Development of Trimusculus, a limpet-like marine snail

Trimusculus reticulatus, or the button-snail, is an unusual marine gastropod that looks like a limpet (i.e. a member of the Patellogastropoda) but is not. It is, in fact, a pulmonate gastropod (it has a lung) - a relative of the common garden snail. Pulmonate species are generally terrestrial or live in fresh water, but some are marine like T. reticulatus. The adults of T. reticulatus are found in the high intertidal underneath the overhangs of boulders or on the roofs of sea caves.

I collected the specimen pictured here from Strawberry Hill, Oregon. As shown, the shell of adult T. reticulatus is about the size of a penny.  The shell is shallow and uncoiled, like that of a true limpet, but unlike shells of most other gastropods.

T. reticulatus lays its eggs in petal-like gelatinous masses around itself (Johnson 1968). These “petals” were detached during collection. Each “petal” is about 5-10 mm in diameter and contains dozens of embryos; the picture has a 5 mm scale bar. The color is indicative of “petal’s” age.  The whitish masses contain embryos in earlier stages of development. In light brown masses the veliger larvae are about to hatch.  The “petals” pictured here were produced sequentially by a single individual.

This is an embryo from one of the whitish ”petals”.  Note the two polar bodies (at four o'clock) trapped inside the oval egg capsule.  Polar bodies mark the animal pole of the embryo.  This embryo is gastrulating (forming the primary gut). The indentation at the opposite (vegetal) pole is the blastopore – the opening of the primary gut, which will become the mouth of the larva.

This is a hatched veliger larva.  Unlike the adult, the larva has a typical coiled shell – a vestige from the evolutionary past. One can also see the velum – the ciliated larval appendage used for swimming – pulled into the shell, the foot with an operculum, and the statocyst – a paired larval balance organ – that looks like a little marble inside a spherical capsule.

Johnson, K.J. (1968) Studies on the feeding, movement, and respiration of the pulmonate limpet Trimusculus (Gadinia) reticulatus (Sowerby) with notes on general morphology, egg masses, and veliger larva. Unpubl. student paper (abstract). Marine Science Center, Newport, Oregon