Elysia chlorotica

Elysia chlorotica is a species of sea slug, also known as the eastern emerald elysia, capable of performing photosynthesis by retaining chloroplasts obtained from the alga Vaucheria litorea (alg) within cells of its digestive system. Belonging to the order Sacoglossa (sap-sucking mollusks), this species can survive for months on sunlight alone, even in the absence of food, thanks to the rare ability among animals known as kleptoplasty (chloroplast theft). It inhabits salt marshes and shallow creek beds along the eastern coast of the United States, ranging from Nova Scotia (Canada) to southern Florida.

Elysia Chlorotica (Generated by Artificial Intelligence)

Physical Characteristics

Elysia chlorotica undergoes significant morphological and color changes throughout its life cycle. Development begins with the veliger larva, a free-swimming planktonic stage equipped with a ciliated velum and a shell. After metamorphosis, juvenile individuals that have not yet fed on algae are brown and exhibit reddish pigment spots on their ventral surfaces.

Adults acquire a bright green color after feeding on Vaucheria litorea and incorporating its chloroplasts. These chloroplasts are distributed throughout the animal’s extensive digestive system diverticula (extensions). Adults typically measure 20 to 30 mm in length, though specimens up to 60 mm have been documented.

The adult form resembles a leaf due to broad, wing-like extensions called parapodia on either side of the body. These structures provide camouflage and optimize light absorption by increasing surface area for photosynthesis. When chloroplasts lose functionality or are shed, the slug loses its green color and turns gray.

Life Cycle and Development

The species’ life cycle lasts approximately 11 months and ends each year in a synchronized mass mortality event. Developmental stages are as follows:

1. Larval Stage: Veliger larvae hatching from eggs are free-swimming and planktonic. At this stage, they do not carry plastids.
2. Metamorphosis: Larvae must come into contact with Vaucheria litorea to undergo metamorphosis into juveniles. Metamorphosis occurs within 1 to 2 days in the presence of the alga, during which the larvae shed their shells.
3. Juvenile and Adult Stage: Immediately after metamorphosis, individuals begin feeding on algae. After approximately 14 days of feeding and continuous contact with the alga, permanent kleptoplasty (plastid retention) begins, and the animal assumes its leaf-like adult form.
4. Reproduction and Death: Hermaphroditic individuals reproduce once per year in early spring. Shortly after laying egg masses, they die en masse, both in natural habitats and under laboratory conditions. Researchers have suggested that these deaths may be caused by a viral agent.

Elysia Chlorotica Life Cycle (Generated by Artificial Intelligence)

Feeding and Kleptoplasty

Elysia chlorotica belongs to the group of “sap-sucking” sea snails mollusks (Sacoglossa). Its diet is primarily based on the alga Vaucheria litorea, though it may occasionally feed on Vaucheria compacta. The slug uses its radula (toothed tongue) to pierce algal cell walls and suck out cellular contents.

During this process, the algal cytoplasm and nucleus are digested, while the chloroplasts are intact and taken up by digestive epithelial cells via phagocytosis. These chloroplasts remain functional within the animal’s cytoplasm for months—up to 10 to 12 months—continuing to perform photosynthesis. During this period, the slug can survive solely on light and carbon dioxide without ingesting external food.

Genetic and Molecular Adaptations

How this symbiotic relationship is maintained remains a subject of intense scientific research. The ability of chloroplasts to continue protein synthesis and function without the algal nucleus raises the question of whether the necessary genes are present in the slug’s genome.

• Horizontal Gene Transfer (HGT): Some studies have demonstrated that photosynthesis-related genes from the algal nucleus—such as psbO and psbA—have been transferred horizontally into the slug’s genome. For instance, the psbO gene, critical for the stability of photosystem II, has been detected in both adult tissues and eggs of the slug.
• Protein Synthesis: Experiments on starved slugs have shown that chloroplast proteins such as D1, D2, and CP43 continue to be synthesized de novo.
• Transcriptomic Response: Upon encountering algae and acquiring chloroplasts, the slug exhibits marked changes in the expression of genes related to the immune system and oxidative stress. An initial increase in oxidative stress response is observed during first contact, but this response becomes balanced as the symbiosis is established.

Habitat and Distribution

Elysia chlorotica is a euryhaline osmokonformer with high tolerance to salinity. It can survive in salinity levels ranging from nearly freshwater (~24 mosm) to highly saline water (~2422 mosm). Its natural habitat consists of salt marsh pools and shallow creeks less than 0.5 meters deep, typically found near its primary food source, Vaucheria litorea.

Reproduction

Elysia chlorotica is a simultaneous hermaphrodite, possessing both male and female reproductive organs. Although capable of self-fertilization, cross-fertilization (mating with another individual) is preferred. During mating, sperm is injected directly into the body surface via hypodermic insemination. Fertilized eggs are laid in long mucus strands and hatch after approximately one week. No parental care is observed.