Mutualism of the Month: Solar-powered sea slugs and their photosyntheic algae

Every month we showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is the relationship between a solar-powered sea slug and a group of photosynthetic algae.

The sacoglossan Placida dendritica obtains chloroplasts from some species of algae, which it stores in branches of the digestive system (the green throughout its body. Photo courtesy of Seth Goodnight.

The sacoglossan Placida dendritica obtains chloroplasts from some species of algae, which it stores in branches of the digestive system (the green throughout its body. Photo courtesy of Seth Goodnight.

You may remember Seth’s post on solar-powered sea slugs, particularly the group of sea slugs called sacoglossans. Sacoglossans are herbivorous sea slugs that feed on algae by puncturing the outer wall of cells and sucking the sap out of them. Some of these sacoglossans keep the chloroplasts (organelles of photosynthetic organisms that turn solar energy and carbon dioxide into sugars and oxygen) alive within their own tissue. The live chloroplasts continue to produce sugars and oxygen within their new home. One species of sacoglossan, Elysia chlorotica, can last up to 9 months on just sunlight after taking in chloroplasts!

Recently another species of sea slug, Melibe engeli, was added to the solar-powered sea slug list. Melibe engeli is not a sacoglossan, rather it’s a nudibranch, a distantly-related group of sea slugs. Unlike their sacoglossan cousins, some nudibranchs keep the algae alive instead of removing the chloroplasts from the algae.

Zooxanthellae are a group of photosynthetic algae found within many groups of organisms including corals, sea anemones, jellyfish, clams, and nudibranchs. Photo source.

Zooxanthellae are a group of photosynthetic algae found within many groups of organisms including corals, sea anemones, jellyfish, clams, and nudibranchs. Photo source.

Normally, nudibranchs obtain their algae through their food: soft and hard corals. Many soft and hard corals (animals) contain live symbiotic algae, called zooxanthellae, within their own bodies, which provide them with oxygen and a carbohydrate source (excreted by the algae) in exchange for protection and a nitrogen source (excreted by the animal host). When certain nudibranch species consume coral with zooxanthellae within, they are able to isolate the live algae and move them to special areas within their own body. The zooxanthellae continue to utilize the sunlight, producing sugars, some of which are absorbed by the nudibranch. Nudibranchs get the added benefit of being a more cryptic color, giving them the ability to hide better in plain sight as they absorb sunlight for their symbionts.

What makes the nudibranch M. engeli special is how it obtains zooxanthellae. It doesn’t steal the zooxanthellae from corals, rather it goes out and acquires its own. The genus (scientific group) Melibe has a specialized "oral hood", which they use to catch microscopic animals out of the water. Melibe engeli uses its oral hood to catch the microscopic zooxanthellae out of the water. The algae is brought into the gut, which branches profusely throughout the body. These fine branches (called tubuli) penetrate the whole body, including the foot, muscles, the ovotestis (think ovary and testes combined), and penis. Algae is found throughout these branches as well as in specialized organs called "cisternae". Cisternae are large balloon-shaped areas of the digestive system, which contain a particularly high level of zooxanthellae. Zooxanthellae are located intracellularly; this means that zooxanthellae are actually incorporated between cells of their host, becoming part of the body of the nudibranch – a super-organism if you will. The algae are happy enough to continue to reproduce asexually within the sea slug, maintaining populations for months! 

The nudibranch Melibe engeli catches microscopic algae and then farms them within its digestive system. The algae produces sugars in return for the protection provided by the nudibranch. Notice the oral hood on the left, which the nudibranch uses to catch prey and algae out of the water. All the colored areas within this animal are parts of the digestive system. Photo source.

The nudibranch Melibe engeli catches microscopic algae and then farms them within its digestive system. The algae produces sugars in return for the protection provided by the nudibranch. Notice the oral hood on the left, which the nudibranch uses to catch prey and algae out of the water. All the colored areas within this animal are parts of the digestive system. Photo source.

The relationship comes with some risks. For one, nudibranchs, a delicacy by many fishes’ standards, have to spend time in lighted regions, exposing themselves to predation. The population of zooxanthellae within the nudibranch need to be controlled or oxygen bubbles may start to build up within the slug. These bubbles can pick the slug off the bottom and float them to the surface of the ocean making them easy pickings for their predators. The oxygen itself can be a threat to the slugs, as oxygen in high concentrations becomes highly reactive, damaging cellular structures including DNA! Slugs can control algae populations through at least two mechanisms: dropping cerata (outgrowths on the top of their body), which contain a large amount of the zooxanthellae, or by defecating the algae.

These costs are certainly outweighed by the positive effects of hosting the zooxanthellae as slugs don’t need to feed in order to grow and reproduce. Slugs can live for at least nine months without feeding, all the while producing fertile eggs clutches. Imagine all the time you’d save not eating, all while spending time in the sun!

To read the original scientific article from the Journal of Molluscan Studies click here.

Don’t forget to check out more mutualisms of the month and all the other great articles on FTDM! Stay hungry!

Check out more mutualisms of the month!