Roy’s take on Nature’s Outliers
Written by Roy Kittrell, June 2024
Here’s to the crazy ones. The misfits. The rebels. The troublemakers. The round pegs in the square holes. The ones who see things differently. They’re not fond of rules. And they have no respect for the status quo. You can quote them, disagree with them, glorify or vilify them. About the only thing you can’t do is ignore them. Because they change things. Sometimes they even change themselves. A lot. Or maybe the world changes around them and they just keep on surviving in spite of it.
Either way, in this month’s Flow Dive Log article we are looking at and appreciating the taxonomic outliers that are found in our oceans and reefs, some of which you may already have seen yourself on our dive trips!
Sea Spiders and Horseshoe Crabs
A lot of people might be surprised to hear that there are arachnids that live in the ocean… and they would be correct! Because there are in fact no spiders or arachnids there.
What do have though are some species of Chelicerates, arthropods in the Subphylum Chelicerata! This is a distinct group, separate from arachnids (who are also arthropods, but in the Class Arachnida).
The first of these chelicerates, the Sea Spiders (class Pycnagonida) can be found across many dive sites in South East Asia, including Malaysia and the Philippines, where I saw this relatively large subject moving across some Xenia soft coral. The ones I’ve seen in Malaysia always seem to be smaller, my theory is that there is a higher level of oxygen in the Philippine waters as they also have larger Skeleton shrimp. As you can see, sea spiders strongly resemble spiders, but are actually a much older group spanning as far back as the Ordovician, some 485 million years ago. Think of them as one of Nature’s first attempts at figuring out how to make spiders and other arachnids… and somehow they’re still around for us to get to see!
Whereas arachnids like spiders have book-lungs on the underside of their abdomen, sea spiders breathe air through their legs! And the colder the water the more oxygen is available, and the more oxygen that’s available the larger arthropods tend to get… which is why there are some species of giant sea spiders found at the polar regions and in the deep sea, a fascinating phenomenon called Polar Gigantism. Sea spiders feed on smaller animals which they can catch in their proboscis.
When it comes to Horseshoe crabs, the other chelicerate found in the ocean, there isn’t much about them that isn’t amazing! They have survived all 5 of the previous major extinctions and shown very little change in their appearance, resembling a large trilobite. Although you’re unlikely to see these while out diving, if you ever do get a chance it is well worth checking out this incredible living fossil.
Striped Eel Catfish
Plotosus lineatus, a species of eeltail catfishes belonging to the family Plotosidae, are relatively common in South East Asia, I think I’ve managed to see them at least once on most of my dive trips with Flow Dive, swarming in a large group and dipping down to the sea bed to feed on the small crustaceans hiding in it.
While fish aren’t uncommon in the ocean, fish from this particular family is. Of the over 3000 types of catfish in the order Siluriformes, almost all are freshwater species, from the Mekong giant catfish found in Thailand to the equally enormous flathead catfish found in the USA. Yet, here this one species is, deciding not only to live in the salt water oceans but go around the reefs hovering up small crustaceans and other animals. Amazing!
I was also surprised to find out recently that this fish is actually venomous; the spines in their fins can sting and cause some pretty severe symptoms if you tried to grab one.
Dugongs
Dugong, also known as a sea cow, is one of the more adorable and beloved inhabitants of Malaysia’s waters. It is a mammal, which places it in the same grouping of other marine mammals such as whales and dolphins, but in the order Sirenia, instead of the order Artiodactyla.
What does that mean though? This means that they are taxonomically distinct from all of the other whales and dolphins. Artiodactyla is an order of mammals that contains many land based mammals, usually ungulates, so animals that bear weight on their toes, like deer or horses. Yes, whales and dolphins are descended from land mammals that walked around on their toes! Some people are even surprised to learn that whales still have a vestigial legs buried deep inside their body.
Dugongs are from a different lineage, so where did they come from? Amazingly, their closest living relatives are actually elephants, in the mirorder Tethytheria! Once you think about it, looking at their cute little faces you can almost start to see the resemblance. Perhaps we should stop calling them sea cows (which are ungulates!) and start calling them sea elephants instead… except that name is already taken by another amazing family of pelagic sea snails by the same name, but I’ll write that article for another day.
Sadly I have yet to encounter these amazing creatures on my travels, but I am hoping someday Mel organises a trip to the places in Malaysia where these majestic creatures live!
The Moorish Idol (Zanclus cornutus)
On doing my research for this article, I was amazed to find the relatively common Moorish Idol fish was an outlier, by the fact that it is the only fish in the entire taxonomic family Zanclidae. Looking through my photos I realised I had some shots of one from Flow’s fantastic trip to Bali in 2023.
Being the only fish in your taxonomic family is fairly odd, especially for the ray finned fish, the most diverse group of vertebrates in the animal kingdom. Consider that in the goby family Gobiidae there are 2,000 species of goby, or that in the family Serranidae which contains sea bass and groupers, there are around 450 species of fish.
Initially described in 1758 by Carl Linnaeus himself, biologists have had more than enough time to find a way to group it into another family, but so far it’s closest relatives are in the surgeon fish family Acanthuridae, from which it is thought to have branched away from. The absence of spines on it’s caudal peduncle is apparently why it’s excluded… amazing!
Mantis Shrimp (Order Stromatopoda)
I know what you’re thinking… Really? Mantis Shrimp are outliers? The relatively common but still quite cool shrimp that I saw on my last 
diving trip with Flow dive? Well first of all yes they are relatively common in South East Asia, you can even order them on the menu in most local fishing towns. But second of all they are stromatopods in the order Stromatopoda, not shrimp in the order Decapoda. They are an ancient form of crustacean that branched off from all of the other decapods 340 million years ago. They are in a sense, Nature’s first attempts at making a shrimp, and they’re still here for us to see today. In fact if you look at a diagram of the class Malacostraca, you will see that there are functionally two main divisions, Stromatopods (called here ‘Hoplocarida’ for reasons known only to taxonomists) and Eumalacostraca, which contains almost all of the other crabs, shrimp, lobsters and any other decapod we would care to know about.
So, the next time you are out on one of Flow’s dive trips across South East Asia 
and beyond, keep an eye out for one of these critters, get a picture if you can, and then amaze your friends, colleagues and tinder dates with all of the amazing facts from this article… happy diving!
Roy Kittrell is an avid naturalist diver and underwater photographer, his work can be found on instagram
@roythedivebro