Spatial Distribution of Hydrobiid Snails in Salt Marsh
along the Skidaway River in Southeastern Georgia with
Notes on their Larval Trematodes
Oscar J. Pung, C. Brad Grinstead, Kraig Kersten,
and Catherine L. Edenfield
Southeastern Naturalist, Volume 7, Number 4 (2008): 717–728
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22000088 O.J. Pung, CS.BO.U GTrHinEsAteSaTdE, RKN. KNeArTstUeRn,A aLnIdS TC.L. Edenfield 7(4):717–772187
Spatial Distribution of Hydrobiid Snails in Salt Marsh
along the Skidaway River in Southeastern Georgia with
Notes on their Larval Trematodes
Oscar J. Pung1,*, C. Brad Grinstead1, Kraig Kersten2,
and Catherine L. Edenfield3
Abstract - Populations of hydrobiid snails and their larval trematode parasites in
salt marsh along the Skidaway River were studied to determine their distribution.
Additionally, the prevalence of larval trematodes infecting the snails was examined
to investigate definitive host distribution patterns on the Skidaway River and to identify
sites for future studies on second intermediate host susceptibility to trematode
infection. To do so, surface sediment and vegetation were collected at low tide from
0.5-m2 quadrats along 20 vertical transects beginning in high marsh at the forest
edge and salt meadow, passing through high, medium, and low Spartina alternifl ora
zones, and ending in the low marsh at creekbed level. Samples were filtered through
sieves to concentrate snails, which were then counted and identified. Two species
of hydrobiid snails, Spurwinkia salsa (4201 specimens) and Onobops jacksoni (136
specimens) were collected. Hydrobiid snails were found in sediments and on plant
stems throughout the S. alternifl ora zones, and snail density was greatest in the
higher Spartina zones. Sediments from the 3 Spartina zones differed with respect to
percent sand, but not percent silt or clay. Salinity and chlorophyll-a levels did not
differ between the 3 Spartina zones, and there was no relationship between hydrobiid
abundance and the abundance of other snail species. The mean prevalence of trematode
infection in S. salsa and O. jacksoni snails was 5.5% and 7.5%, respectively.
Snails were infected most commonly with either an oculate monostome, possibly the
heterophyid Phagicola diminuta, or 2 types of xiphidiocercariae, one of which likely
includes the microphallid Microphallus turgidus. No infected snails were found in
over half of the collection sites, and the distribution of infected snails was patchy
Hydrobiid snails are common inhabitants of soft substrate tidal marshes
along the coast of the southeastern US (Heard et al. 2002, Hershler
and Thompson 1992). Perhaps as a result of their small size, often only
a few mm in length, hydrobiids are inconspicuous and frequently overlooked.
Hydrobiids snails are herbivores that graze on diatoms and other
microalgae found on the surface of intertidal sediments and vegetation
(Jensen and Siegismund 1980). Despite the fact that these deposit feeders
1Department of Biology, Georgia Southern University, PO Box 8042, Statesboro,
GA 30460. 2Department of Biology, Armstrong Atlantic State University, Savannah,
GA 31419. 3Department of Biology, Berry College, Mount Berry, GA 30149. *Corresponding
author - firstname.lastname@example.org.
718 Southeastern Naturalist Vol. 7, No. 4
are a significant component of intertidal communities, their abundance
and spatial distribution patterns in the extensive salt marshes of southeast
Georgia have not been examined thoroughly. This knowledge may prove
important in future evaluations of the impact of real estate development
and other human activities that now threaten the coastal ecosystems of
Georgia (Kleppel et al. 2006).
Hydrobiid snails are also noteworthy because they serve as first intermediate
hosts for a variety of digenetic trematodes. For example, over
50 kinds of trematode larvae have been described in hydrobiids collected
on the coast of France (DeBlock 1980). These parasites, in turn, utilize
economically and ecologically important crustacean and vertebrate organisms
as second intermediate and definitive hosts. Some of the trematodes
that infect hydrobiids in the southeast have been identified (Font et al.
