The Sydney Turban Snail, Lunella torquata (Gmelin,1791) is a medium-sized Turbinid gastropod (bottom of figure below), which inhabits shallow, subtidal, rocky reefs from the mid-north NSW coast to Victoria and also in south-west Western Australia. It is a notable grazer in those portions of the Great Southern Reef. It has previously been known as Turbo torquatus and Ninella torquata.
A smaller related species is the green turban snail, Lunella undulata (previously Subninella undulata, top of figure above). It occupies the same range in NSW and is both subtidal and intertidal. It extends to Tasmania where it grows larger and is the basis of a small fishery. It seems that many aspects of the ecology of both species are common or shared.
A larger, less abundant, sympatric species is Turbo militaris, found from southern NSW to north of Coffs Harbour (figure below).
The archaeological record from shell middens suggests Lunella torquata were a dietary item for first nations people on the southern Victorian coast, but apparently they are rare or non-existent there now.
I have found small juveniles most commonly in turfing algae meadows in very shallow water where they, (and other turbinid juveniles) are recorded as settling out of the plankton. I have found 30mm-40mm individuals grazing on the fronds of the golden kelp, Ecklonia radiata, which is one of the iconic species of the Great Southern Reef. I usually find only larger individuals, ie above about 60mm shell height, in water deeper than 3m. Shells of all sizes above about 50mm, but usually larger ones, are often on open rock bottom around the base of kelp holdfasts under the kelp forest canopy. Large snails are also common among shallow subtidal colonies of the sea squirt, Pyura praeputialis.
At most of my sites, samples come from cryptic microhabitats in gutters and the undercuts of large boulders, or attached to Ecklonia fronds. Snails less than 30mm are difficult to find and seem to inhabit only turf algae microhabitats, often in circular depressions in the rock typically created by sea urchins. So far, I have only found individuals less than 20mm in turf algae meadows in ocean swimming pools constructed on rock platforms (Blue Pool at Bermagui and Mahon Pool in Sydney). Some workers have found very small specimens by sieving the turf algae. This species seems to be nocturnal, and I find more of them out in the open on early morning dives.
Published material on the ecology of my main species exists but is not extensive. The same is true for the other two turbinids in this habitat.
Amy Smoothey, in her doctoral thesis and subsequent published paper (Smoothey,2013) reported that smaller snails were more abundant in turf algae and fringe habitats while larger snails were more abundant in kelp and barren habitats.
Leon Collett apparently produced a doctoral thesis around 1973 on this species (then called Ninella it seems) but I have been unable to locate it. Lindsay Joll documented aspects of reproduction and growth in Western Australia in 1975 in a Masters thesis.
Ettinger-Epstein and Kingsford (2008) found patchy distributions and suggested that shelter (either algal cover or crevices) is an important factor in distribution and abundance. They also found some significant movement (tens of metres) over a two-month period, but most movements seemed to be local (<10m) and this matches my anecdotal observations. Large snails seem to move more than small ones. There was no correlation between the Southern Oscillation Index and the abundance of snails, but they did find higher abundance during a La Nina-dominated period and lower abundance during an El Nino period. Kelp forest cover does seem to be a dominant factor in snail abundance.
I have found similar variability in abundance (Hamer,2022) but I suspect storms (through wave-action, low salinity and sediment runoff) are a major determinant. I will discuss this in future posts.
Average maximum size is around 90mm-100mm, but I have been given a 125mm shell which was washed up on Yarra Bay beach after a storm. Visible gonad development is around 40mm-50mm, so I recognise juveniles as <50mm and adults as >50mm.
The sexes are separate with male gonads being creamy or light brown while females exhibit green gonads. The figure above shows the operculum, foot, digestive system and gonad removed from 5 recaptured snails after boiling. Three male samples are on the left and two females on the right. Snails in this family are broadcast fertilisers, releasing their eggs and sperm into the water column.
Ward and Davis (2002) studied the reproduction and spawning of Lunella populations near Wollongong, Ulladulla and Eden and found spawning at all three sites in autumn-winter 1996, spring-summer 1997 and autumn 1997. Males and females developed and spawned in synchrony, and there was no relationship with water temperature. The sex ratio was 1:1 at Ulladulla and Eden but their Wollongong samples were dominated by large females. Multiple spawning seems to be common and importantly, sites as close as 15kms were different. This is different to most molluscs in south-eastern Australia which seem to have a single spawning event each year, although various limpets may be more like Lunella.
Seinor et al (2023) found extended and variable spawning seasons for Turbo militaris in NSW and lack of synchrony between sites. Males and females were in synchrony at each site. They found factors such as temperature, nutrients and wave action influenced spawning.
In Western Australia, more work on this species has been done than on the east coast.
Joll (1980) formally published the reproductive biology component of his MSc thesis. The more recent reproductive findings of Ward and Davis in NSW were similar to Joll’s: extended spawning seasons with possible multiple spawning; partial spawning with unspawned eggs retained for subsequent spawning; and asynchronous spawning within a population.
