New publication: Critter diversity on the sand

September 14, 2019September 14, 2019 / Maarten De Brauwer / 1 Comment

It turns out that moving halfway across the world and diving into a new job is more time consuming than I expected, so I haven’t been keeping up with the blog recently. I’m slowly starting to get more organised and in the coming weeks I will try to catch up with summaries of papers that I’ve published recently.

The paper “High diversity, but low abundance of cryptobenthic fishes on soft sediment habitats in Southeast Asia” was published almost a year ago in the journal Estuarine, Coastal and Shelf Science. It was one of the key papers of my PhD and describes the diversity of critters on sandy habitats in Indonesia and the Philippines.

If you have ever been muck diving it won’t come as a surprise to you that there is some very exciting marine life to be found on sandy bottoms. When you mention places like Lembeh Strait, Anilao, or Dauin to keen divers – especially photographers – they either get lyrical about the amazing pictures they took there, or will tell you about their plans for visiting any of the above places to go see some crazy marine life.

Species like this painted frogfish (Antennarius pictus) are popular with muck divers

In fact, the popularity of these sandy critters is so great, that the divers visiting Southeast Asia for muck diving bring in more than $150 million of revenue each year, supporting thousands of sustainable jobs! With so much money and jobs involved, it would be normal to expect researchers and conservationists to be interested in knowing which animals live on tropical sandy slopes. Unfortunately, that assumption would be wrong, surprisingly little is known about soft sediment (=sandy) habitats in the tropics. Even basic knowledge such as which animals live where is often unknown.

Luckily things are changing! Scientific interest in “cryptobenthic species” – the small, camouflaged critters this site is all about – is definitely increasing, with excellent work being doing on coral reefs by colleagues from across the world. We are starting to understand just how important they are for coral reefs and how very diverse cryptobenthic species can be.

What I am interested in though, is what is going on with the critters that live away from reefs. Are the critters living on the sand as diverse as those one coral reefs? Which species are most common? What causes species to live in one area, but not another? To answer these questions I set of with my good friend Luke for a 3 month dive survey trip that took us to Lembeh Strait, the north coast of Bali, and the sandy slopes of Dauin.

Surveying soft sediment critters in Dauin. Photo: Luke Gordon

During our survey dives, we not only counted and identified the fish we saw, we also measured a bunch of other factors that could have an effect on the presence of critters. We wanted to know whether depth, benthic cover (growth of algae, coral, sponges etc), or the characteristics of the sediment played a role in which species we found.

So what did we find?

One of the most interesting results is that the diversity (number of species) of cryptobenthic species was very high, higher in fact than the cryptobenthic fish diversity on many coral reefs! In contrast, the abundance (number of individuals) was much, much lower than what is found on coral reefs. To put it in perspective, if a normal coral reef would be an aquarium with 300 cryptobenthic fish of 15 different species crammed inside, soft sediment habitats would be the same aquarium with 30 fish of 16 species.

When looking at environmental factors, one of the most important factors that influenced where species lived, was the characteristics of the sediment. For small critters it makes a big difference whether the sand is powdery fine, or coarse like gravel. There seems to be a middle ground where the size of the sediment seems ideal for many critters. The tricky part is that the characteristics of sediment are in a large part determined by other processes such as currents or wave action. For now it is too early to conclude whether critters are found in these places because of the type of sediment or because of other factors that shape the sediment!

Sediment characteristics play an important role in where critters are found

Luke collecting sediment

The amount of growth on the bottom played a role as well, particularly when algae or sponges were present, which makes sense as it offers variation in the habitat and potential hiding places for some species. Depth differences played a minor role in some regions (Daiun, Bali), but did not make a real difference in Lembeh. The limited effect of depth could partially be due to the fact that we did not survey deeper than 16m (university diving regulations are quite restrictive). It would be a great follow-up study to compare with deeper depths, as I am sure they will give very different results.

What does it all mean?

This study was (as far as I know) the first one ever to investigate the cryptobenthic fish in soft sediment habitats. The unexpectedly high diversity and very low abundance means there is a lot more species out there than what was assumed, but that we have to look much harder to find them. I mostly see our results as a starting point to guide further research. We have only uncovered a fraction of what is out there and are not even close to really understanding how tropical soft sediment systems function. While this provides an exciting opportunity for scientists like me to new research, it also means that we do not yet know how environmental threats such climate change or overfishing will impact species living on soft sediment. We do not know yet if the species that muck dive tourism depends on need protection, or how to best protect them if they do.