1984; Heard 1970, 1976; Heard and Overstreet 1983). One of particular
interest to us is Microphallus turgidus Leigh. This parasite is abundant
in a variety of hosts in brackish water marshes along much of the coast
of the southeastern US. Several hydrobiid snail species, including Litterodinops
monroensis Frauenfeld, serve as first intermediate host for
this parasite and grass shrimp, Palaemonetes Heller, are the second intermediate
hosts in coastal marshes (Heard and Overstreet 1983). Many
different crustacean-eating birds and mammals, including Rallus longirostris
Boddaert (Clapper Rail) and Procyon lotor Linnaeus (Raccoon),
are definitive hosts (Leigh 1958, Heard 1970). Work performed in our
laboratory indicates that the geographic distribution of this parasite in
Palaemonetes pugio Holthuis (Daggerblade Grass Shrimp) may be affected
by salinity levels (Pung et al. 2006) and that infected P. pugio are
more susceptible to predation (Kunz and Pung 2004).
The primary goal of the present study was to determine the spatial distribution
of hydrobiid snails along the readily accessible tidal marshes of
the Skidaway River in Georgia and to investigate the possibility that other
biotic and abiotic parameters characteristic of the collection sites would be
of predictive value with respect to snail abundance. Our secondary goal was
to examine the distribution and prevalence of trematode larvae infecting the
snails as a means of examining the distribution patterns of definitive hosts
and to identify potential collection sites for use in future studies concerning
the biology of the parasite M. turgidus.
Hydrobiid snails were collected during low tide in salt marsh along the
Skidaway River adjacent to Skidaway Island and the Isle of Hope southeast
of the city of Savannah, GA from May 2004 through July 2004. The Skidaway
is a subtropical tidal river (typical tidal range is 2–3 m), a portion of
which is dredged for the Intercoastal Waterway. As the tide rises, water fl ows
2008 O.J. Pung, C.B. Grinstead, K. Kersten, and C.L. Edenfield 719
into the Skidaway River from the Savannah River system via the Wilmington
River in the north and from the Ogeechee River via the Vernon River in the
south. The Skidaway River is surrounded by extensive salt marshes dominated
by Spartina alternifl ora Loisel (Salt Marsh Cordgrass).
Hydrobiid snails were collected from sediment and S. alternifl ora stems
along 20 vertical transects (from 31°59'12"N, 81°01'47"W in the north to
31°56'38"N, 81°03’55"W in the south; Fig. 1) through the intertidal zone
beginning in the high marsh and ending in low marsh at creekbed level. At
Figure 1. Hydrobiid snail collecting sites along the Skidaway River southeast of the
city of Savannah, GA. Snails were collected within 0.5-m2 quadrats along vertical
transects extending through the intertidal zone beginning in the high marsh and ending
in low marsh or at creekbed level.
720 Southeastern Naturalist Vol. 7, No. 4
15 of these transects, snails were collected within a 0.5-m2 quadrate in each
of 3 S. alternifl ora zones (high, middle, and low), and at 5 sites, snails were
collected only in the high zone. The high zone was defined as areas close to
the salt meadow or forest edge with firm sediment and short Spartina (mean
stem length = 42 cm; Table 1). The middle Spartina zone consisted of softer
sediment and medium height Spartina (mean stem length = 64 cm). The low
zone was immediately adjacent to tidal creeks and was characterized by tall
Spartina stems (mean stem length = 94 cm) and very soft, loose sediment.
Samples were also collected from salt meadow and creekbed sediments at
5 transects. Standing water temperatures were recorded, and salinity was
measured with a handheld temperature-compensated refractometer (Fisher
Scientific, Atlanta, GA).
Within each quadrat, plant stems and the top 1 cm of sediment were collected.
The length of 10 Spartina stems from each quadrat were measured.
Seawater was used to filter sediments through a series of 21 cm (8 inch)
diameter brass sieves (ASTM E-11 Specification, Fisher Scientific) stacked
in order of decreasing mesh-size order (2, 1.8, 0.71, and 0.6 mm). All vegetation
was agitated in a bucket containing seawater to dislodge snails, and
the water was then poured through sieves. Material collected in the 1.8-,
0.71-, and 0.6-mm sieves was transported to the laboratory in seawater and
examined under a dissecting microscope (10X magnification). Snails were
counted, removed for identification, and examined for trematode infection.
