Turtle Crap: It’s What’s for Dinner (if you’re a batfish).

The batfish* is one of the most common reef denizens in Southeast Asia, and also one of the most pleasant. They’re large, triangular-bodied fish, and they move with the undisturbed placidity of organisms that are neither predator nor prey. They tend to congregate in schools of anywhere from 10 to 100 individuals, and resemble nothing so much as beautiful, silvery sheep.

*According to most guidebooks, “batfish” is synonymous with “spadefish.” But in reality the spadefish make up an entire family, Ephippidae, of which batfish (genus Platax) are merely the largest component. All batfishes are spadefishes; all spadefishes are not batfishes. Get it, got it, good. (The term “batfish” can also refer to a separate family of crazy-looking bottom dwellers, cousins of anglerfish, but that’s not what we’re talking about here.)

Batfish are often seen glued to the hips of green turtles, the awkward Sancho Panza to the reptile’s Don Quixote. But they’re not squiring the turtles – instead, batfish eat turtle feces, fresh from the source.

That just could not look any tastier than it does.

I haven’t been able to find much information about what, exactly, makes turtle droppings so scrumptious for batfish. Green turtles and batfish have similar diets, heavy on vegetation and small crustaceans, so it could be that batfish simply have an easier time breaking down food that’s been pre-digested; or perhaps the turtles actually produce and excrete some salutary nutrient. Whatever the case, nearly every swimming turtle is accompanied by a batfish escort.

Thanks to their compatible diets, batfish are capable of playing the same ecological role as turtles: top-level lawnmowers. This is important, because in areas that have been denuded of turtles, dugongs, and large fish, harmful seaweed frequently proliferates and chokes out coral. While other herbivorous fish are unwilling or unable to make a dent in the thick vegetative mats, batfish chow down on the seaweed and prevent it from becoming hopelessly overgrown. Without batfish, reefs would be far more vulnerable to weed invasion.

Batfish are also one of many fish varieties whose juveniles differ dramatically in appearance. Young batfish are vertically elongated, as though their entire bodies consist of magnificent tails. As batfish age, they slowly fill out horizontally; they transform from elegant juveniles into ungainly adults.

This youngster imitates an inedible flatworm, and does it well. Courtesy of http://www.seaphotos.com

The reason for their unusual juvenile morphs is mimicry. Although adult batfish are too large to be frequently preyed upon, the young make tasty morsels. Thus they’ve evolved unpalatable shapes to avoid predation. Fascinatingly, each species of batfish imitates something different – for example, young round batfish resemble floating leaves. Other species mimic crinoids (echinoderms known as feather-stars), clumps of seaweed, and poisonous flatworms. (No reports on whether batfish are yet evolving to mimic floating plastic bags, although I suspect it’ll be any day now.)

Apparently there are Ephippidae in Caribbean waters, so we’ll still get our spadefish fix in Honduras. The five species of the genus Platax, however, are endemic to Indo-Pacific waters, so I may have seen my last true batfish for quite a while*.

*In classic fashion, everybody agrees there are five species, but no one can agree about what they’re called. For example, the species that I refer to as the Pinnate Batfish is, according to the website What’s That Fish?, also called the Shaded Batfish, Red Rimmed Batfish, Red-stripe Batfish, Red-faced Batfish, Red-faced Orbic Batfish, Red-finned Batfish, Longfin Batfish, Long-finned Batfish, and Dusky Batfish. I mean, come on.

***

Finally, FishiLeaks is excited to announce the arrival of this month’s Carnival of the Blue, hosted by the estimable Beach Chair Scientist. CotB is a compilation of the month’s best ocean blogging, and my post about moray eel anatomy and behavior made the cut! There’s all kinds of cool stuff in this month’s edition; check it out.

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