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Austrolittorina unifasciata (Gray, 1826)



Lauren Ashley Veary 2018

Summary

Species Summary

Periwinkle snails are common species that are members of the mollusca phylum and the gastropoda class. They are one of the most common species of gastropods found within intertidal zones on coasts all around the world. They have a Gondwanan distribution resulting in a complex taxonomic history. Over time, the classification of periwinkles has become more clear as extensive morphological and molecular studies have been conducted on them.

A. unifasciata, also known as blue periwinkles, are marine snails residing in intertidal zones on the coasts of Australia (Reid & Williams 2004; Figure 1). It is the only species of its genus that is endemic to Australia and is one of the most common gastropods found on Australia coasts. 




Figure 1: Numerous A. unifasciata individuals.

Classification

Kingdom           Animalia

Phylum             Mollusca

Class                 Gastropoda

Subclass           Caenogastropoda

Order                Littorinimorpha

Superfamily      Littorinoidea

Family               Littorinidae

Subfamily         Littorininae

Genus               Austrolittorina

Species             Austrolittorina unifasciata
Source: WoRMS 2018

Physical Description

A. unifasciata snails can be recognised by the shape and colour of their shells even though there can be significant variation in shell morphology between individuals (Reid & Williams 2004). Adult shells are solid and can reach heights of 25mm (Reid & Williams 2004). In figure 2, it can be seen that the shell has a turbinate shape, with the spire being considered tall. The outline of the spire ranges from straight to being slightly convex and is generally a brown colour. The body whorl of the shell is slightly rounded. The periphery of the whorl can be seen to be angled slightly. Shell colour ranges from blue-grey to brown with a spiral pattern being displayed throughout the whole shell. Immature shells display thin brown lines spiralling around the shell (Reid & Williams 2004).

The muscular foot and cephalic tentacles have a yellow-transparent colour. The operculum is more orange in colour than the foot, with a dark stripe around where the operculum meets the aperture. It displays a faint spiral pattern. The aperture inside of the shells is mainly brown in colour.

See figure 2

Figure 2: The physical appearance of A. unifasciata gastropods. Image on the left shows the shell with the head of the snail emerged. The snail is 20mm in length. The image on the right shows a snail retreated into its’ shell and is 12mm in length.

Ecology

Ecology

A. Unifasciata snails are found most abundantly on temperate shores within intertidal rock pools (Reid & Williams 2004). They are more common on coasts that are exposed and can be found spanning from the littoral fringe to the highest spring tide level (Reid & Williams 2004). They exist on any type of hard surface including rocks, concrete and wooden piers (Reid & Williams 2004). They most commonly aggregate into patchy distributions, however this leads to intraspecific competition for food resources (Reid & Williams 2004; Figure 3).


Figure 3: A. unifasciata species found aggregating in rock crevices in the intertidal zone of Moffat Beach, Queensland. Photograph taken by Melita Gaston.


This species generally grazes on algae as well as lichens (Reid & Williams 2004). They feed during high tide and use their radulas to scrape rocks for food. In search for food, they can travel up to 12 metres (Reid & Williams 2004). The most common predators of this species is crabs, birds and other snails (Reid & Williams 2004).

The shell size of this species has been investigated by numerous studies. It has been seen that the largest snails are generally found in the highest and lowest tide marks (Reid & Williams 2004). Studies have also shown that higher density populations have larger shell sizes than those at lower densities (Chapman 1994).

Intertidal Zonation

Background

It has been seen that within a shore, this species extends from the littoral fringe to the highest spring tide levels and has an extensive vertical distribution (Chapman 1994). There is a trade-off between these snails requiring water to prevent desiccation and breed whilst also needing to be out of water to feed. This investigation aimed to determine whether the abundance of A. unifasciata individuals changed depending on the distance from land. It was hypothesised that there would be a larger quantity of individuals found mid-way between the land and the ocean, with there being fewer individuals close to the land and close to the sea.

Methods

To determine where A. unifasciata are more commonly distributed, this species was investigated at Moffatt Beach, Queensland (Figure 4). A transect of nine metres was set up, starting at land and going out towards the ocean. Every one metre along this transect, three rocks were randomly chosen and all A.  unifasciata individuals found on the rock were counted. One metre away from the land represents the high intertidal zone while nine metres away from the land represents the low intertidal zone. Data was analysed using ANOVA with R software. 



