Cryptic speciation in Indo-Pacific gastropod parasites of stony corals
Adriaan Gittenberger
National Museum of Natural History “Naturalis”, P.O.
Box 9517, NL-2300 RA Leiden, The Netherlands
Institute of Biology, University Leiden, P.O. Box 9516, 2300 RA
Leiden, The Netherlands
GiMaRIS, Marine Research, Inventory & Strategy Solutions, 2315
HP Leiden, The Netherlands, Inf...@GiMaRIS.com
Summary
Sympatric speciation seems to have been the major driving force
in the evolutionary history of parasitic gastropods on stony corals.
The diagnostic value of conchological characters in the families
Epitoniidae and Coralliophilidae is discussed and compared with
the usefulness of other, more rarely used morphological characters,
next to molecular, ecological and biogeographical data.
Introduction
The central Indo-Pacific hosts the world’s richest marine
fauna. For this extraordinary high level of biodiversity two main
explanations can be given.
(1) Most Indo-West Pacific species are widespread, with distributions overlapping in the Indo-Malayan centre of maximum diversity.
(2) Many species have complex symbiotic and parasitic relationships that may accelerate speciation and promote diversity. Various questions wait for answers, for example: (a) Has speciation usually occurred in allopatry, so that the areas of overlap between sister taxa should be seen as a result of secondary range-shifts (related to the complex geological history of the area, with varying currents and water/land distributions related to plate tectonics and sea-level changes)? (b) How can gene flow be maintained over very long distances in putatively widespread species, whose pelagic larvae are supposed to be relatively shortlived? (c) Do complex symbiotic relationships among marine organisms lead to co-evolution, co-speciation, and hence elevated biodiversity? To shed some more light on these questions and more specifically on the evolutionary and ecological relationships between Indo-West Pacific species, scleractinian corals (Fungiidae, Dendrophylliidae, Euphylliidae) and their gastropod symbionts (Coralliophilidae, Epitoniidae) were used as model organisms. For both the corals and the gastropods the phylogenetic relationships, ecology and biogeography was studied in detail in an area ranging from the Red Sea to Hawaii.
Methods
About 800 hours were spent underwater to search approximately 60,000
stony coral specimens for endo- and ectoparasitic snails, Coralliophilidae
and Epitoniidae respectively, in various parts of the Indo-West
Pacific, i.e. coastal areas of Egypt (Red Sea), Maldives, Thailand,
Malaysia, Japan, Palau, Philippines, Indonesia and Australia. At
most localities the fieldwork lasted one week to a month. Additional
periods of three and nine months (in 1997 and 2001, respectively)
were spent in the Spermonde Archipelago, off Makassar, SW Sulawesi,
Indonesia, which is situated in the centre of maximum diversity.
The coral diversity here is among the highest in the world (Hoeksema,
2007). The study focussed on mushroom corals (Fungiidae) because
of the known relationships with endo- and ectoparasitic gastropod
parasites (Massin & Dupont, 2003; Robertson, 1963). Therefore
a special effort was made to search all species within this family,
in particular also the less commonly observed ones, which live in
environments that are usually avoided by divers, such as sandy reef
bases, shallow reef flats, areas with strong currents, or murky
water. During the study, in total about 1000 individual coralliophilid
and 3000 epitoniid snails were found in association with the corals
examined. For most of these coral infestations the locality, depth,
microhabitat, parasite species and host species were noted. Specimens
or samples of both the parasites and their hosts were preserved
on 70% or 96% ethanol for DNA-analyses. Subsequently, the markers
CO-I and/or ITS-I&II of about 600 parasitic gastropods and mushroom
corals were sequenced and used for phylogeny reconstructions.
Results and Discussion
Ectoparasitic gastropods of stony corals: Epitoniidae
Most taxa in the family Epitoniidae have been introduced and described on the basis of teleoconch shell characters only, with a special focus on shell shape, the costal and spiral sculpture, the umbilicus and the suture (see e.g. Weil et al., 1999; Nakayama, 2003). The molecular analyses of the present study, however, support the view of Kilburn (1985: 241) that convergent or parallel evolution in these characters may be common among epitoniids (Kilburn, 1985: 241). The species Epifungium twilae (Gittenberger & Goud, 2000) and E. pseudotwilae Gittenberger & Gittenberger, 2005 illustrate this phenomenon (Fig. 1). Even though their broad shells are very similar, they are not sister species. In addition the intraspecific conchological variation in epitoniid species that are associated with corals is very large, with shell shapes that vary between very broad and relatively slender conical as for example in the golden wentletrap species Epidendrium aureum Gittenberger & Gittenberger, 2005 (Figs 2-3) and E. sordidum Gittenberger & Gittenberger, 2005 (Figs. 4-5). Because these teleoconch characters are still commonly used to characterize epitoniid genera and subgenera (e.g. Weil et al., 1999: Appendix II; Bonfitto and Sabelli, 2001; Nakayama, 2003), “epitoniid taxonomy remains in a chaotic state, particularly above the species level” (Kilburn, 1985: 240) and the classification of the genus Epitonium is “very tentative and is aimed solely at grouping together similar species for convenience sake” (Kilburn, 1985: 280). The present molecular studies support the view that the nominal genus Epitonium, as it shows up in most of the recent literature, is polyphyletic (Gittenberger, 2006).
