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The Black Nerite (Nerita melanotragus)

Emerson Blair Pollard 2017


The Black Nerite (Nerita melanotragus) is a species of gastropod which inhabits much of the intertidal zones within the Australasian South Pacific (Queensland to New Zealand) (Przeslawski 2011). N.melanotragus is a medium-sized marine snail ranging from 10-30mm in size, identifiable by its distinct black shell and tan operculum. The Black Nerite tends to aggregate in groups when exposed to increased thermal stress, a common survival behaviour seen within gastropods living in the intertidal zone (Chapperon et al. 2013)

This herbivore actively grazes on a variety of resources within the intertidal zone including plant material, bacteria, and microalgae (Chapperon et al. 2011). During low tide, this species of gastropod commonly attaches to crevices and underneath rocks, where on the incoming tide individuals moves out into the exposed pools to forage for microalgae attached to the surrounding substrate (Rohde 1981). N.melanotragus are a dioecious species and reproduce through internal fertilisation. 

Physical Description

The Black Nerite is a marine gastropod within the phylum Mollusca.  N.melanotragus is a medium-sized gastropod which is 10-30mm in length, and 15-20mm high. This gastropod includes a uniform, smooth black shell with more than 30 fine spiral lines, orange/tan globose operculum, and white aperture. Colour of the operculum is used to separate similar species of Nerita (Nerita atramentosa has an black operculum) (refer to Figure 1).  The Black Nerite’s shell also has a distinct worn down final body whorl. When moving, two black feeding tentacles, along with the outer edges of the muscular foot can be seen protruding out from underneath the shell (Figure 4). The muscular foot of N.melanotragus is black/tan, and is the primary structure used for movement. N.melanotragus has columellar teeth (usually one or two teeth) on the apical end of the shell’s outer lip, a feature found within many gastropods (see Figure 3). The vestigial outer lip of this species’ shell has no obvious crenulation (notches), yet, crenulation of the outer lip is prevalent in other species of Nerita (Spencer et al. 2007), another defining characteristic of this species

Figure 1
Figure 2
Figure 3
Figure 4


The Black Nerite (N.melanotragus) inhabits the intertidal zones within the rocky shores of the tropical and temperate waters of the South Pacific. This animal’s distribution spreads over the coastlines from south-eastern Queensland to New Zealand. The intertidal zone (rocky shore specifically) is an environment which is exposed to the air during low tide, and completely submerged during high tide. Therefore, organisms which inhabit this extremely harsh environment must overcome and adapt to a range of conditions such as: wave/wind exposure, tidal cycles, varying temperature and salinity levels. N.melanotragus are found up to depths of 1-2m (Chapperon et al. 2013). During dry, low tide conditions, N.melanotragus retreats back into its shell behind the operculum, where the animal attaches itself to the surrounding substrate using mucus (Rupert et al. 2004). N.melanotragus will usually attach to sheltered substrate such as crevices and the underneath of rocky platforms, away from the direct sunlight to avoid water loss. N.melanotragus then moves out of these sheltered areas into exposed rocky pools to feed on microalgae attached to the substrate during the incoming tide (Rohde 1981). Movement of this species around the intertidal zone is also heavily dependent on the substrate temperature as they are ectotherms (body temperature is relatively proportional to substrate temperature) (Chapperon et al. 2011). These organisms are found within sheltered and heavily wave-exposed shores, showing the adaptive nature of the Black Nerite’s muscle foot and its attachment to the substrate (Przeslawski 2011).

Although this species is relatively confined by abiotic effects, biotic factors such as predator-prey interactions do also effect the Black Nerite's ecology. The rock shell (Dicathais orbita) is the primary predatory for the N.melanotragus, where the rock shell occupies the rock-pools within the intertidal zone. The rock shell as a common predatory gastropod within the mid-intertidal zone, which it predates upon the Black Nerite whilst it is feeding upon microalgae whilst submerged in the rock-pools (Woodcock & Benkendorff, 2008). It is thought that the distinct dark shell of the Black Nerite is used to camouflage the animal when within these rock-pools, avoiding predation.  

