A brief Evolution of Life in the Ocean through my Lens
Written by Roy Kittrell 16 June 2023.
In most of my articles to date, I’ve focused on highlighting a single kind of animal or species. But in this article, I am going to try to give you a top down view of the reef as a whole, looking at the big picture from a diver’s point of view. By looking at the groups of animals, their history as told by their taxonomic relationships to each other.
It is one thing to go diving and see a shark (which is always a thrilling experience, especially for new divers) but what if you knew that sharks, rays, mantas and even guitar fish were all related in the same class Chondrichthyes, the cartilaginous fish? Your knowledge of the underwater world informs and enhances all of your dives. Knowing what you are looking at, knowing how life evolved into all of the different lifeforms in the ocean, helps you to appreciate diving on a whole new level.
All life on Earth, which started in the ocean, shares a common ancestor. This ancestor, known as LUCA (our ‘last universal common ancestor’) lived a very long time ago, estimated to be as long as 3.8 Billion years ago (with the Earth itself being 4.5 Billion years old). One of the few things we can be certain about of LUCA, is that it reproduced. And its offspring reproduced, and those offspring reproduced, and as life continued for the next 450 Ma and started to take hold on Earth it also evolved and diversified into millions of different types of creatures and animals. The study of this diversity is known as Taxonomy.
Life on Earth is organised into 8 levels of taxonomic hierarchy starting from largest and most general to smallest and most specific. Those levels are Domain, Kingdom, Phylum, Class, Order, Family, Genus and Species
There are three domains of life; Archaea, Bacteria, and Eukarya, that we will focus on and are present in the ocean. The first two of these comprise single celled organisms known as prokaryotes, and here we will pause to look at our first and potentially most important oceanic life form: Phytoplankton.
‘Life, feeds on life, feeds on life, feeds on life, feeds on life, feeds on light.’ – Paraphrased lyrics from a song by the band ‘Tool’
One concept we explore here are Trophic Levels. These refer to a hierarchy of which life feeds on what other life. For example, here on land we have grass, which derives its energy from the sun, making it a primary producer, or ‘Autotroph’. A gazelle wanders by and eats the grass, it is next in the food chain. It is a consumer, or ‘Heterotroph’. A lion hunts the gazelle and eats it, the lion is also an Heterotroph, and so on and so forth.
Phytoplankton and Algae, extremely generic terms for the teeming thousands of kinds of single cellular photosynthesising organisms in the Domains Archaea and Bacteria that inhabit our oceans, are largely autotrophs. They are arguably the most important in the entire biosphere of planet Earth. Some, like Cyanobacteria, are the reason we even have a breathable atmosphere to begin with, by taking in carbon dioxide and exhaling oxygen. According to some sources photosynthesis by these microscopic organisms in the ocean accounts for around 70% of the oxygen in our atmosphere. Others, like the microscopic dinoflagellates, are one of the most important autotroph food sources for the ocean, feeding all kinds of life forms from tiny scallops to gigantic Baleen whales.
Phytoplankton under a microscope
The rest of all multicellular life on Earth, the Eukaryotes, are what we will focus on next.
Of the five kingdoms of Life, the three most relevant to us as divers and amateur naturalists are the more familiar Plantae, Fungi and Animalia.
Plants are found throughout the oceans and reefs as relatively common seaweed, some of which like the Elephant’s Ear serves as food for interesting sap sucking slugs like Shaun the Sheep. Fungi, something that is abundant on land, also exists in the ocean but only to a limited extent. You might see a thin film of slime on a floating log, but you will certainly not see any mushrooms growing underwater.
Outside of these two kingdoms, every other multicellular lifeform in the ocean, and certainly the ones we are most interested in seeing when diving, is classified under Animalia.
It is here when we reach the level of Phylum, that multicellular life really starts to diversify and become interesting.
