Sea slug evolution and conservation – what’s the link?

The two videos in this tutorial focus on the evolutionary lineage level of biodiversity, using the relationships outlined in the tree of life to help inform conservation decisions. Here is a real-life example of such a linkage, featuring those charismatic mollusks called nudibranchs – the oceanic, shell-less (naked) snails also known as sea slugs. Many nudibranchs have astonishing coloration and have become a favorite subject of underwater photographers. It’s easy to see why!

(Image on left: Hermissenda crassicornis, Bolinas, California. Image on right: Hyselodoris sp., Mabini, Philippines.)

These beautiful creatures are using color patterns to warn potential predators that they are toxic or don’t taste very good because they have incorporated into their own bodies toxic and distasteful chemicals from the food they eat.

Other species of nudibranchs take a different approach to avoiding predation – they are more modestly colored to match the backgrounds upon which they live -- they're camouflaged! This is also known as cryptic coloration or crypsis.

(Image on left: Asteronotus mimeticus, Philippines. Image on right: Lomanotus sp., Philippines.)

Nudibranchs occur in most oceans of the world, but reach their greatest diversity in the tropical Indo-Pacific. Research carried out by Dr. Terrence Gosliner, Senior Curator of Invertebrate Zoology at the California Academy of Sciences, his graduate students and post docs, and colleagues around the world, has helped document the tremendous diversity of nudibranch species and their evolutionary relationships to each other. Just how much diversity are we talking about? In the Philippines alone, over 1,000 species of nudibranchs have been documented, and half of them are new to science.

The diversity of species and where they appear in the tree of life are very relevant to conservation management decisions. Here’s an example. Take a look at the three cladograms below that include species from three different parts of the Indo-Pacific: Hawai’i, the Western Pacific, and the Western Indian Ocean. Each area has species of the nudibranch genus Hypselodoris that are endemic to that part of the ocean -- that is, they are found nowhere else. These endemic Hypselodoris species are highlighted in yellow on the cladograms. The species listed in green font are more widespread and not restricted to one particular area. Don’t worry about all of the species names at the top of the cladogram; just focus on where the yellow-highlighted species are and how far down their branch goes on the cladogram.

Hawai’ian:

Western Pacific:

Western Indian Ocean:

If you were a government official or the head of a conservation organization faced with limited resources, which area should be the highest priority for protection based on just these data on one genus of nudibranch? The answer would be the Western Pacific, for two reasons – not only does it have more species of endemics, but it also has representatives of more evolutionary branches, including the deepest branch on the cladogram. Protecting this part of the ocean and those species would mean you were protecting more of the evolutionary lineage, and most likely, a great genetic diversity of species that are also fulfilling different ecological roles in each of the places they occur.

Intrigued by nudibranchs and want to know a little more about their evolution and how that is relevant to biodiversity conservation? Here’s one more cladogram for you. This shows the evolutionary relationship of 19 different species of nudibranchs in the genus Thuridilla. They are found in the all of the tropical oceans. Of these 19 species, 12 are brightly colored (white) and seven are cryptically colored (red). What conclusions can be drawn from this cladogram?

It is clear from the cladogram that cryptic coloration was the ancestral condition in this group of nudibranchs and that bright coloration evolved more recently. But if you look closely, it appears that there might be instances in which there was a switch back to crypsis. Another thing to think about is that it would be great to have more scientists and citizen scientists looking carefully for and documenting the camouflaged nudibranchs as well as the gaudy, more easily seen ones. Who knows what might be out there?! Lastly, the data represented in the cladogram above could be very useful in future conservation decisions. To preserve as much of the evolutionary lineage as possible, it would be important to protect cryptic species as well as the brightly colored ones.