001). In laboratory settings, I. mcgrathi can be observed in Figure 5 attempting to distance itself from light and hide in the crevice of petri dishes while simultaneously moving constantly. When the flatworm was shaded from direct light, it relaxed from the crevice and ventured into the centre of the petri dish, moving at a slower and less erratic pace. Within previous studies, I. mcgrathi was found to consume oyster meat when shaded from the sun, however the increase in predation activity was not statistically significant (O'Connor & Newman, 2001). Other observations found that eggs were laid masses were laid within two days of being kept in a laboratory (Jennings & Newman, 1996). Egg masses were also observed to have been laid in the empty shells of oysters after I.mcgrathi has finished eating (O'Connor & Newman, 2001). The stages of development these platyhelminth go through have been studied, detailing the process of gastrulation and organogenesis for this particular flatworm species (Younossi-Hartenstein & Hartenstein, 2000). After 7-8 days, Göette’s larvae hatch from the egg masses simultaneously (Younossi-Hartenstein & Hartenstein, 2000) (Jennings & Newman, 1996). Unlike other species' Göette’s larvae, I. mcgrathi larvae have been observed feeding on microplankton (Rawlinson, 2014).  How I. mcgrathi develops from the larval form, chooses the oyster prey, enters the shell and then feeds within the shell is unknown. However, when fed oyster meat within a laboratory setting, I. mcgarthi was observed to consume the tissue whole (Jennings & Newman, 1996). Further research into the feeding strategies and behaviour of I. mcgrathi would be beneficial, particularly focusing on how the flatworm enters the shell of the oyster.

Characteristic of the platyhelminth phylum, I. mcgrathi exhibits a primitive, bilaterally symmetrical body plan (Thorp & Rogers, 2011). I. mcgrathi is a free swimming turbellarian platyhelminth, as opposed to the entirely parasitic Neodermata, despite being classed as a parasite itself. This flatworm exhibits a body plan that is unsegmented with three layers (the ectoderm, mesoderm, and endoderm), an incomplete gut, and a lack of a hard structural skeleton (Thorp & Rogers, 2011) (Nielsen, 2011). All Platyhelminthes also exhibit a central nervous system that consists of the orthogon, the anterior brain, and the plexus (Reuter & Gustafsson, 1995). Platyhelminthes can possess a variety of sensory organs, including primitive pigment cup eyes called ocelli (Thorp & Rogers, 2011). I. mcgrathi possess these eyes throughout their body and use them to detect light intensity (Jennings & Newman, 1996) (Thorp & Rogers, 2011). The ocelli are present along the margin of I. mcgrathi’s body in rows of three to four anteriorly and one to two posteriorly; they are also present within the epidermis and on the nuchal tentacles (Jennings & Newman, 1996). The abundance of ocelli present on I. mcgrathi does explain how sensitive they are to light, as are most platyhelminth species.

A pharynx is also characteristic of the phylum Platyhelminth and can be variable in shape and size (Thorp & Rogers, 2011) (Nielsen, 2011). I. mcgrathi’s pharynx has been described as considerably large among the phylum, measuring about half the body length, and contains 20 – 24 complex pharyngeal folds (Jennings & Newman, 1996). A closed gut is expected within the Platyhelminthes, and therefore the pharynx is not only used to hunt, catch, and eat their prey (Thorp & Rogers, 2011), but also to expel solid waste from the body.  As I. mcgrathi lacks a circulatory system, the fine branches found in the gastrovascular cavity are used to move the food through the body and directly to the animal cells (Urry, et al., 2018).

I. mcgrathi are hermaphroditic, possessing the reproductive organs of both male and female flatworms. The gonopores are separated by about 0.5 mm, with the female pore posterior to the male pore (Jennings & Newman, 1996). The vas deferens protrude anteriorly of the gonopores and measure along a third of the pharynx (Jennings & Newman, 1996). The male reproductive organ is simple and small within the deep male antrum; testes are scattered ventrally throughout the body while ovaries are scattered dorsally (Jennings & Newman, 1996).

The egg masses I. mcgrathi lay are described as 20 to 30 mm long, discreet, thin beige layers of film that are variable in size and shape, with each individual egg measuring 0.12 mm in diameter (Jennings & Newman, 1996). The larvae that hatched from these eggs were phototrophioc and survived 11 days without food under laboratory observation (Jennings & Newman, 1996). Polyclad Platyhelminthes have two possible larval forms, Göette’s and Müller’s larvae (Rawlinson, 2014). The larval forms are distinguished by the number of lobes present: Göette’s have four (see Figure 6C) while Müller’s have eight (see Figure 6B) (Rawlinson, 2014). The larvae are completely ciliated with a longer and denser band of ciliation around the body following the sides of the lobes and an apical tuft (Nielsen, 2011). I. mcgrathi’s larval form is that of Göette’s larvae. It can be described as oval, colourless and transparent, measuring around 0.16 mm long, with characteristic ciliation and three eyespots (Jennings & Newman, 1996). The antero-posterior and dorso-ventral axes can be distinguished by the position of the eye spots and brain at the anterior end, and the mouth opening ventrally (Figure 7)(Younossi-Hartenstein & Hartenstein, 2000). 

