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Species Name: 
Modulus modulus Linnaeus, 1758

Common Name(s):
Buttonsnail, Atlantic Modulus

Synonymy:
Modulus filosus Helbling, 1779
Modulus perlatus Gmelin, 1791
Modulus pisum Morch, 1876
Modulus tasmanica Tension-woods, 1877

Species Description:
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 (Houbrick 1980).

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.

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Modulus modulus occurs from North Carolina south to Brazil, and also in Bermuda (Abbot and Morris 1995).

IRL Distribution:
Modulus modulus occurs throughout the IRL system.

III. 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.

Abundance:
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:
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 seagrass surfaces.

Embryology:
Embryonic Modulus modulus exhibit direct development with no planktonic larval stage. Crawl-away juveniles emerge from egg masses after approximately three weeks (Houbrick 1980).