Kenneth A. Hayes and Norine W. Yeung
Hawaii supports one of the world’s most spectacular radiations of land snails (Solem 1984, Cowie 1996a, b). The fauna is disharmonic, with only 10 (Figure 1) of the ~90 land snail families (Cowie et al 1995). The real number of species is difficult to ascertain as most have not been studied in a comprehensive systematic manner for almost a century. Based on the literature and unstudied museum material, Solem (1990) estimated a minimum of 1461 endemic taxa. However, based on published literature, the most current and rigorous estimate was conducted by Cowie et al (1995), recognizing 752 species. The true number is unknown, and probably somewhere between these two estimates. For example, Cowie et al (1995) listed 33 endodontids, but Solem (1976) thought at least an additional 290 await description in the Bernice P. Bishop Museum (BPBM), Honolulu. Despite such discrepancies even the most conservative estimates indicate that Hawaii is an incontrovertible gastropod diversity hotspot. Even more spectacular is that >99% of the species are endemic, many to single islands. The fauna is a biodiversity treasure and evolutionary legacy.
Prior to his death in 1948, C.M. (‘Monte’) Cooke, Jr., in cooperation with H.A. Pilsbry and a handful of other malacologists, dominated the study of Pacific island land snails, particularly the Hawaiian fauna. Although they carried out detailed monographic work on some of the families in Hawaii (Achatinellidae, Zonitidae/Helicarionidae, Amastridae, Pupillidae, Helicinidae), no complete taxonomic treatment of the entire fauna has been completed. With the recent lack of broad focus on the fauna, it is hard to assess the number of species now extinct. However, various estimates, based primarily on limited surveys and extrapolation from studies on achatinellines, suggest 50-90% are lost (Solem 1990, Cowie 2002). A few species probably persist in heavily altered low elevation sites but most of the remaining fauna are hanging on in high elevation refugia protected as reserves or accessible only by helicopter or arduous hiking. This notion was articulated by Pilsbry & Cooke (1914-16:68) nearly a century ago: “The higher mountain slopes of the Hawaiian Islands offer an almost unlimited field of study…of land Mollusca”. Surveys conducted from 2004-2010, while primarily focused on easily accessible lower elevation sites, showed that though the numbers of native snails are surely much reduced estimates of 90% may be unduly pessimistic. But the time left to study and preserve this unique fauna is running out and without immediate intervention this distinctive and important biological treasure may become one of the many poster children of recent extinction--our generation’s Dodo (Solem 1990).
The late Alan Solem of the Field Museum in Chicago and a renowned land snail biologist penned an open letter pleading the case for saving the remaining Hawaiian land snails before they vanish forever (Solem 1990). He set the stage for this plea by first asking how many Hawaiian land snail species remained, and followed with rough estimates for the 10 families represented in Hawaii. Using the collections, publications and notes of C. Montague Cooke Jr. at Bernice Pauahi Bishop Museum in combination with a few publications by others (see references in Solem 1990) detailing the land snail losses he estimated that 65-75% of the fauna was extinct. He ended his impassioned assessment of what remained of the fauna by asking “Who will be the next ‘Monte Cooke’”, and suggesting that “…hope still exists”. He also offered some suggestions as to what needed to be done. First and foremost extensive surveys were needed, something that had not been done widely since the 1930s.
In 2010, a team of malacologists and evolutionary biologist with the necessary motivation and expertise to fulfill the embodiment of the next ‘Monte Cooke’ was assembled to begin answering the questions posed by Solem. Now, more than 25 years later, this team has begun to make inroads into answering the questions of “How many Hawaiian land snails remain?” and “What we will need to save them?”. The team is funded by a National Science Foundation award to address three major goals
1) Complete a comprehensive and systematic survey of the Hawaiian land snail fauna
2) Develop comprehensive phylogenetic hypotheses for all Hawaiian land snail species using material from the above survey and the extensive museum material available
3) Update the taxonomic framework of Hawaiian land snails using an integrated phylogenetic and morphological approach, and describe/redescribe taxa as necessary
This is the first in a series of posts to Molluscan Musings over the next year providing updates on the status on the 10 families of Hawaiian land snails. Here we introduce the families, and provide the background needed for understanding the context of future posts.
The Hawaiian land snail fauna arose as in situ monophyletic radiations following the arrival of a small number of colonizers representing a very few but diverse families. The geographic origins and relationships of most of these endemic lineages remain obscure, largely due to lack of comprehensive study. All but 2 of Hawaii’s land snail families (Hydrocenidae, Helicinidae) are Stylommatophora, the largest and most diverse pulmonate clade, comprising ~90 families with >10,000 described species (Mordan & Wade 2008) found throughout most parts of the world. Their fossil record dates back nearly 300 mya and some Hawaiian taxa belong to families derived from some of the earliest known pulmonates to adapt to life on land in the Palaeozoic (Kondo & Burch 1972, Mordan & Wade 2008).