Life History and Behaviour


N.melanotragus is a herbivore, actively grazing on microalgae, cyanobacteria, and diatoms attached to intertidal substrates. N.melanotragus only feeds when submerged, hence the incoming tide is most favourable as the intertidal zone is inundated at this time (Davey 2000). Once the tide inundates the intertidal zone enough, this species moves from sheltered rock platforms out into the exposed pools. The incoming tide (high tide) also is thought to replenishes food resource deposits within this environment (Chapperon et al. 2011), meaning more microalgae for the Black Nerite when feeding during this period. Like many other marine snails, two cephalic feeding tentacles are constantly touching and feeling the substrate. The extension of the oral hood from the apical end of the animal acts like a vacuum cleaner, constantly attached and sliding across the substrate scrapping up microalgae using its radula covered in rows of teeth (Borell 2004; Rohde 1981).

Reproduction & Development

N.melanotragus is a dioecious (separate sexes) species which spawns almost all-year round in small rock pools on the intertidal zones of eastern Australia, with peak spawning observed throughout summer (Przeslawski 2008; Przeslawski 2011). N.melanotragus becomes sexual active at 20 months of age. After internal fertilisation occurs, females N.melanotragusproduces white, elliptical egg capsules filled with colourless jelly (each containing 20-50 eggs), attaching it to the underside of rocks within a sheltered rock pool (Figure 5). Eggs hatch after 14-15 days after attached to the substrate (Anderson 1962). N.melanotragus larvae can emerge as both lecithotrophic veliger and crawling juveniles; however, crawling juveniles are less frequent (Przeslawski 2011). Intertidal environmental factors such as temperature, salinity, and wave exposure are thought to affect the time in which the N.melanotragus larvae is released. Hence, enabling the species to develop and use multiple modes of larvae development through hatching plasticity. Yet, it remains unknown if N.melanotragus truly displays both of larval forms (Przeslawski 2011). N.melanotragus’ juveniles take two years to reach full adult size (Underwood 1975).

Figure 5

Life History

Recent morphological and genetic evidence has suggested that what was previously known as Nerita atramentosa may have been a combination of two quite distinct species within Australia. Nerita melanotragus and Nerita atramentosahave been consistently confused within past ecological and taxonomic literature. N.melanotragus is located predominately on the eastern coast of Australia, northern New Zealand, and Norfolk Island. Whereas, N.atramentosa is distributed along southern Australia and is often confused with its warmer water relation due to their extremely similar biology (Spencer et al. 2007). One distinction between these animal’s biology is the colour of the operculum. N.melanotragus has an orange/tan operculum, and N.atramentosa has a black operculum.


Aggregation Behaviour

This experimental investigation aims to identify the behavioural response of N.melanotragus individuals towards abnormal thermal stress. It is hypothesised that N.melanotragus individuals under increased thermal condition will show closer aggregation than under normal conditions. N.melanotragus is said to aggregate in large groups during warmer conditions such as a midday low tide. Displaying such behaviour is quite common within intertidal gastropods, and is thought to reduce dehydration, and body temperature of the animals (Chapperon et al. 2012; Chapperon et al.2013).

N.melanotragus individuals were collected from Coolum Beach, Queensland, Australia (26º53’97”S, 153º097’378”E). Ethics approval and collection permits/licenses were not required for this species. Twenty-four individuals were collected during low tide, and identified as N.melanotragus owing to their orange/tan operculum. Only individuals that were between the sizes of 24-28mm in size were collected, this measurement was taken from the anterior to posterior ends of the shell. Specimens were then transported to the closed-circuit marine aquarium at the University of Queensland St Lucia within 2 hours of collection. N.melanotragus specimens were given four days to adjust and settle into the new environment before the first experiment. Three experiments were conducted over a two-week period; experimental set up for all experiments can be seen in Figures 6 & 7.

The preliminary experiment included six specimens evenly placed out (14cm apart) over one smaller tray, inside a larger tray hold hot freshwater (40-50ºC). Specimens were added to the experimental tray once seawater inside the tray hit 39ºC. Experiment was conducted for 10 minutes recording observations on the behaviour of the specimens. After the experiment, these specimens were then transported back into the marine aquarium.

