The land snails of Trindade Island

TrindadeTrindade Island lies about 1140 km off the Brazilian coast. It is the top of a volcanic cone rising 5500 m from the ocean floor, part of the Vitória-Trindade Seamount Chain left by a magma plume.

The island was discovered in 1501 by Portugal, later claimed by England, and finally became part of Brazil in 1822. Many military and scientific expeditions from Europe, North America and, of course, Brazil visited the island. The military expeditions sent to claim and occupy the island brought goats (intentionally) and house mice (non-intentionally) with them. Introduced mammals, especially these two kinds, are extremely destructive to the native fauna and flora of oceanic islands.


The native fauna

Endemic snails of Trindade Island. A: Bulimulus brunoi (shell height 21 mm). B: Naesiotus arnaldoi (10 mm). C: Vegrandinia trindadensis (7 mm).
Endemic snails of Trindade Island. A: Bulimulus brunoi (shell height 21 mm). B: Naesiotus arnaldoi (10 mm). C: Vegrandinia trindadensis (7 mm).

Trindade Island has just a handful of known land snails, some of them are endemic species (i.e., they are known only from this island). The endemic species are: Bulimulus brunoi and Naesiotus arnaldoi (family Bulimulidae), Oxyloma beckeri and Succinea lopesi (Succineidae) and Vegrandinia trindadensis (Subulinidae). The non-endemics are: Lilloiconcha gordurasensis (Charopidae), Pupisoma macneilli (Valloniidae), Gastrocopta barbadensis (Gastrocoptidae) and Vertigo sp. (Vertiginidae; which could be a non-native, introduced species). The single non-native land snail species on the island is Bradybaena similaris (Bradybaenidae), which has been widely introduced worldwide.

Microgastropods of Trindade. A-C: Lilloiconcha gordurasensis (shell width 1.9 mm). D: Gastrocopta barbadensis (shell height 1.9 mm). E-F: Pupisoma macneilli. (shell height 1.2 mm). G: Vertigo sp. (shell height 1.5 mm).
Microgastropods of Trindade Island. A-C: Lilloiconcha gordurasensis (shell width 1.9 mm). D: Gastrocopta barbadensis (height 1.9 mm). E-F: Pupisoma macneilli (height 1.2 mm). G: Vertigo sp. (height 1.5 mm).

And this fauna seems to have gone through hard times during the last few decades. All of these species have only been found on the island as empty shells since the 1980s; not a single snail had been reported alive, native or otherwise. This suggests that many (if not all) of the native land snail species can already be extinct.

The causes for this likely lie with the introduced species. The house mice population on Trindade is estimated at tens of thousands and they might have played a part in the snails’ disappearance: it is well known that mice can voraciously prey upon land snails, especially if other food sources are scarce. Moreover, goats are the main cause of extinctions on many islands. On Trindade, goats were responsible for drastic deforestation, leading to the extinction (or nearly so) of many endemic plant species. This resulted in a loss of habitat and food resources, which might have been the major cause of the disappearance of the native snails.


Rediscovery of living snails

Left: Succinea lopesi, living snail (top) and empty shell (two shells, bottom). Right: shell of Happia sp. in two different views.
Left: Succinea lopesi, living snail (top) and empty shell in two different views (bottom; shell height 12.5 mm). Right: shell of Happia sp. in two views (width 3.5 mm).

In our earliest paper dealing with Trindade’s snails, we predicted that if some land snails still survived, it should be on the island’s peaks, which would have acted as refuges, since introduced species do not usually expand their range into such places. So we were overjoyed to confirm this prediction when more recent collecting on two of the highest peaks on Trindade recovered living snails. Two species were found: Succinea lopesi and a new record for the island, Happia sp. (Scolodontidae).

A great campaign was conducted to eradicate the goats and, after its completion in 2005, the flora seems to be recovering with the aid and constant monitoring of a research team. Hopefully, the recovery of the native flora might allow the snail populations to re-establish themselves. However, it should be noted that no living snails of the other endemic species of Trindade (which lived near the shore), have been found, so the question of whether they still survive remains open.


Bibliography

The preceding text is a summary of the following papers, where this research was published:

  • CUNHA, C.M.; SALVADOR, R.B.; SIMONE, L.R.L. 2015. The terrestrial microgastropods of Trindade Island, Brazil (Gastropoda: Pulmonata). Spixiana 38(1): 139-143.  [PDF]
  • SALVADOR, R.B.; CUNHA, C.M.; SIMONE, L.R.L. 2013. Taxonomic revision of the orthalicid land snails (Pulmonata: Stylommatophora) from Trindade Island, Brazil. Journal of Natural History 47: 949-961.  [PDF]
  • SALVADOR, R.B.; CUNHA, C.M.; SIMONE, L.R.L. 2013. The pulmonate snails of Trindade Island, Brasil. Tentacle : 38-39.  [PDF]
  • SALVADOR, R.B.; SILVA, N.G.; CUNHA, C.M.; SIMONE, L.R.L.; ALVES, R.J.V. 2014. Rediscovery of Living Land Snails on Trindade Island, Brazil. American Malacological Bulletin 32: 140-142.  [PDF]
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