Figure 4: Intertidal zone at Moffatt Beach, Queensland where study was conducted on A. unifasciata individuals.


Results
​Figure 5: The abundance of Austrolittorina unifasciata individuals at different distances from land at Moffat Beach, Queensland. Data points represent a mean of three replicates. Results are significant (df = 8, F = 5.3761, P < 0.05)

Discussion

ANOVA data analysis shows that the distance from the land is significant in determining the abundance of A. unifasciata individuals (Figure 5). Close to the land, numbers remain relatively low until about five metres away. Their abundance is higher at six and seven metres away from the land. However, as the zone starts getting closer to the ocean, their abundance significantly decreases. This data supports the hypothesis in that A. unifasciata individuals are found in higher abundances mid-way within the intertidal zone. These results can be explained ecologically. Too far away from the ocean in the highest parts of the intertidal zone had few individuals of this species as they require water for respiration through their gills otherwise they desiccate (Ruppert 2004). The tide does not reach this high up in the intertidal zone, therefore these snails aren’t found that close to the land. There is also a similar pattern of abundance at the low intertidal zone, close to the ocean. In order to feed, these snails need to move around and graze algae from rocks during low tide (Reid & Williams 2004). They cannot do this if they are submerged in water for long periods of time. 

Conclusion

A. unifasciata snails are found in higher abundances in the mid-intertidal zone as they are able to receive enough air and water that they require for different activities. 

Life History and Behaviour

Sexual Reproduction

This species reproduces through sexual reproduction and has separate sexes. Fertilisation occurs internally as males use a penis to transfer sperm to females (Ruppert et al. 2004). The penis is located on the right side of the head (Ruppert et al. 2004). Once eggs are fertilised, egg capsules are released into the sea by females (Rudman 1996). These egg capsules are transparent, domed and pelagic in nature and are 0.24mm in diameter (Figure 6; Rudman 1996). Each capsule contains one egg that is fertilised and is 0.1mm in diameter (Rudman 1996). These egg capsules are an effective adaptive strategy as it allows the egg to remain protected until veliger larvae hatches and settle on the shore (Williams et al. 2003).

Figure 6: Pelagic capsule of A. unifasciata. Modified from Reid & Williams (2004).

Life Cycle

Once pelagic egg capsules are released into the ocean by the female, veliger larvae hatch from them once they have undergone the trochophore stage within the egg capsule (Rudman 1996). The trochophore stage is the first stage in development whilst the veliger stage is the second stage (Rudman 1996). 

Veliger larvae are swimming larvae and have numerous structures including a shell that protects the visceral organs (Ruppert et al. 2004; Figure 7). The veliger larvae of gastropods develop numerous morphological characteristics of their adult form (Ruppert et al. 2004). These characteristics includes the muscular foot, mouth and eyes. They also have a velum which is a ciliated and protrudes from the shell. It is used for swimming and feeding and can be hidden inside the shell if the individual needs to protect it (Ruppert et al. 2004). A. unifasciata gastropods have planktotrophic larvae, meaning that they have to feed on phytoplankton for a period of time before it is able to metamorphose into a juvenile (Reid & Williams 2004). As it feeds, it develops more of the organ systems needed to become a juvenile (Ruppert et al. 2004).



 

Figure 7: Diagram of the anatomy of veliger larvae of gastropods. Based off of Ruppert et al. (2004).

This species remains as veliger larvae before it is competent enough to settle on the shore as a juvenile. Settlement on the shore can be induced by chemical and environmental cues which tell the individual that conditions are ideal for settlement (Ruppert et al. 2004). 

When developing into a juvenile, the veliger larvae undergoes a process called torsion which is unique to this phylum (Reid 1989). The body twists 180 degrees, allowing the posterior of the body to be bought behind the head in an anterior position (Reid 1989; Figure 8). Once metamorphic competence is reached, the muscular foot is then developed enough to allow the individual to attach and move around a substrate (Reid 1989). 

Figure 8: Diagram representing the process of torsion in gastropods. The eft image is a hypothetical gastropod that isn't torsed. The right image is of a gastropod that has undergone torsion. Based off of Ghiselin (1966).