Snail identification was based on shell and verge morphology (Heard et al.
2002, Hershler and Thompson 1992).
Sediment grain size analyses were performed using the protocol described
by Bouyoucos (1962). One sediment sample from each quadrat was
Table 1. Distribution of the hydrobiid snail Spurwinkia salsa (Salt Marsh Hydrobe) in Spartina
alternifl ora (Salt Marsh Cordgrass) marsh along 20 transects on the Skidaway River in Georgia.
Surface sediment chlorophyll-a levels and sediment characteristics in different Spartina zones
are included. Values represent means ± 1 s.d. A total of 4201 S. salsa snails were collected. In
addition, a few specimens of Onobops jacksoni (Fine-lined Hydrobe) (n = 136) were found in
all three Spartina zones.
Spartina alternifl ora zone
High Middle Low
Number of snails/0.5 m2 113.1 ± 116.2 135.1 ± 144.1 27.9 ± 42.3A
Spartina stem length (cm)B 42.0 ± 8.9 64.1 ± 19.5 94.0 ± 13.5
Chlorophyll-a (mg/m2) 94.7 ± 14.3 85.3 ± 14.3 108.1 ±15.0
SandC 70.7 ± 26.4 53.0 ± 33.0 33.8 ± 19.4
Silt 8.1 ± 9.8 12.0 ± 9.6 17.4 ± 7.5
Clay 21.1 ± 16.8 35.1 ± 24.7 48.8 ± 15.7
ASpurwinkia salsa snail density in the low Spartina zone significantly lower than snail density
in high (P = 0.01) and middle Spartina zones (P = 0.01).
BSpartina stem length significantly different from zone to zone (P < 0.0001).
CSignificant difference between Spartina zones with respect to percent sand (P = 0.02).
2008 O.J. Pung, C.B. Grinstead, K. Kersten, and C.L. Edenfield 721
dried, ground using a mortar and pestle, and sifted through a 2-mm sieve. A
50-g portion was mixed with 100 ml of 5% sodium metaphosphate, diluted
with distilled water to a final volume of 300 ml, and shaken for 24 h (125
rotations/min). Samples were then diluted to a volume of 1 liter in a graduated
cylinder and mixed by inversion. Temperature and concentration scale
(ASTM 152H) soil hydrometer (Fisher Scientific) readings were recorded
40 sec and 2 h after inversion and subtracted from control (0.5% sodium
metaphosphate) readings for calculations.
To measure chlorophyll-a levels, triplicate samples of the top 1 cm of
sediment were collected at each site in cut-off 10-cc syringe barrels and
kept on ice in the dark until frozen. Subsequent pigment extraction steps
were performed in a darkened room. Samples were thawed, ground with
a mortar and pestle for 3 min in 90% acetone, transferred to 15-ml tubes,
refrigerated for 16–18 h, and then centrifuged for 5 min at 1200 x g. The
absorbance of the extract was determined in a Spectronic 21D spectrophotometer
(Spectronic Instruments, Inc., Rochester, NY) at 750 nm
and 664 nm before and after acidification with 1 N HCl to account for
phaeopigments (Lorenzen 1967). Equations for chlorophyll-a calculation
were modified for benthic microalgal concentration following Colijn and
To determine the prevalence of larval trematode infection, individual
snails were incubated up to 5 wk at 30 °C in the wells of 24-well tissue
culture plates (Falcon, Becton-Dickinson, Franklin Lakes, NJ) filled with
brackish water (salinity = 23 ppt). Wells were examined twice a week
with an inverted microscope for the presence of cercariae, after which
snails were given fresh seawater and fed microalgae. In some cases, infection
was determined by microscopic examination of individual snails
crushed under a coverslip. Live cercariae were stained with 0.01% neutral
red, examined using a compound microscope, and identified to type
using Heard (1976), Deblock (1980), Heard and Overstreet (1983), and
Ostrowski de Núñez (1993). Specimens were gently heat-fixed with no
coverslip pressure for measurements.