On the smaller, sympatric Lunella undulata, which I have not studied, Ab Lah et al (2019) found variation in the gonadosomatic index and in characteristics of the foot tissue but noted that in all months, there was good nutritional quality with high levels of protein and polyunsaturated fatty acids. I have no reason to suspect the same is not true for Lunella torquata. Keane et al (2014) also found parallels between Lunella undulata and Lunella torquata.
Taylor and Glover (1999) documented predation on juveniles by the shell boring Muricid snail, Dicathais (Thais) orbita in WA. It is also abundant in Lunella’s habitat here in NSW. I suspect the dead snails I have found with bored holes in the shell fell victim to even larger gastropod predators. A further predator appears to be the common octopus, if empty shells around their lairs are an indication of their prey.
More recent work this century has been associated with extensive research on the Golden Kelp (Ecklonia radiata), ocean warming, tropicalization, and a major marine heatwave in 2010-11. See Coleman et al (2020) and Mulders et al (2022) (In NSW, Mamo et al (2019) found that the second of my two species, Turbo militaris, is probably quite resilient to marine heatwaves and warming.)
Wernberg et al (2008) studied the population structure of Lunella on deep (>10m) reefs across 1000km of WA coast, because most work had been done on shallow, inshore reefs and they wanted to assess population structure at the warmer end of Lunella’s range limit. They found size distributions and abundances varied between reefs and zones, and that densities in offshore, wave-exposed reefs were similar to those of coastal habitats. I have not looked at deep kelp forests in my work so far in Sydney.
As for diet, Lunella torquata was found to graze on all macroalgae offered in “no choice” feeding trials, but gut content analysis of this and other turbinid snails showed consumption of a wide variety of macroalgae in the field. Their conclusion, as I interpret it, was that Lunella is unlikely to do the same grazing damage to a kelp habitat that sea urchins inflict. In NSW, Fletcher (1987) similarly found that removal of turbinids had little effect on the algal community and his discussion therefore focused mainly on sea urchins and limpets.
Small-scale patchiness and high variability seem to be two key issues in turfing algae and kelp communities, and therefore in the biology of these snails. These issues will feature in my discussions in future posts about growth and recruitment.
References
Ab Lah R, D Savins, K Benkendorff, A Dowell. 2019. Temporal variation in condition index and meat quality of Lunella undulata (Turbinidae), in relation to the reproductive cycle. Molluscan Research Sept 2018.
Coleman M A, A J P Minne, S Vranken, T Wernberg. 2020. Genetic tropicalisation following a marine heatwave. Nature Scientific Reports. https://doi.org/10.1038/s41598-020-69665-w
Ettinger-Epstein P, and M J Kingsford. 2008. Effects of the El Nino southern oscillation on Turbo torquatus (Gastropoda) and their kelp habitat. Austral Ecology 33:594-606 htpps//doi:10.1111/j.1442-9993.2007.01824.x
Fletcher W J. 1987. Interactions Among Subtidal Australian Sea Urchins, Gastropods, and Algae: Effects of Experimental Removals. Ecol. Monographs 57(1):89-109 https://doi.org/10.2307/1942640
Hamer G. 2022. Inter-annual variability of recruitment in the turban snails Lunella torquata (Gmelin,1791) and Turbo militaris Reeve, 1848 in Sydney, Australia. Malacological Society of Australasia Newsletter, No 181, May 2022. ISSN 1834-4259.
Joll L M. 1980. Reproductive biology of two species of Turbinidae (Mollusca:Gastropoda). Aust. J. Mar. Freshwater Res. 31:378-390
Keane J P, J M Lyle, C Mundy K Hartmann. 2014. Periwinkle Fishery of Tasmania: Supporting Management and a Profitable Industry. FRDC Project 2011/024
Mamo L T et al. 2019. Resilience of a harvested gastropod, Turbo militaris, to marine heatwaves. Mar. Env. Research 151 https://doi.org/10.1016/j.marenvres.2019.104769
Mulders Y et al. 2022. Patch dynamics driven by wave exposure in subtidal temperate seaweeds are exacerbated by warming oceans. Mar.Ecol.Prog.Series 685:85-95 https://doi.org/10.3354/meps13989
Seinor K Purcell Smith Malcolm. 2023.
Smoothey A F. 2013. Habitat-Associations of Turban Snails on Intertidal and Subtidal Rocky Reefs. PLoS ONE 8(5) e61257. https://doi.org/10.1371/journal.pone.0061257
Taylor J D and E A Glover. 1999. Penetrating the defences: opercular drilling by Dicathais orbita (Mollusca:Gastropoda:Muricidae) on the turbinid gastropod, Ninella torquata. In Walker and Wells (Eds). 1999. The Seagrass Flora and Fauna of Rottnest Island, Western Australia. Western Australian Museum, Perth.
Ward D W and A R Davis. 2002. Reproduction of the turban shell Turbo torquatus Gmelin 1791(Mollusca:Gastropoda), in New South Wales, Australia. Mar.Freshwater Res.,53:85-91 https://doi.org/10.1071/MF00066
Wernberg T, M White, M A Vanderklift. 2008. Population structure of turbinid gastropods on wave-exposed subtidal reefs: effects of density, body size and algae on grazing behaviour. Mar.Ecol.Prog.Series. 362(169-179) doi:10.3354/meps07416