Achatinellidae, Amastridae: These high-diversity families (534 of the 752 Hawaiian species) are 2 of the 4 major endemic Pacific island families, the others being the Endodontidae (see below) and Partulidae (not in Hawaii–Cowie 1992). The Orthurethra, which includes the Achatinellidae, Amastridae and Partulidae, was thought to be ancient, based on morphology (Pilsbry 1900, Kondo & Burch 1972, Nordsiek 1985, Tillier 1989), but molecular analyses show it is a derived clade, probably of Laurasian origin, with the Pacific families representing independent invasions (Wade et al 2006). The Amastridae, endemic to Hawaii, group with the Holarctic Cochlicopidae but support for other relationships within the Orthurethra is poor (Wade et al 2006). The Achatinellidae seem to group morphologically with the Pupillidae, Valloniidae and Pyramidulidae (Solem 1959, 1979, 1981, Tillier 1989) and the Vertiginidae, Enidae or Partulidae in molecular analyses (Wade et al 2006). There are 209 endemic Hawaiian species in the 5 (out of 7) achatinellid subfamilies (Cowie et al 1995), occurring in diverse habitats from dry coastal regions to mid/upper elevation cloud forests. The subfamilies Achatinellinae and Auriculellinae occur only in Hawaii and share a common ancestor derived from one of the more widespread subfamilies.
Pupillidae: The orthurethran Pupilloidea are Holarctic in origin, found in forests and semi-open habitats globally. They are one of the major Pacific island land snail groups with 56 Hawaiian species in 5 genera, placed by Cowie et al (1995) in Pupillidae. Family level classifications differ among authors as do referral of genera to families (Bouchet & Rocroi 2005) and pupillid relationships to other Orthurethra are unclear (Wade et al 2006). Most Hawaiian species are currently in Nesopupinae (Cowie et al 1995), placed by Bouchet & Rocroi (2005) in Vertiginidae.
Endodontidae: The Endodontidae, endemic and highly diverse in the Pacific islands, are represented by only 33 species in Hawaii, but Solem (1976) considered the fauna to include over 200 species. They have been thought of as the most primitive 'higher' pulmonates (clade Sigmurethra) based on morphology (Solem 1976, 1983; Tillier 1989), leading Bouchet & Rocroi (2005) to place them with the Punctidae (see below) in the sigmurethran superfamily Punctoidea. They were not included in the molecular analyses of Wade et al (2006), who found no support for Punctoidea (sensu Bouchet & Rocroi 2005).
Punctidae: Punctids are very small (~1 mm), Holarctic in distribution but with only one monotypic genus in Hawaii. However, there may be up to 9 undescribed species in the BPBM (Solem 1983). Their relationship to the majority of other pulmonates remains uncertain, but Wade et al (2006) recovered a well-supported clade containing the Charopidae, Otoconchidae and Punctidae.
Succineidae: Succineids occur globally, most in damp areas close to fresh water (Pilsbry 1948, Kerney & Cameron 1979, Miller et al 2000) but some in vegetated pockets in dry areas (Baker 1965; Franzen 1985). In the Pacific they inhabit xeric coastal dunes to high elevation rainforest (Holland & Cowie 2007, 2009). Boss’s (1971) data indicate a world total of 171-350 species, a number to be revised. Hawaii has 42 valid species. Succineids originated in the Eocene (Tillier 1989) in an unknown region, though their sister group (Athoracophoridae) is Pacific. There are two succineid subfamilies based on the presence (Succineinae) or absence (Catinellinae) of a penial sheath (Odhner 1950; Patterson 1971), with both present in Hawaii.
Zonitidae, Helicarionidae: Baker (1938, 1940, 1941) was last to monograph this major Pacific island radiation ~70 yr ago, referring to the 2 families as ‘zonitoids’. They have been treated in various ways with some authors including both in the single family Zonitidae (Hausdorf 1998, 2000) and others separating them into multiple families (e.g. Vitrinidae, Euconulidae, Gastrodontidae) (Boss 1982,Tillier 1989, Bouchet & Rocroi 2005). Hausdorf’s (1998, 2000) anatomical analysis recovered the families as sister taxa and together as sister to the superfamily Limacoidea sensu stricto, both treated as part of the larger Limacoidea sensu lato. Bouchet & Rocroi (2005) treated Zonitoidea and Helicarionoidea as distinct but referred the Hawaiian ‘Helicarionidae’ of Cowie et al (1995), who followed Baker’s classification, to the Euconulidae in superfamily Gastrodontoidea, probably their correct placement. Molecular analyses place Zonitidae, Helicarionidae and Euconulidae within the Limacoidea, but none as sister taxa (Wade et al 2006). None of these families is endemic to the Pacific. There are 70 native Hawaiian species.
Hydrocenidae, Helicinidae: These are the only non-pulmonate families, which with 5 other families not present in Hawaii make up the Neritimorpha, one of the earliest terrestrial gastropod branches (Richling 2004). Both are widely distributed in tropical and subtropical habitats, notably in the New World, Indo-Pacific and Pacific islands. In Hawaii there are 2 hydrocenids, but because hydrocenids are tiny (ca.1 mm in shell height) more may yet to be discovered. Recent surveys and comprehensive revisions of helicinids in other regions (Costa Rica, New Caledonia) have revealed undescribed species (Richling 2004, 2009) so more species may yet be added to the 13 known Hawaiian helicinids, most recently revised by Neal (1934).
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Figure 1: Endemic representatives of the 10 families of Hawaiian land snails. A) Laminella aspera (Amastridae), B) Tornatellides sp. (Achatinellidae), C) Kaala subrutila (Helicarionidae), D) Cookeconcha hystricella (Endodontidae), E) Succinea sp. (Succineidae), F) Pleuropoma kauaiensis (Helicinidae), G) Punctum horneri (Punctidae), H) Vitrina tenella (Zonitidae), I) Nesopupa sp. and J) Georissa kauaiensis (Hydrocenidae). Scale bars = 1 mm.