After the juvenile stage has been reached, snails will grow rapidly until the adult stage is reached. This stage is defined by the maturing of sexual structures that are ready for sexual reproduction (Ruppert et al. 2004).

Behaviour

A common behaviour exhibited by A. unifasciata snails is the aggregation of individuals. They are commonly found living in clumps. It has been suggested that this behaviour limits moisture loss. Desiccation is also prevented by the snails retreating back into their shells when conditions are not ideal. The operculum seals themselves into their shell and protects them from external influences (Ruppert et al. 2004).

Studies have shown that A. unifasciata snails exhibit a standing behaviour in response to temperature changes (Lim 2008). If the temperature of the substrate is too high, these snails prevent desiccation by using their holdfast to hold themselves off of the rock (Lim 2008; Figure 9). It is evident that through this technique, the snails do not increase in temperature as the substrate’s temperature increases (Lim 2008). They use this standing position when the temperature of the substrate exceeds 35 degrees Celsius (Lim 2008).



Figure 9: Photo of A. unifasciata in standing posture on a rock at Moffat beach, Queensland.

Anatomy and Physiology

External Anatomy

Shell

The main external structure of A. unifasciata gastropods is their shell. As mentioned in the physical description section, their shell is a solid structure in a spiral pattern (Ponder et al. 2008). It is composed of calcium carbonate which is secreted from the mantle (see internal anatomy). The columella pillar inside of the shell has a slightly concave shape (Reid & Williams 2004).

See figure 10


Figure 10: The shell of an A. unifasciata gastropod that is 15mm in length.

Body

The body of the snail is connected to their shell through a columellar muscle (Ruppert et al. 2004). A defining feature of gastropods is the presence of a muscular foot which is on the bottom of the animal (Ponder et al. 2008). When emerged from the shell, it is used for locomotion and it produces a mucus-type substance so that the snail is able to move more easily (Ponder et al. 2008). In this species it has an ovular shape and has an operculum attached to the end (Ponder et al. 2008). The operculum is what is visible of the snail when it is fully inside of its’ shell. The main function of the operculum is to seal off the snail within its’ shell from external conditions (Ruppert et al. 2004). This species of snail has one pair of tentacles with one eye situated at the back of each tentacle.

See figure 11


Figure 11: Photo of A. unfiasciata body out of its' shell and is 10mm in length.


Radula

Radulas are strong structures used to scrape food resources off of substrates and are expelled from the pharynx (Luckens 1974). The A. unifasciata has a scraping radula which is effective on algal filaments and lichen (Luckens 1974). It is 35 to 48mm in length and has an elongated major cusp with a rounded tip (Reid & Williams 2004).

See figure 12



Figure 12: Radula of A. unifasciata snail that is 10mm in length when coiled.

Internal Anatomy
General

The mantle is a soft tissue that is on the top of the snail surrounding the mantle cavity and is responsible for secreting the components required to create a shell (Ruppert et al. 2004). This includes calcium carbonate and chitin (Ruppert et al. 2004). Numerous internal structures are protected within the mantle cavity including gills, excretory pores and an anus (Ruppert et al. 2004). Gastropods have a reduced coelom that acts as a hydrostatic skeleton that suspends the snails’ organs (Ruppert et al. 2004). Respiration occurs through gills which extracts oxygen from water (Ruppert et al. 2004). Water is pumped through the mantle cavity by cilia surrounding gill filaments (Ruppert et al. 2004). Oxygenated blood is sent from the gills into a muscular atrium (Ruppert et al. 2004). The ventricle then sends the blood to the head (Ruppert et al. 2004). These snails also have a hemocoel which the primary body cavity within the snail (Ruppert et al. 2004). It allows the circulatory system to act as an open system (Ruppert et al. 2004).  

See figure 13

Figure 13: The general anatomy of a marine gastropod. Drawing based off of multiple figures from Ruppert et al. (2004).  


