TWO “TONNIDS” OF SEASHELL

These seashells belong to Tonnidae, a small family of shallow-marine gastropods that live today in tropical habitats, in both shallow and relatively deep waters. These animals have medium size to some rather large shells that are sub-spherical with a low spire and a very large, enveloping last whorl having an aperture (= “opening”) that accommodates their very large "foot," which is used for crawling around and for digging. Tonnid shells are rather thin and fragile. The family name of these gastropods is derived from their having a shell shape resembling wine casks (a.k.a “tuns.”).

Tonnids are carnivores that live semi-faunally (at or near the ocean-floor) in soft-substrates, in both shallow and relatively deep ocean waters. During the day, these animals burrow into the sandy bottoms. At night, they emerge and feed on crustaceans, sea cucumbers (they eat them whole!), and bivalves. Some of the larger tonnid species also capture fish by using their expandable proboscis (mouth area) to also swallow them whole.

The classification (via Wikipideia) of tonnids is:

Kingdom Animalia

Phylum Mollusca

Class Gastropoda

Order Littorinimorpha

Superfamily Tonnoida

Family Tonnidae

Genus Tonna

Two representative large-sized examples are shown below:  

                                                                                     

                                                                 Tonna galea [a.k.a. “the giant tun) 

This species, whose thin shell is characterized by brown bands and yellow/brownish spots, lives in shallow, warm seas of the Caribbean, Atlantic, Mediterranean, and Indo-Pacific waters. It is also found in North Carolina, Argentina, Portugal, and Nambia. It prefers to live on offshore reefs, as well as bays and coves, on rock, sand, or shelly bottoms. Dimensions of illustrated shell: 17 cm, width 11.5 cm. [Note: this species is used as food by some humans].

                                                                                               

                                                              Tonna tesselata

This rather common species, whose thin shell is characterized by white bands with yellow/brownish spots, lives in shallow waters (e.g., offshore reefs, or bays and coves: on rocks in South Africa and the Western Pacific. Dimensions of illustrated shell: height 14 cm, width 12 cm.

Cited Referenences

Mollusca: The Southern Synthesis. Part B (of a two-volume set). Fauna of Australia Volume 5, Part B, CSIRO Publishing, Melourne, (see pp. 802-803). 

Wikipedia. 2025.