Due to low sample sizes, analyses were not performed on data collected
for Onobops jacksoni Bartsch (Fine-lined Hydrobe). The nonparametric
Kruskal-Wallis test was used to compare numbers of Spurwinkia salsa
Pilsbry (Saltmarsh Hydrobe) snails between marsh zones. Mann-Whitney
U-tests were used to identify pairs of means that were significantly different,
and probability values were adjusted for multiple mean comparisons (Wright
1992). One-way analyses of variance (ANOVA) were used to compare
salinity, S. alternifl ora stem length, and chlorophyll-a levels across marsh
zones, and the Tukey-Kramer honest significant difference test was used to
identify pairs of means that were significantly different. Friedman’s 2-way
nonparametric analyses were used to determine if there were differences in
numbers of S. salsa across sediment types and in parasite prevalence across
722 Southeastern Naturalist Vol. 7, No. 4
marsh zones. The relationship between hydrobiid snail density and salinity
and between hydrobiid density and the density of Nassarius obsoletus Say
(Eastern Mudsnail) and Littoraria irrorata Say (Marsh Periwinkle) were
examined using Spearman’s Rho correlation.
Two species of hydrobiid snail were collected: Spurwinkia salsa
and Onobops jacksoni. Most of the snails collected were S. salsa (4201
specimens, 97% of total), while 136 specimens of O. jacksoni were found.
Both species of snails were found in all 3 Spartina zones. At low tide,
88.5% of the snails were collected from sediment and the remainder from
Spartina stems. The density of S. salsa snails differed between zones
(Kruskal-Wallis H = 10.9, df = 2, P = 0.004). Fewer snails were found in
the low Spartina zone than in either the high zone (U-test: S = 244.5, Z
= 2.6, P = 0.03) or the middle zone (U-test: S = 159.5, Z = -3.0, P = 0.01;
Table 1). No hydrobiid snails were found in salt meadow, and only 1 snail
was found in creekbed sediments (5 creekbed quadrats examined). Chlorophyll-
a levels did not differ between the 3 Spartina zones (ANOVA: df = 2,
34, F-ratio = 0.6, P > 0.5). Spartina zones differed with respect to percent
sand (Friedman’s χ2 = 7.82, df = 2, P = .02), but not percent silt (Friedman’s
χ2 = 3.4, df = 2, P > 0.2) or clay (Friedman’s χ2 = 5, df = 2, P > 0.05)
(Table 1). Salinity, which averaged 29.5 ± 2.4 ppt (range = 29–35 ppt), did
not differ between zones, and there was no relationship between hydrobiid
density and salinity. There was no correlation between S. salsa density in
the Spartina zones and the density of other snails (L. irrorata: rs = 0.32,
P = 0.06; N. obsoletus: rs = 0.09, P = 0.78). Spartina stem lengths differed
from zone to zone (ANOVA: df = 2, 41, F-ratio = 44.2, P < 0.0001).
A total of 1035 S. salsa snails and 40 O. jacksoni snails were examined
for trematode infection. A total of 118 S. salsa snails (11.4%) were trematode-
infected, and parasite prevalence did not differ between zones. Three
O. jacksoni snails (7.5%) were infected. The distribution of infected snails
was patchy; the mean prevalence of infection in S. salsa was 5.5% ± 11.8
and ranged from 0% (at 56% of collection quadrats) to as high as 66% at a
single quadrat. The component community of hydrobiid parasites consisted
of 4 types of larval trematode. Sixty-four S. salsa snails were parasitized
with oculate monostome cercariae (mean prevalence = 2.2% ± 6.6, range =
0–39.4%). These parasites were characterized by a pair of eyespots in the
anterior one-third of the body, oral sucker with lip-like spines, no ventral
sucker, and paired penetration glands in 2 diagonal rows on each side of the
body. The body was lanceolate when extended, broadly so at the anterior,
and pyriform when retracted. The body of heat-fixed specimens measured
70–80 μm long by 42–45 μm wide. The tail was 75–90 μm in length, with a
small posterior spine.
2008 O.J. Pung, C.B. Grinstead, K. Kersten, and C.L. Edenfield 723
Fifty-six S. salsa snails were infected with either of two types of xiphidiocercariae
(hereafter referred to as Type I and Type II). Two O. jacksoni
snails were infected with Type I and one with Type II xiphidiocercariae.