Even though many conchological characters do not support the phylogeny
reconstruction of Epitoniidae based on molecular data suggesting
frequent parallelism or convergence, other morphological characters
do. These clades, and more specifically the epitoniid genera Epifungium,
Epidendrium, and Surrepifungium, are supported
by the morphology of the opercula (Figs
6-15), the radulae (Figs
16-25), jaws (Figs 26-35), and
egg-capsules (Figs 36-45). In addition
they are consistent with the host coral-preferences. All Epidendrium
species are associated with cup corals (Scleractinia: Dendrophylliidae)
and all Surrepifungium and Epifungium species
occur with mushroom corals (Scleractinia: Fungiidae) (Gittenberger
& Gittenberger, 2005), with the exception of Epifungium
hartogi (Gittenberger, 2003), which was found to be associated
with bubble corals (Scleractinia: Euphyllidae).
The characters of the opercula, radulae, jaws, and egg-capsules
are more robust to classify the lower rank taxa of Epitoniidae than
are most conchological characters. Therefore, many taxa have been
overlooked until recently. Since 2000, 18 epitoniid species associated
with corals have been described as new to science (Bonfitto and
Sabelli, 2001; Gittenberger et al., 2000; Gittenberger, 2003; Gittenberger
& Gittenberger, 2005) compared to only four such species that
were known before. Three genera were described as new to science
(Gittenberger & Gittenberger, 2005). Most of these species are
specialists associated with only one or a restricted number of coral
hosts. They have large ranges, similar to those of their hosts,
from the Red Sea and the Indian Ocean to the West Pacific. Many
sibling species occur sympatrically, but they are found with different
host coral species. When they do share a host species, they are
not entirely syntopic, occurring at different positions on or below
the host coral (Gittenberger, 2006).
Endoparasitic
gastropods of stony corals: Coralliophilidae
Similar to what was found for the ectoparasitic epitoniid species,
many endoparasitic gastropods of corals, i.e. Leptoconchus
spp. (Gastropoda: Coralliophilidae), can also not be recognized
unequivocally on the basis of their conchology. The intraspecific
variation in shell height/width index is large, shown for example
by the Leptoconchus species found in association with the
mushroom coral Ctenactis crassa (Figs
46-47). This may be the result of phenotypic
plasticity in shell form induced by host coral morphology. Leptoconchus
species lack radulae and have very similar anatomies. Massin and
Dupont (2003) have discussed this poverty of morphological characters
in Leptoconchus species that are associated with mushroom corals,
and described 9 operational taxonomic units with overlapping character
states. These entities can only be distinguished on the basis of
their coral host species. Molecular data show that this group of
parasitic gastropods is even more speciose than Massin and Dupont
(2003) thought. A cryptic, adaptive radiation of 14 coralliophilid
species associated with mushroom corals was unravelled (Gittenberger,
2006). Up to eight species were found sympatrically in the same
geographic area, associated with different scleractinian host species.
Most species can only be characterized on the basis of molecular
and habitat (host preference) data. The species have large ranges
throughout the Indo-West Pacific, obviously depending on the distribution
of their associated host coral species. Most probably there are
many more species in Leptoconchus than hitherto accepted and the
parasite-host relationships are more strict than previously thought.
Conclusions
The combined analyses of molecular, morphological, ecological and
biogeographical data on parasitic gastropods and their coral hosts
collected throughout the Indo-Pacific, show surprisingly speciose,
cryptic, adaptive radiations in the families Epitoniidae and Coralliophilidae.
There appeared to be many more endo- and ectoparasitic gastropod
species associated with stony corals than previously thought on
the basis of shell characters alone. Furthermore, the associations
between these parasites and their host species are far more exclusive
than hitherto perceived. In associations where parasitic species
cannot unequivocally be identified on the basis of their shell morphology,
molecular data (DNA sequences) and host species preferences turned
out to be much more useful for species recognition. For most wentletrap
species the structure of radulae and jaws, the form of the egg-capsules,
and the pattern on the opercula, also appeared to be very valuable
for identifying taxa. For most Leptonchus species no morphological
characters were found to unequivocally characterize them. In the
evolutionary history of gastropod parasites of stony corals sympatric
speciation via host race formation seems to be widespread.
Acknowledgements
I am grateful to Bert W. Hoeksema, Edmund Gittenberger, Merijn Bos,
Pat Colin, Rachel Colin, Hans Ditlev, Mark Erdmann, Jeroen Goud,
Victor de Grund, Bas Kokhoorn, Alfian Noor, Somnuk Patamakanthin,
Somwang Patamakanthin, Bastian, Reijnen, Carlos A. Sánchez,
Niels Schrieken, Frank Swinnen and Nicole de Voogd for their help
in providing information and material used in this study. The research
in Indonesia was sponsored by the Indonesian Institute of Sciences
(LIPI). This research project was supported by WOTRO (grant nr.
W 82-249) with additional funding by KNAW, the Alida Buitendijkfonds,
and the Jan Joost ter Pelkwijkfonds.
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