The first of the main experiments was the treatment experiment, the specimen’s response to thermal stress. Treatment and control experiments had both three replicates; however, only two trays could fit inside the larger tray, meaning two replicates were done, and then the third replicate was done after the first two (under same starting condition). Two plastic trays holding a small amount of seawater in each within the larger tray containing hot freshwater (40-50ºC). Hot freshwater was refreshed between the preliminary and the first experiment. Once the seawater within the smaller trays was heated to 36ºC, 12 new specimens were added to the two trays (6 per tray, 14cm apart), and another 6 specimens were added to the third replicate (18 specimens across 3 replicates). Experiment was conducted for 35 minutes, where observations, distance to the nearest neighbour for each of the six specimens, and the percentage of the specimens that were aggregated at each 5-minute time interval were recorded over the three trays. An aggregation was considered formed when direct contact between two or more specimens occurred (Chapperon et al. 2013); number within the aggregation was recorded and put into a percentage of the sample population (aggregation of 2, 2/6= 33%). Two thermometers were used to observe the water temperature of the trays, and if water was becoming too hot/too cold, warm/cold water was added to the larger tray. The treatment experiment held the seawater temperature at a range between 33-35ºC. After the experiment, the specimens were transported back to the marine aquarium. One week between the treatment and control experiments were given, ensuring that thermal stress endured the treatment experiment did not carry over to the control experiment.

The control experiment had the same experimental set up and parameters as the treatment; however, room temperature freshwater was added to the large tray instead of the hot freshwater. Seawater within the smaller trays were held at 24ºC (room temperature). The same 18 Black Nerite specimens were used in both treatment and control experiments. 

Data collected from the experiments were analysed using statistical t-tests, revealing insignificant results for both percentage aggregated (df= 10, t= 0.68, p= 0.509), and distance to nearest neighbour (df= 14, t= 1.72, p= 0.106). Although the data was found to be insignificant, an obvious relationship between the percentage of aggregate specimens within the treatment environment, evident through Figure 8. Rfor the treatment and control calculated by the graph’s trend line as .918 and .0406, showing a substantial difference. The relationship between the treatment environment and the distance of the nearest neighbour was not seen evident within Figure 9.

Within the preliminary experiment, it was observed that all six specimens did not move from their original positions, yet did stay attach to the substrate. Under such conditions (39ºC), it is common for these animals to enter a heat coma, restricting movement but also continuing to attach to the substrate, reflecting this animal's robust nature (Chapperon et al. 2013).  

The statistical analysis showed the experimental results as insignificant, and therefore the original hypothesis cannot be supported. Despite these insignificant results, the R2 values for specimen’s aggregated (%) do reflect a substantial difference between the treatment and control environments evident though figure 7. Aggregation behaviour is thought to maintain the moisture and reduce the evaporation rate by decreasing the surface-to-volume ratio in contact with the atmosphere (Chase et al. 1980). This similar behaviour is seen within other molluscs such as mussels, by which aggregated mussels exhibit lower body-temperatures by 4-5ºC than solitary mussels (Helmuth 1998). The aggregation behaviour of N.melanotragus does significantly increase water content, also assisting in decreasing the body temperature of the aggregated animals. Not only is this behaviour essential in regulating thermal stress, but aggregations of N.melanotragus could potentially reduce the risk of dislodgement by waves, and effect predation by crustaceans (Chapperon et al. 2013). Within the preliminary experiment, it was observed that all six specimens did not move from their original positions, yet did stay attach to the substrate. Under such conditions (39ºC), it is common for these animals to enter a heat coma, restricting movement but also continuing to attach to the substrate (Chapperon et al. 2013). Investigating such thermoregulatory behaviour could suggest that mobile, intertidal invertebrates like the Black Nerite may be less vulnerable than previously thought to the warming climate.

Figure 6
Figure 7
Figure 8
Figure 9

Anatomy and Physiology


N.melanotragus circulatory system is analogous to other mollusc circulatory systems, yet, owing to torsion, the pericardial cavity and the heart has moved to the anterior visceral mass. Vetigastropods such as N.melanotragus are diotocardian, and only hold one atrium within its circulatory system (this can be seen within Figure 10).

Figure 10


N.melanotragus excretory system is assumed to be similar to other intertidal gastropods. These animals switch between two modes of excretion, depending upon specific environmental conditions; uricotelic during the outgoing tide and ammonotelic (ammonia is end product) on the incoming tide (Ruppert et al. 2004). Nephridium is a blind sac surrounded by hemocoel located on the anterior end of the visceral mass owing to torsion. Kidneys of the animal are located at the end of the sac, connecting to the pericardial cavity with a renopericardial canal. Nephridiopore within the excretory system opens at the rear of the aniaml’s mantle cavity, allowing waste product to be removed by the respiratory water current (Ruppert et al. 2004; Taylor et al. 1988).


