Giant fight: New Zealand’s desperate efforts to save its carnivorous snails

Author: Julián Monge Nájera, Ecologist and Photographer 

Isolated from the world for 80 million years, New Zealand’s giant carnivorous snails never developed resistance to desiccation and lack jaws. Some are in danger of extinction, and desperate measures like spraying poison on their enemies have not worked.

New Zealand giant snails of the genus Powelliphanta come from a very ancient group, present on the Gondwana supercontinent when the first dinosaurs appeared, but the fact that this particular genus is found only in New Zealand suggests that it evolved less than 80 million years ago, when the archipelago was separated from adjacent lands.

For unknown reasons, these snails do not feed on algae or decomposed vegetation. They follow a very different path, they are carnivores, but, having no jaws, which is extraordinary in a predator, they have to slowly scrape the meat with their toothed tongues (radulae). Its main victims are its relatives the slugs (which are nothing more than snails without shells) and earthworms. These snails apparently became predators independently of other carnivorous snails such as Rumina and Euglandina.

Just as their evolution limits the way they process food, it also limits where they can live, because they cannot seal their shell with a membrane to avoid drying out; they spend the day hidden in the ground and come out at night to feed and mate.

Their difficulty to survive in modern times is shared with many other New Zealand species that are the last representatives of old and less competitive groups, extinct in the rest of the world; all have small and isolated populations, are large in body size, have strange diets, and live long lives, but have few descendants¹. 

The natural enemies of these snails include kea parrots (Nestor notabilis), kaka (Nestor meridionalis), and weka birds (Gallirallus australis), which are also endangered and come from ancient, rare, large and long lived species.

In my sketchbook, the giant snail, next to the map of New Zealand and three of its natural enemies. It is also a victim of rats and other imported animals, but its greatest enemy could be humans, who modify its habitats.

The nearly desperate efforts of New Zealanders to conserve their giant snails deserves some space here. New Zealand is an atypical country, isolated from the world, few hardly know that it exists, but it is a country where enormous resources are distributed among a population of just 5 million. In other words, it is a rich country that could invest a lot in preserving its nature.

The first problem with conserving New Zealand’s giant snails is their identity problem; in other words, this genus could have fifty species, but most are not formally described or named. On the other hand, they are slow breeders: it takes them between 5 and 8 years to reach reproductive capacity, and in very cold climates, eggs take up to 12 months to hatch.

Conservation attempts include breeding and poisoning their enemies in the wild.

Raising them in captivity is not very promising, because each hermaphrodite individual lays at most 10 eggs per year (Powelliphanta augusta). They require much space to survive, it is difficult for them to reproduce in captivity, and giving them calcium to develop their shells kills them².

The option of poisoning the rats that eat them has not been very successful either, because if we carefully read the evidence presented by S.J. Bennett, we see that the benefit to snails is doubtful³.

In short, if New Zealand’s giant snails will survive, it will be more likely because their habitats are preserved pristine⁴.

*Edited by Zaidett Barrientos, Katherine Bonilla y Carolina Seas.

Originally published  in Blog Biología Tropical: 14 august 2020

REFERENCES

¹ Daugherty, C. H., Gibbs, G. W., & Hitchmough, R. A. (1993). Mega-island or micro-continent? New Zealand and its fauna. Trends in Ecology & Evolution, 8(12), 437-442.

² Allan, T. E. (2010). Husbandry of the Carnivorous Land Snail, Powelliphanta augusta (Gastropoda: Pulmonata: Rhytdidae) (Thesis). Victoria University, Wellington, New Zealand.

³ Bennett, S. J. (2001). The effects of introduced predators and the invasive weed Tradescantia fluminensis (Vell.)(Commelinaceae) on the land snail Powelliphanta traversi traversi (Powell)(Gastropoda: Pulmonata: Rhytididae) (Thesis).  Massey University, New Zealand.

⁴ Meads, M. J., Walker, K. J., & Elliott, G. P. (1984). Status, conservation, and management of the land snails of the genus Powelliphanta (Mollusca: Pulmonata). New Zealand Journal of Zoology, 11(3), 277-306.