Endangered Hawaiian Tree Snails

From 1999-2002 I worked as a research associate for Dr. Michael Hadfield at the University of Hawaii studying the population biology and managing the captive rearing facility for endangered Hawaiian tree snails. During this time, I also worked closely with Dr. Brenden Holland studying the genetic population structure of these rare Achatinelline snails. The information on this page should provide an introduction to Hawaiian tree snails and an overview of the conservation project. Much of this text was taken from a poster I presented at the Expanding the Ark Invertebrate Science and Conservation Conference, American Natural History Museum, New York, March 25, 2004. Download PDF of poster here.

Summary:

Nearly seventy-five percent of 750 recognized species of terrestrial Hawaiian snails (99% of which are endemic) have disappeared in the past 70 years. Snails in the genus Achatinella are some of the most threatened, and continue to be faced with annihilation by a myriad of threats, especially introduced predators. With low fecundity, limited vagility, slow growth, and late age of maturity, these snails are particularly susceptible. All 41 species of the genus Achatinella were placed on the Endangered Species List in 1981; only 8-9 species are extant. Conservation and management efforts have been ongoing for nearly two decades, and include: captive rearing, monitoring of populations in the field, predator control and exclusion, and the use of molecular genetic data to guide management and recovery plans. Captive populations of most, but not all, species are successful and growing, and laboratory-reared snails have been successfully re-established in the wild. We continue with long-term, mark-recapture studies to monitor populations in the field. Molecular investigations have identified high levels of population structure within one species, Achatinella mustelina. Genetic data are being used to prioritize the placement of predator exclusion fences, identify target populations for captive-rearing, and guide re-introduction and translocation efforts.

Birth Size Shell length: 4.6 mm
Adult Mean Maximum length: 21.4 mm
Age at first reproduction: 4 years
Annual fecundity (young): 4.5
Longevity: 10+ years

Taxonomy and conservation status:

The subfamily Achatinellinae contains Achatinella and the closely related genera, Partulina, Perdicella, and Newcombia, and represents one of the most spectacular radiations in Hawaii (~100 species). The colorful and charismatic snails in the genus Achatinella are endemic to the Hawaiian island of Oahu and were once extremely abundant. There are 41 recognized species of Achatinella and hundreds of historically described varieties. Scientists and shell collectors have been fascinated by this highly diverse group for well over a century; they played a major role in the development of early evolutionary thought, especially the importance of isolation in speciation (Gulick 1872).

Currently, 32 of the original 41 species within this genus are presumed extinct. Remaining populations are severely threatened, small, and fragmented. In 1981, the entire genus was placed on the USFWS endangered-species list, CITES Appendix I, marking the first and only time a taxonomic group above the level of species has been listed. Recovery efforts for the 8-9 extant species began nearly two decades ago and continue in earnest today.

Life history:

Achatinelline snails have evolved unique life-history characteristics that leave them particularly vulnerable to extirpation by predation. This includes very slow growth; late maturity; long life span; and low fecundity (see above). They are also characterized by extremely low vagility, often spending their entire life in one small tree. Hawaiian tree snails are hermaphroditic, nocturnal, and feed on epiphytic fungus scraped from the surface of native vegetation.

Current and historic threats:

Early naturalist and hobbyists began a shell collecting frenzy in the mid-1800’s which precipitated population declines and extinctions that have continued to this day. Shell collection (see picture to right) combined with the clearing of land by Polynesians and early settlers, left many lowland species and varieties extinct by the turn of the 20th century. Habitat destruction and degradation continue today, but the most insidious and intractable threat comes in the form of introduced predators (Hadfield et al. 1993).

In an attempt to control the giant African Snail, the Department of Agriculture introduced Euglandina rosea, a predatory snail, as a biocontrol agent in the mid 1950’s. Euglandina has since become well established; consuming native snails at an alarming rate. Three species of rat (R. rattus, R. norvegicus, and R. exulans) have also been decimating native snail populations across the island.

Captive Rearing, ex situ conservation efforts:

Introduction:
The goal of our ex situ efforts are to remove individuals from severely threatened wild populations and bring them into captivity for propagation. Laboratory populations can then serve as stock for reintroduction or augmenting existing natural populations. The first Achatinella were brought into our University of Hawaii laboratory in 1991. Since that time our captive rearing facilities have expanded greatly.