Both types had simple tails and lacked eye spots and ventral suckers. The
Type I xiphidiocercariae (mean prevalence = 1.3% ± 4.7, range = 0–28.7%)
were characterized by a narrow stylet and 4 pairs of penetration glands extending
into the posterior of the parasite. Type II xiphidiocercariae (mean
prevalence = 2.2% ± 4.8, range = 0–21.3%) had a broad stylet, 2 pairs of
anterior penetration cysts and 2 pairs of lighter staining glands extending
into the posterior of the body. The body size of both xiphidiocercariae was
similar: 100–122 μm long by 25–32μm wide. The tail measured 115–133 μm
Additionally, a single S. salsa snail was infected with both the oculate
monostome and Type II xiphidiocercariae, and one specimen of S. salsa was
infected with sanguinicolid trematode larvae.
Spurwinkia salsa was the most abundant snail collected in the Skidaway
River marshes. This hydrobiid is found along the Atlantic coast from Maine
in the north to Cumberland Island, GA in the south (Hershler and Thompson
1992). In Massachusetts and Maine, S. salsa is reported to inhabit various
low-salinity habitats including high-marsh pools, sediments between marsh
grass stems, and channel and creek bottoms (Davis et al. 1982). In contrast,
we collected S. salsa primarily on and amongst Spartina stems and in small,
open mudfl ats within the Spartina zones. We found virtually no hydrobiids
in creekbeds, and the salinity at all of our collection sites was relatively high
(i.e., ≥29 ppt).
The spatial distribution patterns of deposit-feeding snails in softsubstrate
intertidal communities are not fully understood and likely due to
complex interactions between numerous biotic and abiotic factors (Kneib
1984). These include inter- and intraspecific competition (Levinton 1985,
Levinton et al. 1985), predation (Joyce and Weisberg 1986, Rochette and
Dill 2000), salinity (De Francesco and Isla 2003, Fenchel 1975), water
flow (Levinton et al. 1995), food abundance (Drake and Arias 1995,
Levinton 1985), and sediment type and particle size (Bick and Zettler
1994, Forbes and Lopez 1990, Levinton and DeWitt 1989). We collected
hyrdobiids throughout the Spartina zones of the Skidaway River marshes
and found that hydrobiid density is greatest in the high and middle Spartina
zones. Based on our current findings, we are not able to explain this
apparent zonal preference. Chlorophyll-a levels and salinity did not differ
between Spartina zones, and there was no relationship between hydrobiid
density and that of other snails. We did find that sediment composition,
at least with respect to percent sand, varied from zone to zone. Hydrobiid
724 Southeastern Naturalist Vol. 7, No. 4
growth rates may be greater on sand than on smaller size sediments (Forbes
and Lopez 1990), and snail abundance has been positively correlated with
increasing sediment grain size in at least one instance (Bick and Zettler
1994). Consequently, maximal snail growth on the sandier sediments of the
high and middle Spartina zones along the Skidaway might account for the
higher densities of adult snails we observed. However, other investigators
have shown that hydrobiid snails feed fastest on intermediate-size particles
(i.e., particles smaller than sand) that may also have greater diatom abundance
(Levinton and DeWitt 1989). These sorts of analyses are complicated
by the fact that natural sediment consists of a mix of different size-class
particles and the relationship between sediment size and microalgal abundance
is not clear (Cahoon et al. 1999). Finally, since predation can affect
snail abundance and distribution (Joyce and Weisberg 1986, Rochette and
Dill 2000), greater hydrobiid density in the high and middle Spartina zones
could simply reflect the fact that these zones are inundated with water for
less time than the low Spartina zone and adjacent creekbeds and that, as a
result, snails in the higher zones are exposed to swimming predators, such
as crabs and juvenile fish, for shorter periods of time. We have no evidence
to support this hypothesis.
Hydrobiid snails and other gastropods are obligatory first intermediate
hosts for most digenetic trematodes and, as such, are considered keystone
species for these parasites (Esch et al. 2001). Trematodes, in turn, have dynamic
effects on gastropod communities and can infl uence host reproductive
capacity, distribution, and behavior (Curtis 1987). The component community
of larval trematode species in hydrobiid snails in the Skidaway River
marshes comprises at least 4 types of cercariae. This trematode community is
smaller than that observed in other marine systems (Al-Kandari et al. 2000,
Deblock 1980, Kube et al. 2002). The size of these communities varies both
spatially and temporally (Kube et al. 2002) and is infl uenced by multiple factors.
