The buttonsnail, Modulus modulus, is a small marine gastropod of the
family Modulidae. The shell is top-shaped and low-spired, and is wider
than it is high, consisting of 3-4 or 5-6 strongly convex and angulate
whorls. The body whorl is disproportionately larger then the other whorls.
The margin of each whorl has a ridge or keel formed by a prominent spiral
cord and parallel to 3-4 weaker spiral cords. The aperture is nearly round
and the lip is moderately thin and crenulate (serrate-scalloped). The
columella terminates in a deep notch that accommodates the pallial
tentacles of the inhalant siphon. The shell color is usually yellowish
white, splotched with purple or brown, although this is often obscured by
the periostracum and algal epiphytes in living specimens. The colummela
has a purple tinge and the columellar notch has a purple spot. The
operculum is round, thin, and horny (Houbrick 1980, Abbot and Morris 1995).
The living animal is light green to mossy green in overall appearance. The
head is equipped with a short, bilobe-tipped snout and two thin tentacles.
The fully exposed foot is slightly smaller than the shell diameter
Potentially Misidentified Species
The congener Modulus papei also resides in Florida waters, although
it is primarily a rocky intertidal species (USFWS 2007) and is therefore
unlikely to occur in the same habitat as M. modulus.
HABITAT AND DISTRIBUTION
Modulus modulus occurs from North Carolina south to Brazil, and also in Bermuda (Abbot and Morris 1995).
Modulus modulus occurs throughout the IRL system.
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
Modulus modulus is a small gastropod with shell lengths typically
ranging around 10-25 mm in length (Houbrick 1980).
Houbrick (1980) determined that the life cycle of M. modulus from
the Indian River Lagoon in the vicinity of Fort Pierce lasts for
approximately one year.
Virnstein and Curran (1986) note that Modulus modulus was the third
most abundant seagrass invertebrate in IRL Thalassia testudinum
seagrass beds surveyed by these authors. Prager and Halley (1999) indicate
that M. modulus densities can peak sporadically, and describe
population 'blooms' in the seagrass beds of Florida Bay that appear to b
the result of single recruitment events.
Reproduction is sexual. Sexes are separate and fertilization is internal.
In the Indian River Lagoon population studied by Houbrick (1980), mating
occurred in early winter and egg mass deposition took place in the spring.
Females produce cylindrical gelatinous egg masses that are deposited on
Embryonic Modulus modulus exhibit direct development with no
planktonic larval stage. Crawl-away juveniles emerge from egg masses after
approximately three weeks (Houbrick 1980).
The distribution of this species is restricted to warm-temperate and
subtropical/tropical locations, possibly because it is intolerant of colder
Examination of the NOAA NBI collection records suggests Modulus modulus may occupy a narrower
range of salinities than many estuarine species. Collection information
from these records indicate that buttonsnails have been sampled from
salinities ranging from 23-37 ppt.
Buttonsnails are style-bearing, micrograzing herbivores, feeding primarily on diatoms and other seagrass epiphytes (Houbrick 1980).
Haefner (1990) reports that Modulus modulus dominated the diets of
the portunid crab Callinectes ornatus from Mullet Bay, Bermuda.
Walker et al. (2002, after Randall 1967) list a large number of potential
fish predators of M. modulus, including various grunts, wrasses,
blennies, and puffers.
Modulus modulus inhabits shallow vegetated habitats, and is a
prominent faunal component of many marine and estuarine seagrass
communities (Houbrick 1980, Abbot and Morris 1995). Haefner (1990) notes
the species is common on macroalgae in Mullet Bay, Bermuda.
The shells of dead buttonsnail are an ecologically important resource for
large numbers of hermit crabs such as Pagurus maclaughlinae (Tunberg
et al. 1994, Virnstein and Curran 1986).
Abbot RT and PA Morris. 1995. Shells of the Atlantic and Gulf Coasts and
the West Indies. Peterson Field Guides. Houghton Mifflin Company, NY. 350
Haefner PA, Jr. 1990. Natural Diet of Callinectes ornatus
(Brachyura: Portunidae) in Bermuda. Journal of Crustacean Biology
Houbrick RS. 1980. Observations on the anatomy and life history of
Modulus modulus (Prosobranchia: Modulidae). Malacologia 20:117-142.
Randall JE. 1967. Food habits of reef fishes of the West Indies. Pp.
665-847 in: Studies in Tropical Oceanography, No. 5. Institute of Marine
Sciences, University of Miami, Miami.
Tunberg BG, Nelson WG, and G Smith. Population ecology of Pagurus
maclaughlinae Garcia-Gomez (Decapoda: Anomura: Paguridae) in the Indian
River Lagoon, Florida. Journal of Crustacean Biology 14:686-699.
U.S. Fish and Wildlife service (USFWS). South Florida Multi-Species
Recovery Plan - Ecological Communities: Nearshore and Midshelf Reefs.
USFWS South Florida Ecological Service Office. Available online.
Virnstein RW and MC Curran. 1986. Colonization of artificial seagrass
versus time and distance from source. Marine Ecology Progress Series
Walker SE, Parsons-Hubbard K, Powell E, and CE Brett. 2002. Predation on
experimentally deployed molluscan shells from shelf to slope depths in a
tropical carbonate environment. Palaios 17:147-170.