Fading radiation
Published 29 October 2022
In studies of evolutionary processes, certain radiations of species have become particularly renowned: the finches of the Galapagos, the honeycreepers of Hawaii, the cichlids of the African Great Lakes. Occupying a justified position among these causes celebre are the Partula snails of the Pacific Islands. These small to medium-sized snails, carrying conical shells from one to three millimetres in height, have radiated into over ninety recognized species found from Micronesia in the west to the Society Islands of French Polynesia in the east (Lee et al. 2014).
Partula is one of three genera recognised in the endemic Oceanian family Partulidae. It is distinguished from the other two genera in the family, Eua and Samoana, by features of the male reproductive organs, including a short prostrate, the presence of a flagellum, and having the interior of the penis sculpted into several irregular pilasters broken into rows of rounded tubercles (Schileyko 1999). Individual Partula are hermaphrodites that may be either cross-fertilising or self-fertilising. The rate of self-fertilisation that occurs within a population may vary between species. In the Saipan species P. gibba, self-fertilisation seems to be the norm, but in the Moorea species P. taeniata, self-fertilisation is generally responsible for about one or two percent of births (Cowie 1992). Said births are ovoviviparous with egg-shells being resorbed into the parent prior to emergence. Most Partula species are primarily arboreal and nocturnal with few species regularly descending to the ground to feed.
Individual species within Partula are typically restricted in distribution to single islands or groups of islands. Some islands may be home to multiple species with the best-known examples being on the Society Islands. The island of Moorea was historically home to seven distinct species while species from Raiatea have been assigned to no less than 33 (Cowie 1992). However, species recognition has been complicated by high levels of diversity and interfertility. Individuals of a species may vary significantly in coloration, formation and, in some cases, even chirality. Hybridisation between sympatric species may be common but not universal, with some species hybridising in certain parts of their ranges but not in others. Molecular studies of Society Island species have identified a disconnect between gene distributions and recognised species, indicating a need for further investigation (Haponski et al. 2019).
Broader relationships within Partula exhibit a distinct correlation with geography. The genus’ range across the Pacific Basin is broken by a large central gap with no Partula species found in Tonga or Samoa (partulids are represented on these islands by species of the other two genera). A molecular phylogenetic study of the genus by Lee et al. (2014) identified a division into three basal clades. One included the species from the eastern Pacific between the Society and Cook Islands. Another covered the greater part of the western Pacific, from the Mariana Islands to Fiji. The third clade included the westernmost species found in Palau. This last clade, however, was not directly associated with the other two, being instead placed closer to species of Samoana, and may be misplaced within Partula.
Sadly, most attention given to Partula in recent years has been in the context of tragedy. Carnivorous snails, most notably Euglandina rosea, were introduced to many Pacific Islands in the 1950s in an attempt to control the giant African snail Achatina fulica, itself originally imported to many locations as a food source. Whether these predators had any sort of impact on African snail populations is questionable (Cowie 1992) but there is no doubting their impact on partulids. Within ten years from its introduction, Euglandina rosea had all but eradicated the Partula species of Moorea (a single species, P. taeniata, persists in remote valleys but remains critically endangered). Species on Tahiti, Raiatea, Guam and other Pacific Islands were similarly devastated. Of 51 species currently recognised from the Society Islands, no more than eight currently survive in the wild (Haponski et al. 2019). Another ten species survive as captive populations. Perhaps only a single Tahitian species, P. hyalina, is not under immediate danger of extinction, having been introduced by humans to islands outside its native range (Lee et al. 2014). The Partula radiation has raised many fascinating scientific questions. Many of those, sadly, are now doomed to remain unanswered.
References
Cowie, R. H. 1992. Evolution and extinction of Partulidae, endemic Pacific island land snails. Philosophical Transactions of the Royal Society of London Series B—Biological Sciences 335: 167–191.
Haponski, A. E., T. Lee & D. Ó. Foighil. 2019. Deconstructing an infamous extinction crisis: survival of Partula species on Moorea and Tahiti. Evolutionary Applications 12: 1017–1033.
Lee, T., J. Li, C. K. C. Churchill & D. Ó. Foighil. 2014. Evolutionary history of a vanishing radiation: isolation-dependent persistence and diversification in Pacific Island partulid tree snails. BMC Evolutionary Biology 14: 202.
Schileyko, A. A. 1999. Treatise on Recent terrestrial pulmonate molluscs. Part 3. Partulidae, Aillyidae, Bulimulidae, Orthalicidae, Megaspiridae, Urocoptidae. Ruthenica Supplement 2 (3): 262–436.