Perhaps primary among those is the presence and abundance of infected
definitive hosts (Esch et al. 2001, Gerard 2001). The definitive hosts for the
parasites we observed include many species of crustacean- and fish-eating
birds and mammals. These kinds of hosts are common in the Skidaway
marshes, but their density is low and their distribution sporadic. As a result,
it is not surprising that the parasite distribution in hydrobiid snails in Skidaway
River marshes is variable.
The Type II xiphidiocercariae observed in S. salsa resemble cercariae
of the microphallid trematodes M. turgidus and Microphallus basodactylophallus
Bridgman as described by Heard and Overstreet (1983). As is true
of most trematode cercariae, they are best identified to species level by experimental
infection of second intermediate hosts. Palaemonetes spp. grass
shrimp are second intermediate hosts of M. turgidus, and Callinectes sapidus
Rathbun (Blue Crab) is a second intermediate host of M. basodactylophallus
2008 O.J. Pung, C.B. Grinstead, K. Kersten, and C.L. Edenfield 725
(Heard and Overstreet 1983). As part of a recent host-susceptibility study,
we infected juvenile P. pugio shrimp with Type II xiphidiocercariae found in
Skidaway River hydrobiids (data not shown). Thus, it is probable that a percentage
of the Type II xiphidiocercariae observed in the present study are M.
turgidus. We did not attempt to identify the Type I xiphidiocercariae. Other
microphallid trematodes reported to infect hydrobiid snails in the southeast
include Maritrema prosthometra Deblock and Heard that utilizes O. jacksoni
and other hydrobiids as first intermediate host, Uca minax LeConte (Redjointed
Fiddler Crab) and other fiddlers as second intermediate hosts, and the
Clapper Rail as definitive host (Heard 1976).
Hydrobiid snails in the southeast are also reported to be infected with
heterophyid trematode larvae. For example, Ascocotyle gemina Font,
Heard, and Overstreet utilizes Litterodinops monroensis Frauenfeld as its
hydrobiid host, cyprinodontid and poeciliid fishes as second intermediate
hosts and the Clapper Rail and other fish-eating birds as definitive host
(Font et al. 1984). The heterophyid we observed in S. salsa snails resembles
Ascocotyle (Phagicola) diminuta Stunkard and Haviland as described
by Ostrowski de Núñez (1993). The reported molluscan hosts of this parasite
include the hydrobiids O. jacksoni and Littoridinops tenuipes Couper
(Henscomb Hydrobe), metacercariae encyst on the gills of brackish water
cyprinodont fishes, and the adults are common in coastal herons, Raccoons,
and Clapper Rails (Heard 1970).
In conclusion, hydrobiid snails can be collected readily from sediments
and plant stems in the easily accessible, higher Spartina zones along the
Skidaway River, but we are unable to explain this distribution pattern. The
mean prevalence of trematode-infected hydrobiids in these localities is low,
and their distribution unpredictable at present. In order to obtain M. turgidus
cercariae for future work, we are investigating ways to infect hydrobiids in
the laboratory and surveying the marsh for locations frequently visited by
potential avian and mammalian definitive hosts.
We thank C. Ray Chandler for assistance with statistical analyses, Risa Cohen for
demonstrating chlorophyll-a and sediment grain size determination, Lance Durden
for reviewing the manuscript, Richard Heard and Robert Hershler for advice on snail
identifications, Edward D. Brown for assistance with sample collection, the anonymous
reviewers and the guest editor for their helpful suggestions and criticisms, and
the Skidaway Institute of Oceanography (SKIO) for permitting access to collection
sites on Skidaway Island. We are grateful to Mike Robinson of SKIO for mapping
our collection sites and for sharing his extensive knowledge of the Skidaway River
and its marshes. This study was supported by a grant to O.J. Pung from the Georgia
Southern Faculty Research Committee and a grant to C.B. Grinstead from the Georgia
Southern University College of Graduate Studies.
726 Southeastern Naturalist Vol. 7, No. 4
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