Torsion is the 180º anti-clockwise rotation of the gastropod’s visceral mass, shell, mantle, and mantle cavity, relative to the head and foot (Figure 11). Internal features of the gastropod larvae are then situated directly above the head because of this process (Ghiselin 1966). The head and foot are not altered, along with the visceral mass except for its orientation. Nerves and parts of the gut that pass through the head and foot do become twisted as well. All gastropods are torted or have ancestors in which were torted to some extent. Many theories exist as to why torsion became apparent in the gastropod taxa, yet none are compelling enough for certain understanding (Ruppert et al. 2004).
Figure 11


The digestive system of N.melanotragus includes: a mouth, buccul cavity, esophagus, stomach, intestine, rectum, and anus (Figure 12). Digestion within this animal is both intercellular and extracellular. Extracellular digestion requires numerous enzymes to efficiently break down microalgae. These enzymes are produced by a combination of several digestive organs such as the salivary glands, esophageal pouches, and digestive ceca. Extracellular digestion occurs within the stomach of the digestive tract, whereas intercellular digestion occurs in the digestive ceca. Owing to the torsion of the stomach 180º, the esophasgus and intestine are connected posteriorly and anteriorly respectively (Ruppert et al. 2004).

Figure 12


The muscular foot is responsible for the movement of the Black Nerite, assisting them in feeding, prey capture, and reproduction (Figure 13 shows semi-extended foot). The columellar muscle is connected to this foot, allowing for efficient extension and retraction. Within the sole of the foot are tarsos muscles which also contribute to the gastropod’s locomotion (Ruppert et al. 2004). This sole is extremely flat and broad in N.melanotragus, reflecting the ability to move around a variety of different substrates. The power for locomotion in the Black Nerite is provided by muscular waves moving along the ventral surface of the foot, where these waves are attached to the substrate surface by pedal mucus. This mucus adheres to the substrate, allowing the animal to move/crawl (Denny 1980). An advantage of this mucus is that the animal can chemically change the consistency of the mucus depending upon the animal’s needs (Ruppert et al. 2004). Wave-exposed areas require stickier gel to prevent waves washing them off the substrate, whereas the mucus may be a less adhesive liquid when the animal is situated in sheltered areas.

Figure 13

Biogeographic Distribution

N.melanotragus holds an extremely broad distribution across the Australasian South Pacific over a variety of environmental conditions. The Black Nerite is most prevalent along the eastern coastline of Australia, and northern New Zealand. This species has also been identified within the intertidal communities upon Norfolk Island, Lord Howe Island, and the Kermadec Islands (Figure 14). 

Figure 14

Evolution and Systematics

The Mollusca phyla is considered the largest phylum within the Animalia kingdom in which is separated into six diverse classes: Gastropoda, Bivalvia, Scaphophoda, Cephlapoda, Alsopachophore, and Polyplacophora (Ruppert et al. 2004). Organisms within Class Gastropoda are generally classified by the dorsal placement of the viscera, cephalization and posterior location of the mantle cavity (Brusca & Brusca, 2002). This class includes five well-recognized monophyletic clades, where N.melanotragus is considered under the Neritimorpha. The relationships between Neritimorpha and the other Gastropoda clades is relatively variable throughout studies. However, DNA analysis has identified this clade to be a sister group to the Apogastropoda clade (limpets and operculate snails), and the Caenogastropoda clade (periwinkles) to be a sister group to the Neritimorpha (refer to phylogenetic tree Figure 15)Neritimorpha is characterised by a variety of diverse morphologies such as spiral, conical shells, evolved limpets, and slug-like forms. This clade includes species from a wide range of habitats (intertidal to deep-sea vents), specifically reflecting the extraordinary morphological and ecological diversity achieved by the all organisms within Class Gastropoda (Castro & Colgan, 2010).


Kingdom: Animalia

            Phylum: Mollusca

                        Class: Gastropoda

                                    Subclass: Neritimorpha

                                                Order: Cycloneritimorpha

                                                            Superfamily: Neritoidea

                                                                        Family: Neritidae

                                                                                    Genus: Nerita

                                                                                                Species: N. melanotragus

Common Name: Black Nerite (WoRMS 2011)

Figure 15

Conservation and Threats

N.melanotragus has not yet been assessed by the IUCN, whereby the current status of this species is unknown. Increasing global warming is the greatest threat for any marine invertebrate during the present day, causing habitat loss, reducing essential resources, increased populations of invasive species, and human impacts (Powell 1978). Specifically, for the Black Nerite being an intertidal marine invertebrate, continued sea level rise is gradually reducing their intertidal habitat. However, recent studies have suggested that mobile intertidal invertebrates such as N.melanotragus, are less vulnerable to extreme heat conditions/warming than previously thought. This is because of the intraspecific flexibility of mobile ectotherms to display different behaviours (eg. aggregation), allowing them to buffer heat and reduce desiccation stress (Chapperon et al. 2013). 