The oldest phylums we will encounter in the oceans are the Cnidaria, which contains things like corals and jellyfish, and the Porifera, otherwise known as sea sponges. The mighty Cnidaria, without which reefs wouldn’t even exist, are an ancient phylum. They were present among the first fossils found of life on Earth, and genetic analysis indicates they could date back as far as 700 million years. Cnidaria broadly contain two subphylums; Anthozoa, where we find sea anemones, soft and hard corals, and Medusozoa, under which all jellyfish are found. One of the first insights I hope for readers to gain from this article, is the knowledge that jellyfish and coral are functionally the same form of life.
Sponges, in the Phylum Porifera, a ubiquitous and core part of any reef’s ecosystem, are equally as ancient. Well preserved fossils of sponges are found from 580 Ma with some potential for them dating back even further. A very basic form of life without a central nervous, digestive or circulatory system, Sponges have the unique distinction of being the first group of animals to branch off of the evolutionary tree from LUCA, making them a sister group to all life on Earth.
Our next Phylum is Echinodermata, or the echinoderms, such as sea stars. Echinoderms only exist in the ocean, which means that there is an entire phylum of life on Earth that can only be observed in the wild through scuba diving!
Next, we have molluscs, from the Phylum Mollusca. This contains animals like snails, which also evolved to live on land, but as you will see in the underwater world this is a highly diverse branch of life with many fascinating animals and adaptations in it.
In the Phylum Arthropoda, we have animals with a hard exoskeleton and no spine, for example shrimp. Arthropoda are among the most diverse phylum on the planet, with 86% of all kinds of life on Earth being classified as part of this group.
The Phylum Chordata contains all animals that have a spine and centralised nervous system, including for example, fish. This is another wide ranging and highly diverse phylum containing many of the most sought after animals for divers.
At the Class level, Chordata begin to radiate into many different branches, but the first and probably the largest class are Actinopterygii, or ‘ray finned fish’, so classified because their fins are comprised of thin webs of skin supported by rays of bone. So successful and abundant is this one single class of life, that it contains more than 50% of all vertebrate species on earth.
A stunning example of a ray finned fish seen on Flow Dive’s trip to Lembeh: the Mandarin Fish (Synchiropus splendidus)
Although abundant and diverse now in Earth’s current geological era, Ray Finned Fish are contrasted by another class of fish, the Sarcopterygii or ‘lobe finned’ fish. These are an ancient and extremely important lineage of fish that eventually learned to live on land, and evolved into the Superclass Tetrapoda, a group that contains all which includes amphibians, reptiles, birds and mammals… which means we share a common ancestor with these fish! They have only a few modern species still in the ocean, such as the West Indian Coelacanths, or the Indonesian Coelacanth from Indonesia, of which only a dozen or so specimens have ever been found.
While most of the animals that evolved from Sarcopterygii stayed on land, some evolved to return back to the ocean. One such class of animal is Reptilia, seen in our oceans as Sea Snakes and Sea Turtles. You might think that sea turtles would have come before our smaller and more familiar land based turtles, but their ancestors actually evolved on land in the Jurassic and then returned back to the sea during the Cretaceous. The same goes for the beautiful but deadly Sea Snakes, the most venomous group of snakes in the world, which only evolved to live in the ocean as recently as the Neogene 20Ma until present.
Echinoderms diversify at the Subphylum and Class level. We now have three distinct Subphylums: Asterozoa which contains all of the different sea stars, Crinozoa where you’ll find the crinoids such as feather stars, and Echinozoa where we group all of the sea urchins as well as sea cucumbers… yes, these are in fact echinoderms!
Mollusca diverge into three main classes. Bivalvia, which have two shells such as the Giant Sea clam or the pearl oyster. Gastropoda, a class which contains all of the sea snails and nudibranchs, and Cephalopoda, where you will find Octopus, cuttlefish and squid. It is amazing to think that clams and octopus are so closely related considering they are such different animals
Arthropoda begin to get more interesting. A relatively small subphylum known as Chelicerata contains the classes Pycnogonida, known as Sea Spiders, and Xiphosura, known more commonly as Horseshoe crabs! This subphlum also contains the class Arachnida… which contains all arachnids, such as spiders and scorpions!
