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The Daily Bucket - hiding out on a coral reef [1]
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Date: 2025-04-28
The coral reef ecosystems around Little Cayman island are in as good a condition as you’ll find in the Caribbean, in abundance and diversity, a high density of animals who live in close quarters. That means there’s more competition and predation than in most ecosystems. What’s a critter to do to both find food and stay safe?
Just as there are many feeding strategies and food sources on the reef, animals have many ways to avoid being food for others. This bucket highlights one that is both fascinating and challenging for an observer to see: hiding out.
Note: This report is illustrated by videos. When I’m diving I document almost exclusively with video since I use a GoPro which I carry in my hand and store conveniently in my BC pocket. It suits me for a couple of reasons. One, in general it’s more behavior in nature that interests me than rather than who they are per se (except insofar as their characteristic adaptations drive behavior) and videos are a good way to show behavior. And two, taking good underwater still photos requires “big glass” and lights, which I don’t want to fuss with. However if you want to see good resolution closeup images of the animals in my videos, there are plenty online. I’m including animal names.
There are literally thousands of reef critters who use hiding as a strategy so the following are just a few examples from different situations.
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Sponges grow big in tropical waters, and their vase-like shapes create refuges for small fish and invertebrates. Giant Barrel sponges have ridges and crevices, unique to each individual sponge, that are perfect for small critters to hide in and then venture out briefly when it’s safe to feed. Barrel sponges can grow to 6 feet, with those oldest ones being up to 2000 years old. Given how sponges pull water in from the outside, filtering it for food and sending the cleaned water up and out the big opening, I suspect the tiny critters hanging around the periphery may also be taking advantage of that incoming stream of particles. A sponge filters a phenomenal amount of water, 50,000 thousand times their own volume per day.
In these video clips we see juvenile Blue chromis (Chromis cyanea), juvenile damselfish and angelfish, various gobies, an Arrow crab, (Stenorhynchus seticornis), a Banded coral shrimp (Stenopus hispidus), and other critters. Brittle stars have a particular fondness for sponge refuges so if you want to see a brittle star, look for a sponge.
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The hard reef structure itself grows in many shapes, with lots of crevices, tunnels, overhangs and caves. The animals you most obviously see on a reef are swimming freely around, some paying no attention to divers, like ones in the video below. Most wrasses, butterflyfish, filefish, snapper, pufferfish, boxfish, angelfish, damselfish, surgeonfish, jacks, groupers have that lifestyle, having other adaptations than hiding for predator protection. This scene of a Hawksbill turtle cruising through a school of Creole wrasses is typical of critters who just live out and about in plain view:
But other fish are more circumspect, and you have to seek them out in crevices or holes. Or under a ledge like these Fairy Basslets (Gramma loreto), which I ended up somersaulting to look at :)
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Besides the hard reef itself, a significant component of the coral reef ecosystem is the sand/rubble flats. Waves and surge erode coral into sediment which collects behind the reef crest forming an important habitat. Sand flats may not look like much, being white and apparently featureless, but if you look closely there’s a lot going on. “Barren” Underwater Sand Flats More Biodiverse Than We Thought. Many creatures burrow into the sand, are tiny, or are so well camouflaged you can swim right by not noticing them, like this Peacock flounder (Bothus mancus).
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Among my favorite fish on sand flats are these two:
The Sand tilefish (Malacanthus plumieri) have almost translucent coloring and undulate their fins gracefully. They build burrows out of rubble and don’t venture far from them to feed (on benthic invertebrates mostly). In this video we can see a male and female with adjacent burrows. The males are larger and highly territorial, he’ll only allow females within his territory. I watched this pair from two directions. The female was quite a bit more skittish.
Yellow-headed jawfish (Opistognathus aurifrons) are among the cutest fish I know. They build burrows too, but theirs are vertical, and they retreat into them tail-first at the slightest sign of danger. Jawfish are territorial too, and each has their own burrow within a circle of a few feet they defend. That’s why it was very surprising to see two tiny jawfish in one burrow. Perhaps when they are very young, sharing is ok.
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Eels are classic hiders. They vary in lifestyle and appearance. Morays are the big ones, hiding out in crevices in the daytime (mostly) and roaming the reef to hunt at night. Garden eels in contrast are small and live in burrows in sand flats. Garden eels occur in colonies, poking their heads up to snatch at plankton drifting by. You can often tell which way the current is running by how the garden eels are pointing. These eels are extremely shy and if you approach they withdraw into their burrows. This video has clips of a Spotted moray (Gymnothorax moringa) and a field of Brown Garden eels (Heteroconger longissimus).
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And last but not least, I have two videos of Caribbean reef octopus, (Octopus briareus). Octopus are very elusive, especially in the daytime, when they tuck themselves into their dens. Octopus eat conch and other shellfish so look for a pile of empty shells and you might find an octopus den. Invariably it’s their eyes you’ll see, looking back at you. It’s a bit unnerving, but wonderful — octopus are incredibly smart and when you lock eyes you can feel that mind considering you.
Since octopus only roam around freely at night, to hunt, it was surprising to see one just resting on a rubble flat once. That octopus was the most relaxed I’ve ever seen. We all watched it napping for a while, and eventually it woke up, heading off to a coral knob to tuck itself in. The dive guide offered to take a close view with my GoPro, which I was happy to hand off to him (that’s the first clip). The second clip shows another diver who had to rise up out of the way when the octopus came her way. She was obviously a very experienced diver (meaning good buoyancy control and no flailing around), which likely reassured the octopus. As I said, they are very smart creatures. A lot of divers come to this reef and we were probably not the first it had seen.
In general, diversity and abundance have declined in the Caribbean as it has on all coral reefs so finding a spot like this is a reminder of the lushness such ecosystems used to have everywhere. Even around Little, where much of the reef is protected and not subject to development pressure, the corals are affected by bleaching and disease, global problems. It does help that the island is fairly remote. Many divers visit but since it takes a lot of effort to get to Little, over the many years I’ve been going there, the divers I’ve seen treat the reef and its inhabitants with care and respect.
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Back here in the PNW islands it is partly cloudy and continued dry today. Temps are seasonable. Precipitation is below normal for April.
What’s up in nature in your neighborhood?
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