Amin, S et al. 2014, ‘Assembly and annotation of a non-model gastropod (Nerita melanotragus) transcriptome: a comparison of De novo assemblers’, BMC Research Notes, vol 7, pp.1-8.


Anderson, DT 1962, ‘The reproduction and early life histories of the gastropods Bembicium auratum (Quoy and Gaimard)(Fam. Littorinidae), Cellana tramoserica (Sower)(Fam. Patellidae) and Melanerita melanotragus (Smith)(Fam. Neritidae)’, Proc. Linn. Soc., vol 398. pp.62-68.


Battershill, CN & Bergquist, PR 1984, ‘The influence of Biorhythms on Sensitivity of Nerita to Pollutants at Sublethal Levels’, Oil & Petrochemical Pollution, pp. 31-38.


Borell, EM et al. 2004, ‘Induced resistance in intertidal macroalgae modifies feeding behaviour of herbivorous snails’, Oecologia, vol 140, pp.328-334.

Brusca, RC & Brusca, GJ 2002, 'Invertebrates', Sinauer Associates, vol 2.

Campbell, D 2012, “Morphology”, Encyclopedia of Life, Accessed 30th May 2017.

Castro, LR & Colgan, DJ 2010, ‘The phylogenetic position of Neritimorpha based on the mitochondrial genome of Nerita melanotrgus (Mollusca: Gastropoda)’, Molecular Phylogenetics and Evolution, vol 57, pp. 918-923.


Chapperon, C et al. 2013, ‘Thermally mediated body temperature, water content and aggregation behaviour in the intertidal gastropod Nerita atramentosa’, Ecol Res, vol 28, pp. 407-416.


Chapperon C & Seuront, L 2012, ‘Keeping warm in the cold: On the thermal benefis of aggregation behaviour in an intertidal ectotherm’, Journal of Thermal Biology, vol 37, pp.640-647.

Denny, M 1980, 'The role of gastropod pedal mucus in locomotion', Nature, vol 285, pp.160-161.


Dietl, GP 2004, ‘Reduced Competition and Altered Feeding Behavior Among Marine Snails After a Mass Extinction’, Science, vol 306, pp.2229-2231.


Grove, S 2007, ‘Vicariance, Dispersal and the strange case of the Tasmanian Black Nerites’, The Tasmanian Naturalist, vol 129, pp. 34-36.

Powell, AW 1979, 'New Zealand Mollusca: Marine, Land and Freshwater', Collins: Auckland. 

Przeslawski, R 2011, ‘Notes on the egg capsule and variable embryonic development of Nerita melanotragus (Gastropoda: Neritidae)’, Molluscan Research, vol 31, pp. 152-158.


Rohde, K 1981, ‘Population Dynamics of Two Snail Species, Planaxis sulcatus and Cerithium moniliferum, and their Trematode Species at Heron Island, Great Barrier Reef’, Oecologia, vol 49, pp. 344-352.


Rossini, RA et al. 2013, ‘Feeding ecology of the seagrass-grazing Nerite Smaragia souverbiana (Montrouzier, 1863) in subtropical seagrass beds of eastern Australia’, Journal of Molluscan Studies, vol 80, pp.139-147.


Spencer, HG et al. 2007, ‘Taxonomy and nomenclature of black nerites (Gastropoda: Neritimorpha: Nerita) from the South Pacific’, CSIRO Invertebrate Systematics, vol 21, pp. 229-237.


Uribe, JE et al. 2016, ‘Phylogenetic relationships among superfamilies of Neritimorpha (Mollusca: Gastropoda)’, Molecular Phylogenetics and Evolution, vol 104, pp.21-31.

Woodcock, SH & Benkendorff, K 2008, 'The impact of diet on the growth and proximate composition of juvenile whelks, Dicathais orbita (Gastropoda: Mollusca)', Aquaculture, vol 276, pp. 162-170.