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).
References:
Baker,
H.B. 1938. Zonitid snails from Pacific islands. Part 1. Southern genera of
Microcystinae. Bernice P. Bishop Museum Bulletin 158: 1-102, pls. 1-20.
Baker,
H.B. 1940. Zonitid snails from Pacific islands. Part 2. Hawaiian genera of
Microcystinae. Bernice P. Bishop Museum Bulletin 165: 103-201, pls. 21-42.
Baker,
H.B. 1941. Zonitid snails from Pacific islands. Part 3. Genera other than
Microcystinae and 4. Distribution and indexes. Bernice P. Bishop Museum
Bulletin 166: 202-370.
Baker,
R.E. 1965. Catinella arenaria Bouchard Chantereux at the Braunton Burrows
National Reserve, Devon. Proceedings of the Malacological Society of London 36:
259-265.
Boss, K.J.
1971. Critical Estimate of the Number of Recent Mollusca. Occasional Papers on
Molluscs 40(3): 81-135
Bouchet,
P. & Rocroi, J.-P. 2005. Classification and nomenclator of gastropod
families. Malacologia 47: 1-397.
Cowie,
R.H. 1992. Evolution and extinction of Partulidae, endemic Pacific island land
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& Miller, S.E.), p. 347-372. SPB Academic Publishing, Amsterdam.
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R.H. 2002. Invertebrate invasions on Pacific islands and the replacement of
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Invasions 3(3)[2001]: 119-136.
Cowie,
R.H., Evenhuis, N.L. & Christensen, C.C. 1995. Catalog of the native land
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Franzen,
D.S. 1985. Succinea vaginacontorta Lee (Gastropoda: Pulmonata: Succineidae).
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B.S. & Cowie, R.H. 2007. A geographic mosaic of passive dispersal:
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Holland,
B.S. & Cowie, R.H. 2009. Land snail models in island biogeography: a tale
of two snails. American Malacological Bulletin 27(1-2): 59-68.
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M.P. & Cameron, R.A.D. 1979. A field guide to the land snails of Britain
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& Burch, J.B. 1972. Archaic land snails of the Pacific islands.
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M.P., Stevens, L.E., Busch, J., Sorensen, J.A. & Keim, P. 2000. AFLP and
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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.
