Wildlife enthusiasts within the diving community are treated to a huge array of different forms of life, specialised to thrive within the different ecosystems they inhabit. Immediate focus is always drawn to the large and to the majestic: popular marine icons such as the sea turtle, the manta ray, the dolphin, and recently the world is even opening its collective heart to the pinnacle predator, the shark.
However, recently we have seen growing fascination with hidden, cryptic, often transparent gems of the marine world. The inhabitants of the weird and wonderful micro world are beginning to catch the public eye and the diversity that lies within.
Particular celebrities of this new and relatively unexplored world are the colourful and charismatic nudibranchs, a term which is commonly used (incorrectly) to include all sea slugs. As these critters grow in popularity and photographers continue to capture their almost synthetic looking colouration and texture, so does our knowledge of what they are and what they do in our marine world, which turns out to be almost as diverse as the estimated 7000 different species that exist just within the Indo-Pacific.
Sea slugs, like all slugs and snails of the world, are part of a taxonomic group known as gastropods which are in turn part of a much larger collection of soft bodied invertebrate organisms grouped under the phylum ‘Mollusca’. (Read more about marine invertebrates and their taxonomic classification here). The marine world hosts an incredible variety of gastropods, and those that are considered sea slugs are grouped under the term ‘opisthobranch’. This term has recently been questioned as a valid taxonomic term but we will continue its use until an alternative is proposed.
Opisthobranchs are gastropods that have evolved to live in almost every marine habitat we know of, from the waters of the Antarctic, to the incredible diversity we see on equatorial coral reefs. They have been found around every country in the world that has a coastline and more are being discovered every year.
Opisthobranchs are hermaphroditic (with reproductive organs of both genders in their anatomy). They feed on organisms that have natural defenses such as stinging cells or toxins and incorporate those defenses into their own bodies, creating their own protection. Most species have evolved to feed on very few, very specific prey species such as specific types of coral or toxic sea sponges. Some species have evolved to feed on other species of slug and then absorb the prey chemical defences into their own arsenal.
IMPORTANCE AND RESEARCH
The majority of research conducted on sea slugs involves either Taxonomic research or biomedical research. The important role of ecological research has yet to be a dominant output of sea slug science, but in recent years an increasing effort to expand our understanding has lead to some interesting ecological discoveries; This includes the potential use of monitoring sea slug ranges in mapping climate change impacts in the North American region, the potential role of sea slug aggregations in nutrient and bio-control of certain organisms, and recent work at our program involving the potential role of sea slugs as food for some large corals. In early 2015 the NHRCP team discovered a mushroom coral consuming a sacoglossan sea slug and further research on the relationship has yielded interesting results. However, though there will be some time before this is completed, the initial observation paper can be found here and read about here.
Biomedical research conducted on sea slugs stems from their complex chemical make up and adaptations. Their deterrent chemistry has resulted in the production of rare or novel chemical compounds that have been found to possess incredibly effective properties in certain fields of medicine. These include anti-microbial and anti-parasitic traits, along with promising compounds to aid against the everlong fight against cancer. Most of these discoveries are still awaiting human trials. Additionally a number of sea slug species, especially some of the larger species such as pleurobranchs and sea hares, have played an important role in neurobiological research due to the relative size of many neurons and neural ganglia, which is useful in both neurological studies and teaching purposes.
Unlike thousands of other species of gastropod that evolved to live on land and fresh water environments, sea slugs have evolved in a completely marine habitat. This means sea slugs have had millions of years to adapt and diversify into the thousands of ecological niches found in marine environments throughout the globe. However due to this expansive distribution and diversity, the classification of sea slug species has been turbulent for many decades with relationships and a hierarchical structure being more complex and convoluted than can be easily imagined. For this reason, the taxonomy of sea slugs is in turmoil and now the role of molecular technology in species identification promises to both clarify and uproot previous held understandings of sea slug classifications.
True sea slugs are divided into a number of taxonomic orders, however 5 are perhaps the most popular/dominant with regards to scuba divers and naturalists. To read our recent published work on sea slug diversity and importance on Koh Tao and Thailand, read the link here. The five major orders of Sea Slug to be found, particularly on Koh Tao are as follows:
- Sacoglossans – Sap Sucking slugs that feed on macro algae. These slugs are diverse vegetarians and are famous for the ‘kleptoplasty’ ability that many species have which allows them to keep ingested chloroplasts active inside their own bodies. To translate, this means many species can effectively become ‘solar powered’ with minimal need to actively feed for varying lengths of time.
- Nudibranchs – Naked Gill slugs are the most diverse and well known sea slugs in the marine environment. They are among the most colourful and charismatic of sea slugs and largely feed on sponges and other toxic/deterrent food sources that can be used in its own defences. The huge variety of shapes and sizes of nudibranchs make them among the most popular in the diving community.
- Cephalaspideans – Headshield/Tailed/Bubble shell slugs are largely carnivorous with several being omnivorous. Many species within this order are still in the process of losing their shell, a remnant of the snails they evolved from, with some that have retained shells large enough for the whole body of the animal and others with small, relatively useless shells. These slugs are also highly charismatic in colouration but are less diverse and often more cryptic than nudibranchs.
- Pleurobranchs – Side Gill slugs are actually closely related to nudibranchs, however many species look completely unlike their distant cousins. Some pleurobranchs are able to grow to significant sizes, often as large as a dinner plate. These slugs are typically less diverse than the other slug species discusses so far but nonetheless make up an important portion of the sea slug world.
- Anaspideans – Sea Hares are, regardless of what their name suggests, important herbivorous slugs. These animals have the ability to secrete toxins and deterrent fluids from their mantle and many species even have the ability to produce a noxious ink to repel predators. Like pleurobranchs, some species of sea hare can get extremely large, and can feed on large amounts of algal matter throughout their life times.
Finally, if you want to see some of the beautiful sea slugs that roam the muck habitat of our island,
check out our video here: MucKosmos 2015 – Slugs