The other subphylum of Arthropoda that we see in the ocean are an extremely diverse subphylum Crustacea, where we find many different kinds of invertebrates. Of the 10 classes, Malacostraca is one of the largest, containing all crabs, lobsters, shrimp, and other more recognisable decapods which I will talk more about later. The other classes would not be familiar to the casual observer as many of them are tiny planktonic
Copepod parasites on the side of a whip coral goby
The Evolution of Intelligence
Intelligence, such as the ability to communicate using language or the complex use of tools, is rare in nature. Of the two branches of the tree of life, only vertebrates, such as dolphins, whales, birds and even humans, have demonstrated this cognitive leap forward. They all did this by developing a complex brains that require a lot of energy to maintain and bodies connected to them by a central spine.
But intelligence also evolved along another branch: the Cephalopods. Intelligence evolved in creatures such as the Octopuses and Cephalopods, in a way entirely separate to vertebrates. Their brains are distributed throughout their body; each limb of an octopus functionally thinks for itself, and they can camouflage their bodies and communicate with one another through their incredible colour-changing ability. They have demonstrated the ability to learn, to remember things, and even use tools, as is seen with the Coconut Octopus Amphioctopus marginatus.
What this all means is, Cephalopods are likely the closest thing we will ever have to coming into contact with an alien intelligence, and the only way to interact with them in the wild is through diving.
A coconut octopus (Amphioctopus marginatus)
To read more about Cephalopod intelligence, I highly recommend the book ‘Other Minds’ by Peter Godfrey-Smith.
Malacostraca diversify a lot at this level, and contain the Order Decapoda, meaning ’10 footed’. It would seem strange to look at a creature like a crab and see that it only has 6 feet, but if you look closely they functionally have 10 limbs, all of which can have different functions. If you’re a true crab in the infraorder Brachyura, you might decide to use 6 of them for walking and two pairs for large claws and eating.
It is often said that nature abhors a vacuum. But it is also true that nature seems to abhor a lack of crabs. I am referring here to the fascinating phenomenon that happens in Decapoda, known as ‘carcinisation’, a process by which decapods keep evolving into crabs! A great example of this (and something I have written about in a previous article) are the infraorder Anomura, which contain hermit crabs
and the beautiful porcelain crabs. These are not ‘true’ crabs, but in fact squat lobsters that evolved into a crab-like form. Anomura also contain another amazing organism: the Coconut Crab, the largest land-based arthropod on planet Earth!
Shrimp, from the infraorder Caridea, are another hugely diverse Infraorder that are very well known and hugely important to the reef ecosystem.
Mantis shrimp or Stomatopoda are found now in their own order, distinct from Caridea shrimp having branched off and evolved separately from other decapods as far back as 350 Ma ago. This gave them time to develop some truly remarkable features, for example the most complex eyes in the entire animal kingdom, or smashing appendages that boil water when they ‘punch’ their prey.
Chondrichthyes diversifies here, the Orectolobiformes or Carpet Sharks now form their own order, so called because many of their species resemble Persian carpets, such as the wonderfully named Wobbegong shark. While most species in this order are small, surprisingly the largest fish in the ocean, the mighty Whale Shark (Rhincodon typus) is related to them.
Anguilliformes, or eels, diverge away from the other Ray Finned Fish into their own order. Morays, garden eels, and ribbon eels all share a common ancestor with most if not all of the reef fish that you will see on your various dives.
The extremely beloved and well known order Cetacea is another example of a branch of vertebrates that returned to the ocean. This order contains all whales and dolphins, which descended from a long line of placental mammals that resembled Hippopotamuses only as recently as 50 Ma ago. Another order of mammals, the Sirenia, contains Dugongs, a beloved and popular although listed as ‘Vulnerable’ underwater mammal that is found throughout SE Asia and East Africa.
The Caridea shrimp diversify again at this level and contain the Family Palaeomonidae, under which we find the vast majority of the most popular shrimp divers look for in the ocean.
Another family, Caprellidae, also known as Skeleton shrimp, appear at this level and are taxonomically distinct from the other arthropoda. They are not shrimp, and are in fact a type of fascinating and highly evolved amphipod in the order Amphipoda.