We might not know yet what the future holds for sand-specialists like this whiteface waspfish (Richardsonichthys leaucogaster), but I am hoping to find out!

Fluo time

November 27, 2016November 27, 2016 / Maarten De Brauwer / 2 Comments

If this isn’t the first time you’ve read this blog, you probably know I am interested in the phenomenon of biofluorescence. I’ve previously talked written about what it is and what it might be used for. In the near future I’ll be tell you all about the details what I was actually doing. But I realized I haven’t shared any pictures recently that show just how beautiful and otherworldly it can be. So here is a random selection of fluo shots I took over the last two years. Enjoy!

A bubble snail (Hydatina physis) photographed in Lembeh Strait, Indonesia

Thorny seahorse (Hippocampus histrix) in Bima Bay, Indonesia

West Australian Seahorse (Hippocampus subelongatus) in Perth, Australia

Amazing coral in Raja Ampat, Indonesia

Reptilian Snake Eel (Brachysomophis henshawi) in Amed, Indonesia

Lizardfish (Synodus sp.) in Lembeh Strait, Indonesia

Cockatoo Waspfish (Ablabys sp.) in Lembeh Strait, Indonesia

Sea spider (Pycnogonid sp.) in Tulamben, Indonesia

Painted frogfish (Antennarius pictus) in Lembeh Strait, Indonesia

Juvenile Painted frogfish (Antennarius pictus) in Dauin, Philippines)

Barred moray (Echidna polyzona) in Nusa Kode, Indonesia

Flamboyant Cuttlefish

October 7, 2016October 7, 2016 / Maarten De Brauwer / 9 Comments

It’s been a long time since I put a critter in the spotlights, so it’s time for one of my personal favourites: the Flamboyant Cuttlefish! I am definitely not the only one to like this amazing little animal, for most divers it is high in the top 10 of critters to see. With good reason as well, Flamboyant Cuttlefish are one of the most beautiful and intriguing inhabitants of sandy dive sites.

Flamboyant Cuttlefish (Metasepia pfefferi) striking a pose

The small cephalopod many divers call “Flamboyant Cuttlefish” are in fact two species: the Flamboyant Cuttlefish (Metasepia pfefferi) and the Paintpot Cuttlefish (Metasepia tullbergi). The first one lives in Malaysia, Indonesia, Papua New Guinea and the north of Australia. The Paintpot Cuttlefish is found further north, from the Gulf of Thailand all the way up to southern Japan. Both species are classic muck dive critters, they only occur on muddy or sandy bottoms, so you will have to move away from coral reef to encounter them.

flamboyant-cuttlefish3 So why does this animal deserve the effort of searching sandy plains for days on end, in the hope catching a glimpse of it? To start with (the name is a bit of a give-away) they are very flamboyant critters. We are talking yellows, pinks, blacks and whites, all at once! If that wasn’t enough, they often change their colours into “traveling waves”, even more so than normal cuttlefish or octopuses. From my experience, smaller Flamboyant Cuttlefishes have the brightest colours and make the most extravagant displays. When I write small, I do mean really small: adults do not grow much bigger than 8cm. They ideally sized Flamboyant Cuttlefish for the best colour-show would be around 3-5 cm!

If finding an animal that size seems difficult, you might also want to consider the following. When they are not disturbed, hunting, or mating, Flamboyant Cuttlefish are anything but flamboyant. In their “standard” state, they blend in perfectly with their background, so they will be a mottled grey, brown, or black. For this reason some divers or photographers are tempted to disturb the animal to better see its colours. It is obvious that this is a bad idea, as it will stress out the cuttlefish. Be patient instead, observe it for a while and you might even be rewarded by seeing it hunt small shrimp, lay eggs, or even mate!

Flamboyant cuttlefish showing its colours

The same animal only a few seconds later

Another thing that is special about them is that they do not swim, but walk across the bottom. To do so they use two of their arms and an adaptation of their mantle. They can swim, but only do so when they’re startled and over short distances. I could easily spend an entire dive watching these guys wander across the seafloor, little blobs of colour on a quest to eat as many shrimp as possible. It might be because of the awkward way they walk, but I always imagine them to be mildly grumpy animals. A bit like an old man with stiff joints who can’t walk that well, grumbling to himself about how the terrible weather…

A juvenile Flamboyant cuttlefish (M. pfefferi) walking across the rubble

An interesting mystery surrounding these animals is whether or not they are toxic. Their colours would suggest some form of aposematic colouration, in the same way nudibranchs advertise their toxicity with bright colours. Some authors have even suggested flamboyant cuttlefish might mimic nudibranchs such as the Ocellate Phyllidia (Phyllidia ocellata). I have however, not read a single bit of conclusive evidence of this. It seems to be one of these “facts” people have assumed, written about and then it just got copied. To date there seem to be no papers out their describing whether or not Flamboyant Cuttlefish really are toxic, and which toxin they would produce. If anyone would have come across that information, I would be very grateful if you could share it with me and the rest of the world.

