In the Grip of Sleep

Sleeping filefishes Indo-Pacific and Caribbean
“Sleeping” filefishes Indo-Pacific and Caribbean

One of our favorite suggestions for fish watchers in the Caribbean is to look for small one-to four-inch Slender Filefish, Monacanthus tuckeri, hiding out within gorgonian bushes, where they shelter from predators, feed, and even more delightful, sleep or rest at night by biting down on polyps for stability – especially helpful when the current runs. This behavior turns out to be a common family trait. In the Caribbean, we have also observed the Whitespotted Filefish, Cantherhines macrocerus,  Orangespotted Filefish, C. pullus, and the Scrawled Filefish, Aluterus scriptus, holding on with their mouths while tucked into the reef. In the Indo-Pacific, we’ve seen Bristle-tailed and Strapweed Filefishes exhibit the same behavior. It appears that filefishes adopt this behavior early in their life cycle – long before they settle onto the reef: 

Larval filefish holds onto a coral polyp
Post-larval filefish, probably Aluterus sp., holds onto a coral polyp. Image used with permission, courtesy of Deb Devers
A young filefish holds onto a jelly
Young filefish, possibly Aluterus monoceros. Used with permission, courtesy of

Blackwater diving has become increasingly popular over the past five years, and social media has given us unprecedented opportunities to see what our fellow divers are observing while drifting at night in offshore waters around the world. A few years back, I noticed the first of many blackwater images of filefishes holding onto things, mostly jellies. Some looked quite young, like the one taken by Deb Devers, of a filefish holding onto a coral polyp. In others, such as the photo by Songda “Wowie” Cai, the filefish appears to be much older. When the image by our friend Nicola Kundrun popped onto my screen two weeks ago, things became even more interesting. This filefish in her image was holding onto a seahorse! Nikki admits she thought she was photographing a filefish latched onto a piece of sea grass but after downloading the photo, her sprig of grass turned out to be a seahorse. 

Larval Filefish holfing onto a seahorse
Seahorse, seagrass…just catchin’ a ride. Tiny larval filefish is holding onto a seahorse! (used with permission @Nicola Kundrun).

While we haven’t personally seen anything as entertaining as a filefish hitchhiking on a seahorse, we did capture photos and video of a tiny filefish holding onto a jelly during a night drift in Balayan Bay, Philippines, this past February:

For me, this was coming full circle from an earlier observation that really intrigued me. In 2008, we published a magazine article and video about the Slender Filefish sleeping on the reef while holding onto a gorgonian polyp. After observing other filefishes in both the Caribbean and the Indo-Pacific doing the same, we updated the video and wrote a 2012 blog post here on BlennyWatcher. After reading our post, Gal Eyal, sent me a paper published in Coral Reefs, documenting what he calls “Teeth-anchorage” in sleeping filefish in the Red Sea. It was the first time we heard of an official name for the behavior. Hunting online, I found a paper written in 1998 by Jeff Childs, documenting what he calls “nocturnal mooring and parking behavior” involving several species of filefishes in the Gulf of Mexico. His paper describes two “sleeping” behaviors – one, holding on with their mouths, and the other, parking themselves in places, out of the current, on the oil platform where he was diving.

Aquarists have long been aware of their filefishes sleeping by holding onto something with their mouths or wedging themselves behind structures in the tank. During a night dive in Indonesia, we saw one species, Pseudaluttarius nasicornis, using its file to lock into a sponge. There was a strong current and this was on a muck site in Lembeh Strait, without a nearby reef, so the filefish tucked itself into a sponge, deploying what Childs calls “parking behavior”.

Pseudalutarius nasicornis, Rhinoceros leatherjacket
Pseudalutarius nasicornis, Rhinoceros leatherjacket uses its spine to hold itself in the current

One of the many joys of fish watching is seeing something for the very first time. It might be a new fish for your life list or a behavior you’ve never noticed before. But one of my greatest pleasures is observing something I’d once read or heard about, with the hope of one day seeing it for myself . “Knowing that something exists” – is one of Ned’s cardinal tips for successful underwater hunting.

In the early 90s, I made a point of reading Dee Scarr’s “What’s That” columns in each issue of Dive Training magazine. I always made a mental note of the wonderful things she wrote about. The closing paragraph in her October 1995 “Fantastic Filefish” article recounted finding a sleeping filefish, “I looked more closely and realized that the filefish had a bit (a polyp, perhaps) of the gorgonian in its mouth. The fish was clearly sleeping, not in the midst of a nibble. It must have grabbed the gorgonian to keep itself in position overnight!” 

Note to self: look for sleeping filefish.

I wasn’t much into diving after dark until about 2005 (that’s a story for another time) so it was nearly a decade after reading Dee’s article, on a night dive in Bonaire, before I saw my sleeping filefish. I was actually attempting to show a cryptic needle shrimp to a friend, when the gently swaying filefish caught my eye. Dee’s description, filed away for over ten years, popped into mind. And sure enough, leaning close we could see the slumbering Slender Filefish had its tiny fish mouth firmly clamped onto a gorgonian polyp!

Slender Filefish sleeping in a gorgonian (Bonaire, 2008)

We spent the next few night dives inspecting every gorgonian we passed, and as often happens, found what we once thought a novel behavior, more common than imagined. In a two-night survey fanning through every gorgonian on Buddy Dive’s house reef, we located a dozen sleeping fish.Fortunately, our night lights didn’t seem to disturb their rest. And best yet, every fish could be counted on to be in the same location night after night, which made sharing the discovery with others a dream. “In the Grip of Sleep” our filefish article published in the February 2008 issue of Scuba Diving magazine added fun and knowledge to Dee’s legacy.

References: “What’s That : Fantastic Filefish “ by Dee Scarr, October 1995, Excerpt, courtesy of Dive Training magazine

Childs, J. 1998. Nocturnal Mooring and Parking Behavior of Three Monacanthids (Filefishes) at an Offshore Production Platform in the Northwestern Gulf of Mexico. Gulf of Mexico Science 16 (2).

“In the Grip of Sleep”  Ned and Anna DeLoach with Paul Humann, Scuba Diving, February 2008  Youtube: and

Eyal, G., Eyal-Shaham, L. & Loya, Y. “Teeth-anchorage”: sleeping behavior of a Red Sea filefish on a branching coral. Coral Reefs 30, 707 (2011).

Adams, Reid & Adams, Ginny & Hoover, Jan & Armbruster, J.. (2003). Oral Grasping: A Distinctive Behavior of Cyprinids for Maintaining Station in Flowing Water. Copeia. 2003. 851-857. 10.1643/i202-261.1. 

“Those Fabulous Filefish!” Scott Michael, Tropical Fish Hobbyist, Sept/Oct 2015