Nervous System

The torsion process during development has led to the formation of a visceral loop (Ruppert et al. 2004). This visceral loop connects the head, the foot and the visceral mass (Ruppert et al. 2004). The ganglia are found anteriorly and is paired to ventral cords (Ruppert et al. 2004). The foot contains statocysts which are responsible for the detection of movement (Ruppert et al. 2004). The eyes of this species are situated at the back of the cephalic tentacles and have displayed to only be capable of detecting changes in light sensitivity (Ruppert et al. 2004). Tentacles are used as a touch sensor (Ruppert et al. 2004).

See figure 13
Digestive System

The torsion process (see Life Cycle) causes the anus to be located above the snail’s head. The radula is the main feeding structure used to scrape algae filaments off of rocks (see External Anatomy). This species has complete gut structure, with the torsion process resulting in the anus being located close to the head within the mantle cavity (Ruppert et al. 2004). The radula brings food into the mouth inside of the buccal cavity (Ruppert et al. 2004). The oesophagus then leads into the stomach which leads to the intestine and ends at the anus. Enzymes throughout the digestive system break down the food, with structures allowing for absorption of nutrients (Ruppert et al. 2004).

See figure 13
Reproductive System

All members of the Caenogastropoda subclass have separate sexes. Reproduction occurs sexually by males transferring sperm to females via a penis (see Sexual Reproduction).

The penial vas deferens opening of A. unifasciata gastropods is considered an apomorph as it is a unique trait to phew genera (Reid 1989). The opening is subterminal whilst it is normal terminal in most littorinids (Reid 1989). The vas deferens is attached to the penis as well as the prostate and assists in transport of sperm to the penis from the prostate (Reid 1989). In males, the penis is bifurcate as it split in two (Reid 1989). There is the presence of a penial glandular disc as well as a single mamilliform gland as seen in figure 14 (Reid & Williams 2004).


 

Figure 14: A. unifasciata penis. Modified from Reid & Williams (2004).

The main reproductive structures of females include an ovary, a seminal receptacle and pallial oviducts (Reid & Williams 2004). Sperm storage and egg capsule production are done by the seminal receptacle (Ruppert et al. 2004). The bursa in situated in an anterior position and is responsible for receiving the sperm (Reid & Williams 2004). Sperm storage and egg capsule production are done by the seminal receptacle (Reid 1989).

See figure 15


 


Figure 15: A. unifasciata female reproductive system. Modified from Reid & Williams (2004). 

Biogeographic Distribution

A. unifasciata is commonly distributed on the shores of Australia and is found all along the eastern and western coasts below a latitude of 20 degrees. (Reid & Williams 2004; Figure 16). They are the only species within it’s genus that is endemic to Australia (Reid & Williams 2004) Their distribution on northern coasts is limited.  It has been suggested that this is due to the shores being more sheltered (Reid & Williams 2004).



Figure 16: Biogeographic distribution of A. unifasciata shown in red. Based on Reid & Williams (2004).

Evolution and Systematics

Mapping the history of A. unifasciata snails has been challenging as they have often been confused with other species (Williams et al. 2003). Fossils of this and similar species are very rare to find due to them living in high energy environments (Williams et al. 2003). Previously, this species as well as others of the littorinidae family were grouped together under one large genus known as Nodilittorina (Reid 1989). In older studies, this species has been named Nodilittorina unifasciata, for example in a study done by Reid (1989). More recent molecular data has displayed that all of these species should actually be grouped within three different genera with Nodilittorina being a polyphyletic group (Williams et al. 2003). Austrolittorina was one of these genera (Williams et al. 2003).

There are five species within the Austrolittorina genus and have been found spread out amongst Australia, New Zealand and South American shores (Williams et al. 2003). The breaking up of Gondwana has been suggested to be the cause for this international distribution (Williams et al. 2003). The divergence of Australian and South American species has been shown to be synchronous with the breakup of Gondwana approximately 40 to 70Ma (Williams et al. 2003). The divergence of sister species within the Austrolittorina genus has been estimated to have occurred between 10 to 47Ma (Williams et al. 2003). A. fernandezensis has molecularly been shown to be the sister species to A. unifasciata (Reid & Williams 2004; Figure 17).



Figure 17: Molecular phylogenetic tree of Littorininae. Species highlighted in yellow is Austrolittorina unifasciata. Adapted from Williams et al. (2003). 