As is often the case with small critters, we don’t know very much about them. But that doesn’t have to stop us enjoying looking at them. Which is why I couldn’t resist adding a video of one of these guys. The title is a bit too sensational, but the footage is great, enjoy!

Black sand: Origins

July 6, 2016 / Maarten De Brauwer / 9 Comments

The species I study are found in a very specific habitat. Unlike many (dare I say most) marine biologists who work in the tropics, I hardly spend any time at all on coral reefs, mangroves or seagrass beds. I estimate that since starting my PhD, roughly 80% of my dives were done over sand. No coral, no rocks, hardly any distinctive feature at all, just wide expanses of sand. Mind you, not just any kind of sand, the critters I look for seem to be found most often on black sand.

A black sand beach in Lembeh Strait, Indonesia

For those who have never had the pleasure of diving over black sand or those used to diving in places like Egypt, Maldives or the Great Barrier Reef, the idea of a black sand beach or dive site might seem strange or hard to imagine. When reading the words “tropical beach”, most of us imagine powdery white sand, turquoise water, and a bunch of palm trees added for good measure. But I am happiest starting my dive from a beach as black as my (soul) wetsuit and dive boots. The first time you see a true black sand beach is mesmerising and even a bit alienating, it somehow doesn’t seem right. The water looks grey instead of blue, darker, and less inviting than those beautiful blue lagoons from travel brochures. Until you put your head under water and start looking around…

But why are some beaches black? What is the difference between the powdery white sand of Maldives and the pitch black sands of Lembeh or Hawaii? After the ICRS conference in Honolulu last week I decided to go have a closer look at the origins of my preferred study systems: active volcanoes! There are few places on this world better suited to do this than on Hawaii’s big island. Accompanied by Jamie, a marine scientist specialising in underwater soundscapes, I set off to find out how close to a volcano one needs to get before the hairs on your legs get scorched off 😉

Lava flow making its way through the forest

Turns out the answer is: not very close at all. In what was the highlight of our trip we flew in an open-doors helicopter over a lake of magma and a lava stream running down a mountain. Even hovering 50m above the lava you can feel the incredible heat emanating from the stream! The Kilauea volcano is one of the most active volcanoes in the world, and has been erupting since 1983. The result is an amazing landscape of solidified lava, covering vast expanses of the island. During our 3 day stay in the Volcano National Park, we visited lava tubes, hiked over solidified lava lakes, were awestruck by the raw spectacle of watching a volcanic crater at night, and got sunburned ambling across a frozen lava wasteland that was a village until only few year ago. While we were in Hawaii, there were no lava streams entering the ocean, but there were plenty of places where it recently did.

Fresh lava flow in the ocean

Liquid lava flowing in the ocean is one of the two ways black sand beaches can be formed. The lava is so hot that when it runs into the cold water, it causes explosions that shatter tiny fragments of volcanic sand/glass around. This process can happen so fast that a black sand beach can be formed overnight! The size of these fragments can be from small boulders to actual fine sand, dependent on the temperature of the lava and the water. The dark colour of the sand is caused by the mixture of minerals inside the lava, the minerals giving the darker colours are mostly iron oxides such as magnetite.

The other way black sand beaches are formed, is through erosion of black volcanic rocks by rivers, which then carry the black grains of sand down to the ocean. Depending on which other types of rock the rivers flow over other minerals will be added to the mix, resulting in sand colours that can range from pitch black to dark brown or even beige or green. For any geology-inclined readers, this site has got all the details you could possibly want to know about black sand.

Black sand + dive boots

So to summarise: black sand ecosystems are formed by some of the most epic geological processes in the world, magma from the centre of the earth erupting and shaping the world as we know it. The powdery white sand beaches of Maldives on the other hand, are mostly formed by the build-up of whole lot of parrotfish poop***. An interesting process for sure, but give me an epic black sand beach over a pile of fish poop any day 😉

***Technically speaking parrotfish poop, eroded corals, shells and bits of other stuff, but I’m taking some freedom here 😉