An animal that can photosynthesize genuinely exists in our oceans. This sea slug defies the traditional understanding of animal biology by harnessing sunlight to produce its own food. Its ability blurs the lines between the animal and plant kingdoms, presenting an example of biological adaptation.
Unveiling the Photosynthetic Sea Slug
These creatures are sacoglossan sea slugs, marine gastropod molluscs. They are often mistaken for nudibranchs due to their similar appearance, but sacoglossans are different, primarily in their feeding habits. A well-known example is Elysia chlorotica, commonly called the eastern emerald elysia.
Elysia chlorotica is typically small to medium-sized, ranging from 20 to 30 millimeters in length, though some individuals can grow up to 60 millimeters. Juveniles are initially brown with red pigment spots, but they transform into a bright green as adults after feeding on algae. This green coloration comes from the chloroplasts they acquire, which are distributed throughout their extensively branched gut. These slugs inhabit shallow marine environments, such as salt and tidal marshes, creeks, and pools, usually at depths less than 0.5 meters. Their habitat is closely tied to their diet, which consists almost exclusively of the intertidal alga Vaucheria litorea.
How Sea Slugs Steal Photosynthesis
The sea slug’s ability to photosynthesize stems from a process called “kleptoplasty,” named from the Greek word for thief. When sacoglossan sea slugs feed on algae, they selectively retain chloroplasts, the organelles within plant and algal cells responsible for photosynthesis. The slug punctures the algal cell wall with its radula, a specialized scraping structure, and then sucks out the contents, much like using a straw.
These kleptoplasts are integrated into the slug’s own digestive cells, filling the extensively branched digestive tubules of the slug, visible as a branching pattern resembling leaf venation. Once incorporated, these kleptoplasts continue to function, performing photosynthesis and producing sugars that can provide energy for the slug. While many kleptoplastic organisms need to replace their plastids periodically, Elysia chlorotica is unique. Its chloroplasts remain photosynthetically active for an extended duration, with some individuals sustaining them for up to a year without further feeding on algae.
Why This Ability is Extraordinary
A multicellular animal performing photosynthesis is rare in the animal kingdom. While some animals engage in symbiotic relationships with whole algae, the sea slug’s direct retention and utilization of chloroplasts is a distinct adaptation. This challenges the traditional boundaries between plant and animal biology, as animals typically obtain energy by consuming other organisms. The ability to convert light energy into chemical energy, usually exclusive to plants, algae, and certain bacteria, makes these sea slugs unusual.
For Elysia chlorotica, the long-term maintenance of these foreign organelles was once thought to involve horizontal gene transfer, where the slug incorporates specific genes from the algal nucleus into its own genome. This genetic integration would theoretically allow the slug to produce proteins for maintaining and repairing the stolen chloroplasts. However, recent genomic analyses of Elysia chlorotica egg DNA have found no evidence for extensive horizontal gene transfer into the slug’s germline that would fully explain the long-term functioning of these chloroplasts. The mechanisms allowing these kleptoplasts to remain active for such extended periods within the slug’s cells are still being investigated.