Science Friday 001: Who Ate That?

Dive curious. Discover what others miss.

Mission Recap

Something is always eating on a coral reef. We don't always get to see it happen, but if we know what to look for, we can often find the clues left behind.

That was the mission for our first Science Friday: Who Ate That?

Rather than simply looking for marine life, we spent two dives looking for evidence of feeding and asking a simple question: Who ate that?

Reading the Reef

One of the goals of Science Friday is to get us thinking about a coral reef as more than a collection of fish, invertebrates, and substrate. It is a community of organisms constantly interacting with one another. Feeding is one of the easiest ways to begin observing those interactions because even when we don't witness it first hand, often clues are left behind.

Parrotfish Were Here

One of the signs we were looking for was evidence of parrotfish feeding. Instead of simply spotting a parrotfish swimming across the reef, we looked at the reef itself. Parrotfish use their fused, beak-like teeth to bite and scrape algae from hard reef surfaces. Depending on the species and feeding strategy, those bites can leave visible marks behind. Once you know what those marks look like, they start showing up everywhere. Parrotfish are important herbivores as their bite marks open up new space on the rock for coral larva to begin to colonize.

Something Ate the Urchins

We also found evidence of a very different kind of feeding. During the dives, we encountered the remains of pencil urchins, collector urchins and banded urchins.

One urchin test (shell) had a gaping hole in its underside, while another had been broken roughly in half. Obviously, something had happened. But unlike our parrotfish evidence, we couldn’t determine the animal responsible. Finding an organism's remains doesn't necessarily tell us who ate it. A broken urchin test is evidence. Determining what produced that evidence requires more information. Could it have been predation? If so, what predator could produce that type of damage? Could something have happened to the test after the urchin died?

We don't know from the evidence we observed alone.

Part of learning to read a reef is separating what we observed from what we think may have happened. Sometimes the best answer underwater is simply, “we don’t know yet.”

What we do know is many sea urchins in Hawai’i protect themselves during the day by wedging into crevices, hiding beneath reef structure, or relying on long, thick, or venomous spines to discourage predators. After dark, many species emerge from shelter to graze on algae. Nighttime feeding may reduce their exposure to some daytime predators, but urchins still face threats from animals such as the bigeye emperor, seven-eleven crab, and giant porcupinefish, all of which are known to feed on sea urchins or parts of them here in Hawaiʻi.

Then We Caught One in the Act

Not everything remained a mystery. We did encounter a crown-of-thorns sea star actively feeding on coral. Crown-of-thorns sea stars are corallivores or predators that consume living coral tissue. Rather than biting pieces from the colony, a crown-of-thorns feeds externally. It positions itself over the coral, everts its stomach onto the colony and digests the coral tissue outside its body before absorbing the nutrients. It is a very different feeding strategy from the scraping of a parrotfish or whatever happened to our urchins.

What We're Still Wondering

At the end of every Science Friday, we want to leave with another question. This week's came from Emma, visiting us from Texas. She was curious about the cushion sea star we came across near the end of our second dive.

Hawaiʻi's cushion sea star is itself a corallivore. Generally, they prefer cauliflower coral over several other Hawaiian coral species. Just like crown-of-thorns sea stars, cushion sea stars feed by everting their stomach over their food. The unassuming cushion sea star Emma noticed wasn't simply sitting on the reef. She had found another answer to the question we'd been asking all day: Who ate that?

Start Looking for the Evidence

You don't necessarily need to witness an interaction to start understanding what is happening on a reef. Look at the clues such as a scrape across the reef, a broken urchin test, a patch of recently exposed coral skeleton and ask: What happened here?

The more you ask that question, the harder it becomes to simply swim over a reef without wondering what happened before you arrived.

Next Science Friday:
Mission 002: Masters of Disguise

This coming Friday, August 28th, 2026, we're changing the question.

Instead of looking for evidence of animals eating, we're looking for animals trying very hard not to be seen.

