Three posts ago—while discussing the newly discovered symbiosis between green algae and spotted salamander embryos—I referenced an article about my favorite group of invertebrates, nudibranchs (sea slugs), which inspired today’s installment.
Nudibranchs are simply spectacular. The incredible diversity of shapes and color is almost intoxicating.
They are breathtaking!
I find myself mesmerized and end up contemplating how such exquisite organisms could have ever evolved. The safest way to go through life is to be unseen, which is why there are so many cryptic species. But if you have something to back it up, sometimes your best bet is to make a statement, be seen, and advertise to potential predators in no uncertain terms that you are dangerous (i.e., toxic). This is called aposematic coloration, and one of the best examples are the ‘poison dart frogs’ (Dendrobatidae).
Note: It is actually more appropriate to call the group ‘poison frogs’, since only one species (Phyllobates terribilis) was actually used for poison darts. But I digress…
There are two schools of thought on how aposematism could have evolved: either the toxicity came first and the coloration second, or the coloration first and the toxicity second. Either way you are faced with the conundrum that aposematism only works when the predators have learned to avoid the brightly colored individuals, but up until that point the predators would preferentially target the brightest individuals and disproportionately removing them from the gene pool. However it works, it works. So what defenses do nudibrachs bring to bear? More on this later…
As amazed as I am by the physical splendor of these little mollusks, it pales by comparison to the complex interactions they share with other organism.
Note: It is actually more appropriate to call the group ‘poison frogs’, since only one species (Phyllobates terribilis) was actually used for poison darts. But I digress…
There are two schools of thought on how aposematism could have evolved: either the toxicity came first and the coloration second, or the coloration first and the toxicity second. Either way you are faced with the conundrum that aposematism only works when the predators have learned to avoid the brightly colored individuals, but up until that point the predators would preferentially target the brightest individuals and disproportionately removing them from the gene pool. However it works, it works. So what defenses do nudibrachs bring to bear? More on this later…
As amazed as I am by the physical splendor of these little mollusks, it pales by comparison to the complex interactions they share with other organism.
In the previous post I referenced an article about a species (Elysia chlorotica) that has a symbiotic relationship with algae. Well I have to admit, that is not entirely true. This nudibranch doesn't actually play host to algae, but rather assimilates the chloroplasts from algae it ingests. This relationship is call subcellular endosymbiosis (subcellular, because the relationship only involves organelles rather than entire organisms). The chloroplasts can survive within the slug's body for up to 10 months, and all the while they are actively photosynthesizing. In a very real sense this nudibranch is solar-powered and can complete its entire life cycle (including reproduction) on photosynthesis alone, although they continue to graze to collect new chloroplasts. But photosynthesis is a complex biochemical reaction that requires many specialized enzymes to complete, so simply possessing chloroplasts isn’t enough. Somewhere in its evolutionary history Elysia chlorotica actually assimilated algal genes (15 have been described so far), and there are likely more that have yet to be discovered. Gene transfer between species is common among single-celled organisms, but this represents the first time it has been described in multi-cellular life.
Many species (at least those belonging to the aeolid infraorder) possess an astounding defense mechanism that is strikingly similar. These nudibranchs defend themselves via nematocysts (or cnidocytes) located in the dorsal body wall (protrusions called cerata). But nematocysts are the hair-trigger ‘ballistic’ defensive (or offensive) organelles containing toxins found within specialized cells (cnidoblasts), unique to cnidarians (phylum Cnidaria; e.g., jellyfish, sea anemones, hydroids, corals, and ctenophores).
So how the heck did a cnidarian organelle end up on the back of a sea slug?
Well it’s a complex process, but it goes a little something like this… Nudibranchs graze on sea anemones, and are protected by their mucus that prevents the anemones’ nematocysts from firing. Once ingested, intacellular discs called ‘spindles’ form a physical barrier preventing nematocyst discharge in their bodies. The encapsulated organelles are then transported to the cerata, where they are deposited in the outermost layer of tissue. When this process is complete, the stolen nematocysts are then referred to as (quite appropriately) kleptocnidae (heh heh… science words are fun). This too is an example of subcellular endosymbiosis.
In my undergrad invertebrate zoology course I was shown a series of cross-section photos illustrating the actual progression of nematocysts through the nudibranch body wall. It was incredible and I’m sorry I was unable to find similar images for this post. I’ll keep digging for a sequential series and put them up if/when I come across them.
So there you have another beautiful example of the intricate complexity of life. Nature will always find interesting and creative ways to deal with life’s many challenges.
So how the heck did a cnidarian organelle end up on the back of a sea slug?
Well it’s a complex process, but it goes a little something like this… Nudibranchs graze on sea anemones, and are protected by their mucus that prevents the anemones’ nematocysts from firing. Once ingested, intacellular discs called ‘spindles’ form a physical barrier preventing nematocyst discharge in their bodies. The encapsulated organelles are then transported to the cerata, where they are deposited in the outermost layer of tissue. When this process is complete, the stolen nematocysts are then referred to as (quite appropriately) kleptocnidae (heh heh… science words are fun). This too is an example of subcellular endosymbiosis.
In my undergrad invertebrate zoology course I was shown a series of cross-section photos illustrating the actual progression of nematocysts through the nudibranch body wall. It was incredible and I’m sorry I was unable to find similar images for this post. I’ll keep digging for a sequential series and put them up if/when I come across them.
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