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Shell-color polymorphismof intertidalgastropods in Chuuk State, Federated States of Micronesia
南太平洋海域調査研究報告
No.53（201３年２月）
OCCASIONAL PAPERS No.53（February 2013）

Shell-color polymorphism of intertidal gastropods in Chuuk State,Federated States of Micronesia
KAWAI Kei
Research Center for the Pacific Island, Kagoshima University
Abstract
I investigated the relationship between shell color of Nerita plicata gastropods
and habitat background color in Piis and Romanum Islands, Chuuk State, Federated
States of Micronesia. The brightness of habitat background color ranged from 10 to80, whereas the brightness of shell color varied from 40 to 98. The shell color wasusually brighter than the habitat background color, suggesting that N. plicata inhabits
backgrounds that are darker than its shell color. Only snails with brighter shell colors
inhabited brighter backgrounds. By contrast, darker backgrounds were inhabited bysnails with several types of shell color. My findings indicate that N. plicata may adapt
to different environments by means of shell-color polymorphism.
Keyword: background, polymorphism, shell color

Introduction
Color and color-pattern polymorphism within animal species may be attributed
to different modes of selection, such as selection for crypsis (e.g., BYERS 1990), sexualselection (e.g., ENDLER 1983), climatic selection (e.g., MIURA et al. 2007), or frequencydependent selection (e.g., O LENDORF et al. 2006). Intertidal gastropods live undervariable temperature and salinity regimes, and face changing wave action, habitat
type, prey type, desiccation and predation risks, and other environmental factors(HUGHES 1986; LITTLE and KITCHING 1996). Nerita plicata is a gastropod distributed
within the reef region of the upper intertidal zone of the Indo-Pacific Ocean, and it is
known to exhibit shell-color polymorphism (ABBOTT and DANCE 1985; NEVILLE 2003).
High intertidal neritid species, including N. plicata, are known to be better temperature
regulators than low intertidal species (VERMEIJ 1971a). In addition, N. plicata changes
the direction of its shell to face the sun at the time of emersion (WARBURTON 1973).
These behaviors may help N. plicata adapt to changing temperatures in the upperintertidal zone. On the other hand, visual selection by predators may underlie shellcolor polymorphism in N. plicata (BYERS 1990; McKILLUP and McKILLUP 2002).
The species inhabits substrates of varying background color, including basalt rocks,
limestone, and beach rocks, in the upper intertidal regions of the tropical Pacific Ocean(VERMEIJ 1971b).
In the present study, I investigated the relationship between shell color of N. plicataand habitat background color in Piis and Romanum Islands, Chuuk State, Federated
States of Micronesia.

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KAWAI Kei

Materials and Methods
This study was conducted in August 2011, in Piis and Romanum Islands, Chuuk
State, Federated States of Micronesia.
The shell color and habitat background color of each snail were recorded using adigital camera. The shell color and habitat background color varied according to timeand weather conditions. Therefore, to adjust the quality of the photographs, standard
colors (black and white substratum) were also photographed on each occasion. The
colored photographs were adjusted to monochrome color using Photo Shop. For eachshell area and habitat background area, the brightness of 3 points was calculated byPhotoshop, and each value was adjusted using following formula:
Adjusted value = (sample brightness - standard black brightness) / (standard whitebrightness - standard black brightness) × 100
A value of 0 indicated that the color was fully black; a value of 100 indicated thatthe color was fully white.
Results
The brightness of habitat background color ranged from 10 to 80, whereas thebrightness of shell color varied from 40 to 98 (Fig. 1). The shell color was usuallybrighter than the habitat background color. Only snails with brighter shells inhabitedbrighter backgrounds. By contrast, darker backgrounds were inhabited by snails withseveral types of shell color.

Fig. 1 R elationship between brightness of shell color and brightness of habitat
background. A value of 0 indicates that the color is fully black; a value of 100
indicates that the color is fully white.

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Shell-color polymorphism of intertidal gastropods in Chuuk State, Federated States of Micronesia

Discussion
In the present study, only snails with brighter shells inhabited brighter
backgrounds. By contrast, darker backgrounds were inhabited by snails withseveral types of shell color. This finding indicates that N. plicata exhibits shell-color
polymorphism, and that this polymorphism is strongly related to habitat backgroundcolor.
Temperature can greatly affect the metabolism of gastropods (e.g., HUGHES
1986). Metabolic activities may be accelerated if the body temperature increases
within a range that does not impede normal physiology. The difference in bodytemperature between N. plicata individuals living in beach rock (white-colored background)and basalt rock (dark-colored background) environments was 1-2 ℃ (Kawai, personalcommunication). Similar differences in body temperature have been recorded
between banded and unbanded morphs in Cepaea and other snail species (JONES 1973;RICHARDSON 1974; HEATH 1975). When the body temperature of the land snail C.
hortensis was increased from 25 ℃ to 26 ℃ , oxygen consumption increased from 2.6%
to 4.5% (STEIGEN 1979). This finding indicates that a small temperature increase may
have a relatively small effect on physiology in the short term. However, a long-term
temperature differential may significantly influence rates of growth, reproduction,mortality, and other physiological conditions.
Many other environmental factors affect the frequencies of shell-color morphs
in gastropods. Juveniles of a muricid snail, Concholepas concholepas, exhibit shellcolor plasticity depending on the coloration of prey species (M ANRÍQUEZ et al. 2009).
Frequencies of shell-color morph may depend on parasites and predation (BYERS
1990; MCKILLUP and MCKILLUP 2002), with environmental gradients in temperature,salinity, and wave action frequently affecting the evolution of color polymorphisms(ETTER 1988; SOKOLOVA and BERGER 2000; MIURA et al. 2007). Shell-color polymorphism
in some gastropods results from complex interactions between selective forces (JONESet al. 1977); the observed shell-color polymorphism of N. plicata may be derived fromcomplex interactions between several such selective forces.
N. plicata is thought to have a comparatively long, free-swimming larval stage(KANO 2006), suggesting that the genetic differentiation underlying the observed shell
variation in proximate populations is not large. In the present study, I observed similar
trends in the frequency distribution of shell-color morphs between Piis and Romanumislands (Figure 2). However, the frequency of shell color variation differed according
to habitat background brightness. This finding implies that genetic differentiation may
contribute to shell-color polymorphism in N. plicata populations. To elucidate suchgenetic influences, I intend to conduct further detailed analyses using microsatellite
DNA markers.
Acknowledgements
I would like to thank Mike and the staff of the Chuuk State Government for theirkind support.
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