I. mcgrathi is endemic to Australia, found across Queenslandand New South Wales coastal areas (O'Connor & Newman, 2001) (Jennings & Newman, 1996) (Davie, 2011). There are three speciesof oyster that I. mcgrathi has been observed to predate on: Pinctada imbricata (O'Connor & Newman, 2001), Saccostrea glomerata and Crassostrea gigas (Cox, Kosmeyer, O'Connor, Dove, & Johnstone, 2012). These three species of oyster are known to be farmed specifically at Port Stephens, New South Wales (Gifford, Dunstan, O’Connor, & Macfarlane, 2005)  (Honkoop & Bayne, 2002). Outside of commercial oyster leases, in relation to wild oyster populations, there have been some sightings in Moreton Bay, Queensland (Jennings & Newman, 1996). I. mcgrathi has never been reported outside of oyster populations, however the movement of their larval form within and between populations of oysters has not been studied and therefore remains unknown.

The evolution of platyhelminth larval forms

The most striking development of I. mcgrathi and related species is the larval stage. Majority of the platyhelminth phylum undergo direct development suggesting that this was the preferred lifestyle for the ancestral form (Adell, Martin-Duran, Salo, & Cebria, 2015). However, it has been observed that three groups of free-swimming Platyhelminthes undergo indirect development and have a larval stage; this includes the polyclads with both Göette’s and Müller’s larvae, but also two other larval forms known as Luther’s and Kato’s larvae (Adell, Martin-Duran, Salo, & Cebria, 2015) (Rawlinson, 2014). Distinct from the adult form, the larvae do have features that are lost during metamorphosis. The lobes and ciliary tufts are the most obvious, but the larvae also has an apical organ that is not present in the adult form (Rawlinson, 2014); further research into the features of larvae in comparison to the features of the adults, understanding what is lost and what is retained, is needed to understand the entirety of platyhelminth metamorphosis. The strategies of these larvae once they are hatched from the egg casing are very different. Müller’s larvae have been observed to be planktotrophic while majority of the Göette’s larvae are lecithotrophic (Rawlinson, 2014). There is one species that possesses Göette’s larvae that has been observed to feed on microplankton and that is I. mcgrathi; the need for this species larvae to feed remans unknown, however it should be considered an area that needs to be further researched (Rawlinson, 2014). What has I.mcgrathi Göette’s larvae evolved that allows it to feed unlike other species Göette’s larve?

Systematics and Classification 

Phylum: Platyhelminthes
Subphylum: Rhabditophora
Order: Polycladida (Faubel, 1983)
Family: Stylochidae
Genus: Imogine (Marcus & Marcus, 1968)
Species: mcgrathi (Jennings & Newman, 1996)

There is not much data available regarding the occurrence of I. mcgrathi across the cost of Queensland and New South Wales, suggesting that not many samples have been collected or even observed. Because I. mcgrathi numbers are unable to be quantified, the need for conservation of these organisms is unknown. Additionally, the role of these organisms in an ecosystem outside of their position as a parasite to rock oysters is also unknown.

Outside of threats to I. mcgrathi specifically, this flatworm can pose a threat to rock oyster populations, especially in terms of commercial aquaculture. I. mcgrathi has been observed preying on three common commercial species of oyster (O'Connor & Newman, 2001)(Cox, Kosmeyer, O'Connor, Dove, & Johnstone, 2012), and has also been observed laying egg masses within the empty shells (O'Connor & Newman, 2001). As there is still little known about I. mcgrathi behaviour in relation to oysters (e.g., how they enter the oyster shell and how they feed on the oyster), the effect of this flatworm on oyster aquaculture is unknown. Research has shown that methods of controlling biofouling within oyster leases can work to remove I. mcgrathi; cold shock through immersion in hypersaline baths, or commonly known as “super salty slush puppy,” has proved to be effective against soft-bodied organisms like I. mcgrathi (Cox, Kosmeyer, O'Connor, Dove, & Johnstone, 2012). Other research has also concluded that hypersaline baths and freshwater baths were also effective treatments for I. mcgrathi infestation in oyster leases (O'Connor & Newman, 2001).

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