The Rounded Porcupinefish (Cyclichthys orbicularis)

I fell in love with this image and knew this little guy deserved a blog post all to himself. Meet the Rounded Porcupinefish (Cyclichthys orbicularis), a fascinating member of the Diodontidae family. With only about 20 known species across seven genera, these spiny oddballs are a relatively small group in the ocean, but their charm is undeniable.

Closely related to pufferfish, porcupinefish share a similar defense mechanism: when threatened, they inflate their bodies by swallowing water or air, causing their spines to stand upright and deter predators. It’s an effective and dramatic defense strategy that has earned them their balloon-like nickname.

A Personal Encounter:

I’ve had the joy of encountering these calm, curious fish while diving in Indonesia and the Maldives. They’re larger than expected, typically around 12 inches in length and surprisingly serene. I remember hovering nearby, watching as their expressive eyes studied me, with what seemed like genuine curiosity before they slowly drifted away.

The image above, captured by Nick Hobgood, perfectly captures that quiet curiosity. The fish’s large, detailed eyes almost seem to tell a story. To me, this photo reflects the magical stillness and wonder of underwater life.

Tetrodotoxin (TTX) Uptake in Porcupinefish:

Tetrodotoxin is a potent neurotoxin famously found in pufferfish, but what about their spiny cousins?

A 2006 study by Noguchi and colleagues tested TTX uptake in liver slices of various species. Porcupinefish (Diodon holocanthus and D. hystrix) exhibited minimal TTX accumulation (approximately 0.1 µg/mg), in contrast to highly toxic pufferfish, which stored over ten times as much.

This suggests that while some porcupinefish can carry small amounts of TTX, they are far less dangerous than their puffed-up relatives.

Habitat, Behavior, and Diet:

According to FishBase, porcupinefish like Cyclichthys orbicularis are circumtropical, meaning they’re found in warm oceans across the globe. They inhabit depths of 2 to 200 meters, most commonly between 2 and 35 meters, and prefer coral reefs, lagoon areas, and soft-bottom habitats.

They are nocturnal feeders that hunt at night for hard-shelled prey such as snails, sea urchins, and crabs. During the day, they are slow swimmers that shelter in rocky crevices or under coral to avoid predators.

Juvenile Development and Swarming Behavior:

A 2019 study commissioned by the USGS and led by David R. Robertson found that juvenile porcupinefish can form large pelagic swarms, drifting in the open ocean during early life stages. As they grow, they settle closer to reef habitats, often during nighttime hours. Despite visual differences between juveniles in different regions, genetic testing shows that these are all the same species, just morphologically diverse.

Ecosystem Role and Parasites:

Porcupinefish play important roles in marine food webs. As mid-level predators, they help control benthic invertebrate populations. As prey, they are hunted by larger animals such as dolphinfish and reef sharks. They also host a variety of marine parasites, including copepods and trematodes.

Final Thoughts:

Whether you're a diver, a marine biologist, or someone who loves the ocean’s mysteries, this species is a reminder that the sea still holds wonders, some spiny, slow, and full of soul.

References:

Froese, R., & Pauly, D. (Eds.). (2024). FishBase. Diodon holocanthus summary page. Retrieved from https://www.fishbase.se/summary/Diodon-holocanthus.html

Kuiter, R. H. (2014). Fishes of the Maldives: Indian Ocean. Atoll Editions.

Myers, P., et al. (2021). Diodon holocanthus. Animal Diversity Web. University of Michigan Museum of Zoology. Retrieved from https://animaldiversity.org/accounts/Diodon_holocanthus/

Noguchi, T., Arakawa, O., & Takatani, T. (2006). TTX accumulation in pufferfish. Toxicon, 48(7), 713–723. https://doi.org/10.1016/j.toxicon.2006.07.014

Noguchi, T., Onuki, K., & Arakawa, O. (2011). Tetrodotoxin poisoning due to pufferfish and gastropods, and their intoxication mechanism. Marine Drugs, 9(12), 2483–2516. https://doi.org/10.3390/md9122483

Robertson, D. R. (2019). Field Guide to the Juvenile Porcupinefish (Diodon holocanthus): A Circumtropical Swarm Species. Smithsonian Tropical Research Institute. Retrieved from https://biogeodb.stri.si.edu/caribbean/en/thefishes/species/3906

Su, F. Y., Bushong, E. A., Deerinck, T. J., Seo, K., Herrera, S., Graeve, O. A., Kisailus, D., Lubarda, V. A., & McKittrick, J. (2017). Spines of the porcupine fish: Structure, composition, and mechanical properties. Journal of the Mechanical Behavior of Biomedical Materials, 73, 38–49. https://doi.org/10.1016/j.jmbbm.2017.02.029