Conservation and Threats

Living in intertidal zones exposes A. unifasciata snails to many environmental and anthropogenic threats. One of the biggest threats to this species is a rise in sea level. Intertidal zones are very vulnerable to small changes in sea level (Thorner et al. 2014). Global warming has been predicted to cause a rise in sea level. As erosional processes aren’t dominant in intertidal zones, there is limited ability of habitat renewal within short periods of time (Thorner et al. 2014). For this reason, the biodiversity within these zones is vulnerable to habitat loss. Thorner et al. (2014) conducted a study to investigate how rising sea level would impact intertidal zones at a small spatial scale. They modelled a rise in sea level, with the worst-case scenario being a rise of 1m (Thorner et al. 2014). This scenario reduced the intertidal habitat in all of the sites investigated (Thorner et al. 2014). Smaller changes in sea level had variable effects on different habitat types (Thorner et al. 2014). A. unifasciata snails require water for reproduction and respiration but they feed when they aren’t submerged in water (Ruppert et al. 2004). Therefore, if sea levels rise and intertidal zones of Australia become immersed, these snails won’t be able to survive (Thorner et al. 2014). 

Conservation efforts of this species is limited, and they have not been assessed by the IUCN Red List (IUCN 2017). Marine Protected Areas (MPAs) have been used as effective strategies to conserve biodiversity within a marine environment (Banks & Skilleter 2002). However, these MPAs often do not include intertidal zones which contain important biodiversity including gastropods such as A. unifasciata (Banks & Skilleter 2002). This is due to coastlines often being viewed as a boundary between ocean and land habitats instead of being considered as an environment of equal importance (Banks & Skilleter 2002). Intertidal zones are fine scale habitats which need to be studied in depth in order to produce MPAs that are effective at conserving important intertidal species (Banks & Skilleter 2002).

References

Banks, S. A. and Skilleter, G. A. (2002). Mapping intertidal habitats and an evaluation of their conservation status in Queensland, Australia. Ocean & Coastal Management 45, 485 – 509.

Chapman, M. G. (1994). Small-scale patterns of distribution and size-structure of the intertidal littorinid Littorina unifasciata (Gastropoda: Littorinidae) in New South Wales. Australian Journal of Marine and Freshwater Research 45, 635-652.

Ghiselin, M. T. (1966). The adaptive significance of gastropod torsion. Evolution 20, 337 – 348.

Lim, S. L. (2008). Body posturing in Nodilittorina pyramidalis and Austrolittorina unifasciata: A behavioural response to reduce heat stress. Memoirs of the Queensland Museum 54, 339– 347.

Reid, D. G. (1989). The comparative morphology, phylogeny and evolution of the gastropod family Littorinidae. Philosophical Transactions of the Royal Society of London 324, 1 – 110.

Reid, D. G. and Williams, S. T. (2004). The Subfamily Littorininae (Gastropoda:Littorinidae) in the Temperate Southern Hemisphere: The Genera Nodilittorina, Austrolittorina and Afrolittorina. Records of the Australian Museum 56, 75 – 122.

Rudman, W.B., 1996. A note on the egg capsule of Nodilittorina unifasciata from eastern Australia. Molluscan Research 17, 111 - 114.

Ruppert, E. E., Fox, R. S. and Barnes, R. D. (2004). Invertebrate Zoology: a functional evolutionary approach. 7th Edition – Thomson Brooks/Cole.

The IUCN Red List of Threatened Species (IUCN) (2017). Austrolittorina unifasciata. Viewed 30 May 2018, <http://www.iucnredlist.org/search>

Thorner, J., Kumar, L. and Smith, D. A. (2014). Impacts of Climate-Change-Driven Sea Level Rise on Intertidal Rocky Reef Habitats Will Be Variable and Site Specific. PLoS ONE 9, e86130.

Williams, S. T., Reid, D. G. and Littlewood, D. J. (2003) A molecular phylogeny of the Littorininae (Gastropoda: Littorinidae): unequal evolutionary rates, morphological parallelism and biogeography of the Southern Ocean. Molecular Phylogenetics and Evolution 28, 60-86.

World Register of Marine Species (WoRMS) (2018). Austrolittorina unifasciata. Viewed 28 May 2018, <http://www.marinespecies.org/aphia.php?p=taxdetails&id=446843>