Our mission is to find organisms using different strategies to avoid being noticed. Camouflage is only the beginning. We'll look at how color, patterns, body shape, behavior, habitat, and positioning can help animals disappear into a reef even when they're right in front of us.

Reserve your August 28th spot here!

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I began my journey into Scuba as a very nervous participant. I did not consider myself a very adventurous person and was coerced by a significant other to try Scuba. I was living in the pacific Northwest. Surrounded by cold and mysterious salt water bays, sounds and canals. I was fascinated by everything about the underwater world except going into an environment inhospitable to human life. When I finally was convinced to sign up for an open water scuba class I was terrified. I read the PADI manual cover to cover twice before the first class (this was before eLearning existed) I was sure there was so much technical skill and memorization I’d need to master to make it out alive from my first dive. The night before my first session I called the instructor to tell him this just wasn’t for me. I was terrified and I was pretty sure I did not have the ability to survive the pool session the next day. He asked me to please give it a try. He didn’t want to break his record of no student quitting on him and would I just get in the pool and see what happens. No pressure to continue after that. I reluctantly agreed. I over thought every skill in the pool and awkwardly made it through the skills development sessions. My instructor kept reminding me “relax this is fun”. So far to me the stress of getting it right was over shadowing any fun to be had. The weekend finally came to do the open water check out dives. We were going to a popular “easy” shore diving spot called Illahee state park. It was a pebble beach with a long wooden dock with tall tar covered pilings with muscles and seaweed growing off of the stretches exposed from the low tide. It was early fall so air temperature was mild but still we poured ourselves into double layer 7mm farmer John style wetsuits to keep warm In the frigid 44 degree water of the puget sound. 7 mm hoods gloves and boots and I no longer felt, looked or moved like a human. Our class of 5 waddled towards the sea fins in hand following our instructor and his assistant. As we donned Fin’s and defogged our masks I was mildly panicking trying to reenact everything I’d learned and imagining (dreading) going below the black surface of this frigid water. The plan was to start descending near a piling for reference. We did our buddy checks. BWRAF which of course I’d memorized from my panic studying. The other students raised their deflators. And carelessly dropped below with the dive pros. I added more air to my BC and started to cry. The assistant instructor stayed below with the other comfortable divers and my instructor re emerged from below. Did you forget which button to push to release air he asked? “No” I said “but I think I’m pretty cozy right here. Did you see how pretty the sky is today? Up here. Where we can breathe air? Without monitoring it? “ He laughed and asked if we could just put our regulators in and hold the pier and just let our heads go under. See what it’s like down there. No pressure. We can come right back up. Well ok I could handle that. As I released the air from the BC I found I didn’t drop out of control like I’d feared. In fact I could control it by exhaling. I was still pretty wide eyed and in my head but it was ok. Then Jim my instructor pointed at the piling. There on a slimy black mound of algae was the most fluttery, colorful nudibranch you could imagine. Just slowly sliming his way along the pier. He had spiral spikes coming from his head and soft feathers coming from his back. I’d never seen anything like it. I moved closer and saw scattered below that was starfish in multiple colors laying over each other in a kaleidoscope of colors. Below that anemones opened and closed as they swayed in a gentle back and forth motion. 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I’ve worked as a dive instructor for my entire career and now get to provide an environment for other budding instructors to live their passion and bring more new divers into the world of SCUBA. To this day my favorite part of teaching diving is to those that it doesn’t come easily to. Bring me your terrified, your nervous, your overthinking land lovers that are curious about what it’s like beneath the sea and willing to try something they are scared of. When my good friend Christine expressed interest in giving diving a try I was drop jawed. Since I’d known her she could barely get on a boat. Swimming in a 4 foot swimming pool conjured up fears of the megaladon for her. Perfect I thought. I’m always up for a challenge. Her drive to learn scuba was to be part of her family’s adventures since they were divers. Also she wanted to prove to herself she could face her fears. For many fear of deep water runs deep. Since it’s my passion to help new divers overcome fears and find peace underwater I wanted to learn more about where these fears come from. Here are the main reasons people fear deep water: