Nudibranchs: The Sea’s Plant-Animal Hybrids

Nudibranchs are among the most fascinating and diverse creatures in the ocean. These soft-bodied mollusks, also known as sea slugs, come in a dazzling array of shapes, colors, and sizes. But what makes them even more remarkable is their ability to blur the line between plants and animals. Some nudibranchs can take up chloroplasts, the organelles that enable plants to convert light into energy, and use them to perform photosynthesis. Others can absorb cells and components from other organisms, such as algae, sponges, or jellyfish, and incorporate them into their own bodies. These abilities give nudibranchs an edge in survival and adaptation, but also raise intriguing questions about the nature and evolution of life.

How Nudibranchs Perform Photosynthesis

Photosynthesis is the process by which plants and some other organisms use light to produce organic molecules, such as sugars, that serve as their food. Photosynthesis requires chloroplasts, which are specialized structures that contain chlorophyll, the pigment that gives plants their green color and captures light energy. Chloroplasts are found only in plant cells and some algae, but not in animal cells. So how can nudibranchs, which are animals, perform photosynthesis?

The answer is that some nudibranchs can take up chloroplasts from the algae that they eat and store them in their own cells. This process is called kleptoplasty, which means “stealing plastids”. Plastids are a group of organelles that include chloroplasts and other types that are involved in different metabolic functions. By stealing plastids from their food, nudibranchs can benefit from the extra energy that photosynthesis provides, especially when food is scarce or when they need to grow or reproduce. Some nudibranchs can keep the stolen plastids functional for weeks or even months, while others can only use them for a few days.

However, kleptoplasty is not a simple matter of swallowing and storing plastids. Nudibranchs have to overcome several challenges to make it work. First, they have to avoid digesting the plastids along with the rest of their food. To do this, they have specialized cells in their digestive system that can recognize and protect the plastids from being broken down by enzymes. Second, they have to maintain the plastids in a suitable environment within their own cells. This means providing them with enough light, water, carbon dioxide, and other nutrients that they need for photosynthesis. Third, they have to prevent the plastids from being attacked by their own immune system, which may see them as foreign invaders. To do this, they have to either hide the plastids from their immune cells or suppress their immune response.

These challenges suggest that kleptoplasty is not a random or accidental phenomenon, but a result of a long and complex evolutionary history. Nudibranchs that can perform kleptoplasty belong to a subgroup called sacoglossans, which have a distinctive feeding habit. They use a sharp structure called a radula to pierce the cell walls of algae and suck out their contents. This allows them to selectively ingest the plastids and avoid the rest of the algal cell. Sacoglossans have evolved this feeding habit over millions of years, and along with it, the ability to perform kleptoplasty. However, not all sacoglossans can perform kleptoplasty, and not all kleptoplasts are equally efficient. This suggests that there is a lot of variation and experimentation in this process, and that it is still evolving.

How Nudibranchs Steal Genes from Other Organisms

Photosynthesis is not the only way that nudibranchs can benefit from other organisms. Some nudibranchs can also take up cells and components from other animals, such as sponges, corals, or jellyfish, and use them for their own advantage. This process is called kleptocnidy, which means “stealing stinging cells”. Stinging cells, also known as nematocysts, are specialized structures that contain venom and a harpoon-like device that can inject it into prey or predators. Nematocysts are found only in cnidarians, a group of animals that include jellyfish, corals, anemones, and hydra. Nematocysts are one of the most complex and sophisticated weapons in nature, and they are very effective in defense and offense. So how can nudibranchs, which are not cnidarians, use nematocysts?

The answer is that some nudibranchs can take up nematocysts from the cnidarians that they eat and store them in their own cells. This process is similar to kleptoplasty, but with some differences. Nudibranchs that can perform kleptocnidy belong to a subgroup called aeolids, which have a distinctive body feature. They have numerous projections on their back called cerata, which are extensions of their digestive system. The cerata are where the nudibranchs store the nematocysts that they acquire from their food. By doing so, they can use the nematocysts for their own defense, and deter predators from attacking them. Some nudibranchs can also use the nematocysts for offense, and inject venom into their prey or rivals.

However, kleptocnidy is not a simple matter of swallowing and storing nematocysts. Nudibranchs have to overcome several challenges to make it work. First, they have to avoid triggering the nematocysts when they eat them. To do this, they have to either secrete mucus or chemicals that can inhibit the nematocysts from firing, or use their radula to carefully remove the nematocysts from the cnidarian tissue. Second, they have to transport the nematocysts from their mouth to their cerata without damaging them. To do this, they have specialized cells in their digestive system that can recognize and protect the nematocysts from being digested or destroyed. Third, they have to activate the nematocysts when they need them. To do this, they have to either stimulate them with a nerve signal or a chemical signal, or rely on the external stimuli that trigger the nematocysts.

