Snail or Clam? The Curious Case of the Family Juliidae

Everyone knows that clams are mollusks, and sea snails are mollusks as well. The most obvious difference between them lies in their shells. In the common understanding, animals with a single coiled shell that move using a muscular foot belong to the class Gastropoda—the snails and slugs. In contrast, animals with two hinged shells that move using a hatchet-shaped foot are classified as Bivalvia, the clams and their relatives.

Yet within the vast molluscan lineage exists a group that seems to defy this tidy distinction. These unusual creatures combine features of both worlds, transforming themselves into what appears to be a sea snail wearing a pair of clam shells. They belong to the remarkable family Juliidae.

Figure 1. A living Juliidae photographed in a shallow lagoon of Réunion Island in the Indian Ocean.（Image source：Alexandre Laporte， CC BY-SA 4.0 ）

Members of Juliidae are part of the superorder Sacoglossa within the superfamily Oxynooidea. Their bodies are typically green and equipped with a flattened foot, eyes, tentacles, and a radula. At the same time, they possess two shell valves joined by a hinge, giving them the appearance of clams with eyes. Faced with such an animal, one might naturally wonder whether these organisms represent an ancestral form of bivalves. The answer, however, is no. Their resemblance to clams arises purely through convergent evolution. They remain unmistakably gastropods. Evidence for this lies in their anatomy: unlike clams, they lack siphons and retain a well-developed head bearing a radula, eyes, and tentacles.

Figure 2. Nineteenth-century hand-drawn illustration of a Juliidae shell, exterior view.（Image source：George Washington Tryon, Jr.，CC0 1.0 ）

Figure 3. Nineteenth-century hand-drawn illustration of a Juliidae shell, right interior view.（Image source：George Washington Tryon, Jr.，CC0 1.0 ）

Figure 4. Nineteenth-century hand-drawn illustration of a Juliidae shell, left interior view.（Image source：George Washington Tryon, Jr.，CC0 1.0 ）

Compared with their bivalve counterparts, Juliidae appeared relatively late in evolutionary history. Their fossils first appear near the transition between the Paleocene and the Eocene. Early fossil discoveries led researchers to interpret them as bivalves and even to assume that the entire group had long since vanished. That assumption changed dramatically in 1959, when a living member of the family was discovered in Japan on the alga Caulerpa. This species, known in Japanese as タマノミドリガイ and scientifically named Berthelinia limax, reshaped the understanding of both taxonomy and paleontology. After this discovery, additional species of Juliidae were identified across the world. Today, species from nine genera are known.

Figure 5. Juliidae with the alga Caulerpa.（Image source：djnugent，CC0 1.0 ）

Their clam-like shell is only one of their unusual traits. These animals also possess another striking ability—photosynthesis. In the molluscan world, photosynthetic capability is not entirely unprecedented; some mollusks, such as giant clams, maintain photosynthetic partnerships. The strategy used by Juliidae, however, is quite different. Instead of hosting algae directly, they steal chloroplasts from algal cells and store them within their own tissues.

To accomplish this, their planktonic larval stage is extremely brief. The larvae quickly settle onto algae, where they begin feeding. Using their radula, they scrape algal cells and absorb the chloroplasts into their own cells. Once this process is complete, the animal can harness these captured chloroplasts to perform photosynthesis.

The story of Juliidae offers a fascinating glimpse into the evolutionary possibilities of gastropods. Their form challenges simple expectations about what snails and clams should look like. More broadly, they remind us that biological diversity often escapes the boundaries we attempt to impose upon it. Under the persistent influence of evolution, life repeatedly explores pathways that seem almost unimaginable.

Author: Rodrigo

References:

1. MolluscaBase eds. (2021). MolluscaBase. Juliidae E. A. Smith, 1885. Accessed through: World Register of Marine Species at: http://marinespecies.org/aphia.php?p=taxdetails&id=196330 on 2021-06-09.

2. Taxonomic edit history Kawaguti S. & Baba K. (1959). "A preliminary note on a two-valved sacoglossan gastropod, Tamanovalva limax, n. gen., n. sp., from Tamano, Japan". Biological Journal Okayama University 5(3-4): 177–184.