Sacoglossan sea slugs are marine gastropods recognized for their vibrant colors and distinctive, often leaf-like, body shapes. These small mollusks, ranging from 1 to 3 centimeters, are a type of sea slug. They navigate underwater with a soft, often translucent body, featuring sensory tentacles called rhinophores on their head. Some species also display wing-like extensions called parapodia or cylindrical projections known as cerata on their dorsal surface.
The Unique Ability of Kleptoplasty
Sacoglossan sea slugs possess a unique ability called kleptoplasty, which literally means “stolen plastids.” This process allows certain species to retain functional chloroplasts from the algae they consume. These chloroplasts, known as kleptoplasts, continue to perform photosynthesis within the slug’s cells.
When a sacoglossan feeds on an algal cell, it ingests the cellular contents but selectively digests everything except the chloroplasts. These intact chloroplasts are then incorporated into specialized cells lining the slug’s digestive diverticula, small pockets along the digestive tract. Within these cells, the stolen chloroplasts continue to convert sunlight into sugars, providing the slug with a supplementary energy source.
This physiological adaptation is temporary, as the kleptoplasts eventually degrade over time, requiring the slug to feed again to acquire new ones. The duration of functional chloroplast retention varies among species, ranging from a few days to several months, with some species like Elysia chlorotica maintaining them for up to ten months. Some sacoglossans can even survive for extended periods solely on the photosynthetic products of their acquired plastids, especially when food is scarce.
Diet and Habitat
Sacoglossan sea slugs are specialized marine herbivores, feeding on various species of algae. Their diet often includes siphonous green algae, such as Caulerpa or Vaucheria litorea. To consume these algae, sacoglossans possess a unique radula, a chitinous ribbon of teeth found in most mollusks.
The sacoglossan radula is distinctive, featuring a single row of pointed teeth adapted for piercing the cell walls of algal filaments. Once the cell wall is punctured, the slug uses its pharynx muscles to suck out the internal cellular contents, including the chloroplasts. Worn radular teeth are not expelled but are instead stored and reabsorbed in a ventral sac called the ascus, from which the name Sacoglossa is derived.
These sea slugs inhabit shallow, coastal marine environments worldwide, including coral reefs, seagrass beds, and rocky shores. Their presence is tied to the distribution of their specific algal food sources. Many species are camouflaged among the algae they feed on, their body color often matching the chloroplasts they ingest, which aids in avoiding predators.
Sacoglossan Diversity and Life
The sacoglossan superorder encompasses around 284 recognized species, showcasing diverse forms, sizes, and colors. Some sacoglossans, like those in the families Cylindrobullidae and Juliidae, retain an external shell, while many others are shell-less, often resembling leaves or having ruffled body edges. Their coloration can be green, brown, or red, depending on the type of algae they consume and the chloroplasts they incorporate.
Sacoglossans are slow-moving creatures, relying on camouflage to blend into their algal surroundings. Some species also employ defensive mechanisms, such as secreting distasteful compounds derived from their algal diet. When disturbed, certain sacoglossans can detach body parts, a process known as autotomy, as a means of escape.
These mollusks are simultaneous hermaphrodites, meaning each individual possesses both male and female reproductive organs. Mating involves reciprocal sperm exchange, with individuals facing opposite directions. Sacoglossans lay their eggs in gelatinous masses or coiled strings, often deposited on their algal food source or nearby objects.
In marine ecosystems, sacoglossans play a role as herbivores, influencing algal growth by feeding on specific species. Their specialized diet and unique adaptations, such as kleptoplasty, make them a distinct component of the food web. While they are a food source for some predators, their ability to sequester compounds from algae can also offer a degree of protection.