These challenges suggest that kleptocnidy is not a random or accidental phenomenon, but a result of a long and complex evolutionary history. Nudibranchs that can perform kleptocnidy belong to a subgroup called aeolids, which have a distinctive phylogenetic origin. Aeolids are derived from a group of sea slugs that have lost their shells and evolved the cerata as a replacement. The cerata have multiple functions, such as respiration, digestion, and defense. The ability to perform kleptocnidy is one of the adaptations that aeolids have developed to enhance their defense. However, not all aeolids can perform kleptocnidy, and not all nematocysts are equally useful. This suggests that there is a lot of variation and experimentation in this process, and that it is still evolving.

How Nudibranchs Challenge Our Understanding of Life

Nudibranchs are not only beautiful and diverse, but also fascinating and mysterious. Their ability to perform kleptoplasty and kleptocnidy challenges our conventional understanding of life and its boundaries. How can animals perform photosynthesis, a process that is typically associated with plants? How can animals use the cells and components of other animals, without being rejected or harmed by them? How can animals acquire new traits and functions, without changing their own genes?

These questions have intrigued scientists and inspired new research in biology and evolutionary theory. Some of the topics that nudibranchs have shed light on include:

• Horizontal gene transfer: This is the process by which organisms can exchange genetic material with other organisms, without being related to them or reproducing with them. Horizontal gene transfer is common among bacteria, but rare among multicellular organisms. However, some nudibranchs have been found to contain genes from algae or cnidarians, which they have acquired through kleptoplasty or kleptocnidy. These genes may help the nudibranchs to maintain or activate the plastids or nematocysts that they have stolen, or to produce chemicals that protect them from predators or parasites. Horizontal gene transfer may be a way for nudibranchs to enhance their adaptation and diversification, without relying on mutations or recombination.

• Endosymbiosis: This is the process by which one organism lives inside another organism, and forms a mutually beneficial relationship with it. Endosymbiosis is the origin of plastids and mitochondria, which are organelles that were once independent bacteria, but became incorporated into the cells of eukaryotes, the group of organisms that include plants, animals, and fungi. Endosymbiosis is a major event in the evolution of life, as it enabled the emergence of complex and multicellular organisms. However, endosymbiosis is not a one-time event, but a continuous and dynamic process. Some nudibranchs may represent a form of endosymbiosis, as they harbor plastids or nematocysts that are still functional and independent, but also dependent on the nudibranchs for protection and transport. Endosymbiosis may be a way for nudibranchs to acquire new functions and traits, without changing their own genome or morphology.

Why Nudibranchs Matter for Science and Society

Nudibranchs are not only fascinating creatures, but also valuable sources of scientific and social inspiration. Their ability to perform kleptoplasty and kleptocnidy has implications for various fields and applications, such as:

• Biotechnology: Nudibranchs may provide novel insights and tools for biotechnology, which is the use of biological systems or processes for industrial or medical purposes. For example, nudibranchs may help us to understand how to engineer or manipulate plastids or nematocysts for biofuel production, drug delivery, or gene therapy. Nudibranchs may also help us to discover new compounds or molecules that have potential uses in medicine, agriculture, or cosmetics. Nudibranchs are known to produce a variety of chemicals, such as toxins, pigments, or pheromones, that have diverse biological activities and functions. Some of these chemicals may be derived from the plastids or nematocysts that they have stolen, or from the genes that they have acquired. Biotechnology may be a way to harness the power and diversity of nudibranchs for human benefit and innovation.

• Ecology: Nudibranchs may provide important clues and indicators for ecology, which is the study of the interactions and relationships between organisms and their environment. For example, nudibranchs may help us to monitor and assess the health and status of coral reefs, which are among the most diverse and productive ecosystems on Earth, but also among the most threatened by climate change, pollution, and overfishing. Nudibranchs are closely associated with coral reefs, as they feed on and live among the corals and other reef organisms. Nudibranchs may reflect the changes and impacts that affect the reef, such as bleaching, disease, or invasion. Nudibranchs may also help us to understand and conserve the biodiversity and resilience of coral reefs, as they represent a unique and diverse group of organisms that have adapted to various environmental conditions and challenges. Ecology may be a way to appreciate and protect the beauty and importance of nudibranchs and their habitats.

• Education: Nudibranchs may provide inspiring and engaging examples and models for education, which is the process of acquiring and sharing knowledge and skills. For example, nudibranchs may help us to teach and learn about various topics and concepts in biology, such as photosynthesis, endosymbiosis, horizontal gene transfer, evolution, adaptation, and diversity. Nudibranchs may also help us to stimulate and foster curiosity, creativity, and critical thinking, as they challenge and expand our understanding and imagination of life and its possibilities. Nudibranchs may also help us to develop and promote awareness, appreciation, and responsibility for the natural world and its wonders. Education may be a way to celebrate and share the fascination and mystery of nudibranchs and their stories.

Nudibranchs are the sea’s plant-animal hybrids, and they are amazing. They can perform photosynthesis and steal genes from other organisms, and they can inspire us in science and society. They are more than just colorful and cute sea slugs, they are living wonders and treasures. They deserve our attention and admiration, and they may have more secrets and surprises to reveal. Nudibranchs are the sea’s plant-animal hybrids, and they matter.