Rafting

Histrio histrio, Sargassum fish (NOAA)

 

A few years back, maybe more than a few years, my friend Rich asked if I wanted to go rafting. He had gotten some tickets for a white-water rafting trip. I had never done such a thing, and, when offered a free ticket, I said yes. It was a great trip. Rafting, eating lunch by the riverside, and then more rafting and some floating in the quiet parts of the river.  Rafting then was all the rage, and I could see why. What fun. When I read the title of a paper in the journal Biological Invasions, So you think you can raft?, my answer was the same as the one I gave Rich some years ago: Yes. Then I read the second part of the title, traits that enable fish to survive and disperse with floating objects.

 

Although the paper is not about the rafting that first came to mind, it is also not specifically about the fish that second came to mind: the sargassum fish, Histrio histrio, which has adapted to floating with chunks of sargassum weed. The fish does appear in the paper; however, despite these adaptations, this fish is not the focus of the paper. Rather than focusing solely on Histrio, which is another paper I would like to read, the authors examine why some reef fishes survive long journeys on drifting objects, whereas most do not. The paper then is a review of what is known of the state of rafting fishes. 

Here is some of the natural raft material, Sargassum weed.

Besides acting as rafts, Sargassum blooms can cause beaches to be littered with the huge weed mats:

A little bit of Sargassum on the beach. That was sarcasm.

The sargassum fish looks less like a fish and more like a fragment of seaweed that grew eyes. Its mottled yellows and browns match the algae perfectly. Small fleshy appendages break up its outline, turning the body into a frond. Camouflage here is not decorative; in a habitat with little cover beyond the algae itself, blending in is survival.

Sargassum weed and a Sargassum fish. 

Unlike most pelagic fishes built for speed, the frogfishes, which contain the Sargassum fish and other fish that don’t look like fish, are built for patience, the sit-and-wait lifestyle. Its pectoral fins are limb-like, allowing it to climb through the weed. There is no chasing down prey. It waits. Extending from the top of its head is a modified dorsal spine tipped with a lure. A subtle twitch and curious juveniles of other species, small crustaceans, anything investigating the movement can disappear in a sudden, vacuum-like strike. The mouth opens, water and prey rush inward, and the illusion of seaweed snaps shut.

 

Its entire life unfolds adrift. Eggs are released in the rafts of weed. Juveniles remain in the weed. Adults rarely leave it. This is a pelagic species that behaves like a reef ambush predator, because its reef is a moving habitat carried by currents. 

Woops, got waaaaay off track, back to the study:

This study of rafting fishes sits at the *node of invasion biology, climate change, and plastic pollution. Floating debris, especially the new plastics in town, becomes more abundant and long-lived, serving as a mobile habitat. According to authors studying the Western Mediterranean, plastics have become the largest source of rafts. These rafts cross oceans, possibly delivering coastal fishes far beyond their historical ranges.

Note: *node is a word I don’t often use, but I realize it came to mind because I have just been discussing phylogenetic trees with my students, which contain nodes.

 

Their literature review of rafting fish species identified 75 studies and 460 species associated with floating objects. However, only 103 species appeared in more than three studies, suggesting consistent rafting among those 100-ish species. Most research has focused on drift algae, with comparatively less attention to the man-made stuff, especially plastics. So, rafting is way more popular than the old data tells us! At least that is the premise.

 

According to the authors, although rafting fish are more likely to have laterally compressed (deep) bodies, there does not appear to be a single ideal rafter. Deep-bodied species can dart into crevices and maneuver through tight structures. Fast, open-water predators such as dolphinfish or amberjacks use rafts as waypoints in an otherwise featureless ocean. Schooling adds another layer. Rafts appear to function as meeting points, helping fish form and enlarge protective aggregations. Schooling is strongly associated with rafting.

 

Rafts may amplify classic schooling benefits, such as predator confusion, improved foraging, and enhanced detection, by concentrating individuals around a visible landmark.

All interesting information, but the crux of the study is that rafting can provide a pathway for potentially invasive species to cross oceans. Rafts are the vector in this case. Especially enhanced in a tsunami. Tsunami-generated debris has been known to carry fish across the Pacific. And plastics, being pervasive and persistent, offer a huge mode for more rafting. More floating plastic debris = more opportunities for long-distance dispersal. Besides our golden child, the sargassum fish, identifying which fish are predisposed to rafting may help anticipate future range shifts and invasions in a warming, debris-filled sea.

Plastics acting as a fish raft.

Back to the Sargassum fish, the ultimate rafting species. Oh, wait, now I’m making this post too long, more on this awesome fish later. 

 

Sources and Further Readings:

Photo credits, in order: NOAA, Wikipedia, Virginia Institute of Marine Science, Bryce Groark, H Oxenford.

Benadon C, Carlton JT, Atalah J, & Nelson P. 2026. So you think you can raft? Traits that enable fish to survive and disperse with floating objects. Biological Invasions 28 Article 40. https://doi.org/10.1007/s10530-025-03751-w

Carlton JT, Chapman JW, Geller JB, Miller JA, Carlton DA, McCuller MI, Treneman NC, Steves BP, Ruiz GM. 2017. Tsunami-driven rafting: transoceanic species dispersal and implications for marine biogeography. Science 357: 1402–1406/

 

Riera F, Grau A, Grau AM, Pastor E, Quetglas A, Pou S. 1999. Ichthyofauna associated with drifting floating objects in the Balearic Islands (Western Mediterranean). Scientia Marina 63: 229–235.