Methods:
Snails are maintained in screened terrariums on the live branches of two common host trees (Metrosideros polymorpha and Freycinetia arborea). See image to right. Terrariums are kept in commercial environmental chambers set for 12 hour day/night cycles at 20º and 16º C. These temperatures match the average highs and lows for an elevation of ~650 meters in the mountains of Hawaii (Hadfield et al. 2004). A custom built plumbing system ensures that a fine spray of water is delivered to each cage three times a day to simulate rainfall. Twice a month, branches in each cage are replaced, cages are sterilized, and all snails are censused. Additionally, diet is supplemented with cultured fungus (Cladosporium sp.) grown on calcium enriched potato dextrose agar.

Results and discussion:
As of March 2004, there were 1288 snails in the captive rearing facility, representing 15 species from the subfamily Achatinellinae (1018 individuals from 7 species of Achatinella). Only two extant species of Achatinella are not represented by captive populations (A. fulgens and A. concavospira); and at least one species in the laboratory is believed to be extinct in the wild (A. apexfulva). We have recently established nine new captive populations of one species, A. mustelina. Molecular data was used to guide the expansion of our facilities to include these additional populations. .Most, but not all species of Achatinella in our laboratory are thriving.

Field Studies, in situ conservation:

Mark-recapture investigations of Achatinella spp. have been ongoing for the past 20 years. Monitoring of several long-term field sites has been vital to gain demographic and life-history information and to document population declines. It is possible from ground shell surveys assess what predators have been in an area. Then, if the threat of predation is detected, targeted control measures can be initiated in an area. One newly developed method of in situ predator control are specially designed fences built around existing populations of Achatinella to protect them form rats and Eulandina rosea (see picture to right of snail exclosure in Pahole Natural Area Reserve, Oahu). Two predator exclusion fences have been recently built on Oahu, thus far they appear effective and populations within them are rebounding.

Conservation Genetics:

The phylogeographic structure of fragmented Hawaiian tree snail populations was investigating using cytochrome oxidase I (CO1) mitochondrial DNA sequences. Genetic variation between and within populations was extensively surveyed for one species, A. mustelina (Holland and Hadfield 2002). From 1999-2001, 90 individuals from 18 populations were sampled using a non-invasive technique. A surprising amount of intraspecific genetic structure was found, and based on reciprocal monophyly six evolutionary significant units (ESUs) were defined for this species (Figure to right, click for larger version). ESUs are currently being used to prioritize the placement of predator exclusion fences and guide captive propagation efforts. Subsequent genetic research on many more species of Hawaiian tree snails has illuminated evolutionary history and colonization events (Holland and Hadfield 2004).

Conclusions:

1. Remaining populations of the once abundant and highly diverse endemic Oahu tree snails are small, highly fragmented, and severely threatened by introduced predators.

2. Captive rearing efforts have been largely successful and laboratory-born snails have been used to successfully reestablish wild populations in the field.

3. Successful in situ management requires regular monitoring of populations, and active predator control and exclusion.

4. Molecular studies have augmented our knowledge of population structure, systematics and evolutionary patterns; data are being used to guide in situ and ex situ conservation efforts.

References:

1. Gulick J.T. (1872). “On Diversity of Evolution under One Set of External Conditions.” Journal of the Linnean Society (Zoology) 11, 496-505.

2. Hadfield, M. G., B. S. Holland, K.J. Olival (2004). "Contributions of ex situ propagation and molecular genetics to conservation of Hawaiian tree snails." Experimental Approaches to Conservation Biology. University of California Press.

3. Hadfield, M. G., S. E. Miller, et al. (1993). "The decimation of endemic Hawaiian tree snails by alien predators." American Zoologist 33: 610-622.

4. Holland, B. S. and M. G. Hadfield (2002). "Islands within an island: phylogeography and conservation genetics of the endangered Hawaiian tree snail Achatinella mustelina." Molecular Ecology 11: 365-375.

5. Holland, B. S. and M. G. Hadfield (2004). "Origin and diversification of the endemic Hawaiian tree snails (Achatinellidae: Achatinellinae) based on molecular evidence." Molecular Phylogenetics and Evolution (In Press).

6. Kobayashi, S. R. and M. G. Hadfield (1996). "An experimental study of growth and reproduction in the Hawaiian tree snails Achatinella mustelina and Partulina redfieldii (Achatinellinae)." Pacific Science 50(4): 339-354.