Ray Finned Fish have some interesting and likely familiar families; the Syngnathidae are an offshoot containing seahorses and pipefish that have some interesting adaptations. Seahorses, such as the stunning Pygmy Seahorse Hippocampus bargibanti, sacrificed their tails for a prehensile limb that allows them to grasp onto corals and seaweeds, and evolved brood pouches that allowed both the males and females to take part in pregnancy and child-bearing.
Pygmy Seahorse (Hippocampus bargibanti)
Ogcocephalidae evolved to live a very benthic lifestyle on the seafloor, and contain a myriad of popular and very colourful Frogfish found throughout Asia Pacific, and also the utterly bizarre looking Batfish such as the red-lipped Ogcocephalus darwini.A skeleton shrimp with babies attached to a small strand of seaweed
Another family of fish, the Scorpaenidae, evolved to have highly venomous spines for defence, in fact they are known as some of the most venomous animals in the world, for example the aptly named Synanceia horrida.
The photogenic family Ostraciidae which is comprised of Box Fish sacrificed the sleek and streamlined bodies of normal fish to have odd, box-like forms that allowed for quick turns and fast get-aways from predators.
While most of these families evolved to live close to the sea floor, at least one other notable family the Molidae containing the Mola-Mola or Sun fish became a huge sea-faring species at up to 2m across, and are found worldwide.
At the genus level, Palaeomonidae shrimp diversify hugely and contains multiple fascinating groups that are highly specialised, from living communally on echinoderms like those in the genus Periclimenes, to the amazing Bumblebee shrimp in the genus Gnathophyllum. All are extremely popular and highly sought after by underwater photographers. Zenopotonia contains both the species rex, also known as the emperor shrimp, an extremely photogenic and popular species normally found living on top of sea cucumbers, and soror the seastar shrimp which is found on sea stars.
An emperor shrimp (Zenopotonia rex) living on a buried sea cucumber
Another genus of shrimp Phyllognathia contains both the stunning ceratophthalma or Spiny Tiger Shrimp, and the rarer and more elusive simplex shrimp, which is a highly prized animal to see on any diving trip. Hymenocera contains picta Harlequin shrimp, both a stunning looking shrimp and a voracious hunter of starfish.
The Species level of taxonomy is the one area where there is a fairly formal definition: if two animals can mate with each other and that offspring is also fertile, then those two animals are the same species. When two species mate at the genus level they can still create offspring however that offspring will be infertile; a land based example of this is the tigon, the infertile offspring of a tiger and lion.
One hugely diverse and colourful group of Gastropoda you will definitely see a lot of when you are diving, are the order Nudibranchia! Within this taxonomically complex area, over 3000 species grouped largely into two main clades (informal groupings) the Dorids and Aeolids, these molluscs have evolved to have gills that are open to the water and not covered, hence the English translation of their latin name ‘naked gills’. While nudibranches no longer have a shell (and instead protect themselves by being toxic to predators) they still have one in their larval planktonic stages which is shed before becoming adults.
Nudibranches have evolved some astonishing abilities. Perhaps one of my favourites is Shawn the Sheep, or Costasiella kuroshimae, which feeds on elephant ear seaweed and incorporates the plant’s cells into its body (known as ‘kleptoplasty’ to do photosynthesis!
“If you can imagine a nudibranch, it probably exists” – Roy Kittrell
This article has been something I thought about and researched for many months leading to it being published; it is in some way a love letter to the Ocean as a whole. Even as long as it became, there is quite simply no way to condense all of the potential species that I could have into even this long of an article, but my hope, is that it inspires you to appreciate all of the different forms of life available in the ocean. From innocuous and otherwise boring sponges, to the near imperceptible but hilarious looking idiomysis shrimp, and not just focus on the usual main attractions such as turtles and sharksTwo Spiny Tiger Shrimp (Phyllognathia ceratophthalma) fighting over a small brittle star
I like to think that Nature is fractally interesting. That is, the more you look into it, the more interesting it seems to become. After 4 years of diving with Flow Dive Center I have never ceased to find new and amazing animals to learn and fascinate myself about. Come join our upcoming dive trips as we look for encounters with these fascinating creatures!