Physiological color change, as opposed to morphological color change, is a rapid change in color (seconds to hours) due to the movement of chromatosomes (pigmentary organelles). Chromatosomes reside in chromatophores (pigmentary cells) which, for fish, mostly reside in the dermis (Heflin et al. 2009; Stuart-Fox and Moussalli 2009; Fujii 2000). This rapid color change may assist some animals in escaping detection from predators. Rather than undergo a whole-body color change, the animal may temporarily display disruptive coloration, which is defined as the use of one or more colors to disrupt the outline of an individual to reduce detection by other organisms (Stevens and Merilaita 2009). The short-term use of disruptive coloration may be advantageous if color change is costly to maintain. This anti-predation strategy has been documented in fish, such as the blackspotted rockskipper (Entomacrodus striatus), and in invertebrates, like the common octopus (Octopus vulgaris) (Heflin et al. 2009; Hanlon 2007). Such strategies are examples of crypsis, which has been defined as any organismal trait that reduces the risk of detection by other organisms when visible (Stevens and Merilaita 2009). Research on color change as a defense mechanism in aquatic animals is limited, with most scientific publications focusing on cephalopods and freshwater fish (Hanlon 2007; Armbruster and Page 1996).

This report documents previously unreported physiological color change in a marine fish species, Arothron meleagris. Five individuals of the spotted morph of the guineafowl puffer, Arothron meleagris (Lacepede, 1798), were observed to rapidly form patterns of irregular bands overlaying their typical color pattern of evenly dispersed spots on the body (Fig. 1a; Supplementary Table 2). All videos were recorded with a GoPro Hero 8 camera (GoPro, Inc., San Mateo, CA, USA) while freediving to a depth of ~5 m. These observations occurred between November 24, 2022, and December 2, 2022, in rock and coral reefs off the coast of Cuajiniquil, Costa Rica.

Fig. 1
figure 1

Video stills showing two instances of the progression of rapid color change in (a) the same individual Guineafowl puffer (Arothron meleagris) in Costa Rica. When disturbed in the water column, (b) this individual sought refuge and within seconds of becoming stationary, curled in its fins; (c, d, e), and presented a blanched banding pattern on its snout, brow, back, and under the eyes (Supplementary Fig. 1)

Full size image

Initial observations of one individual were made on December 2, 2022, at Isla Cocinera at a depth of 4 m. Upon approach, this individual retreated under a rock overhang with its right side exposed (Fig. 1b). The caudal fin was pulled to one side, the dorsal fin was down, and the pectoral fins were stationary. Blanched bands appeared on the back, brow, snout, and under the eyes muting the white spots (Fig. 1c). During the 10 s while the observing diver surfaced for air, the individual had not moved from the area, but it began undulating its fins and the banding disappeared. As the diver approached the pufferfish, the specimen retreated between the rocks of the overhang until it settled under a different overhang and, facing outward towards the opening of the second overhang, proceeded to reposition its body until its tail was wedged underneath. It quickly pulled in all its fins and sat stationary, at which point the banding coloration again became visible (Fig. 1d, e; Supplementary Video 1-2).

Similar observations were made of four additional individuals using the same methodology (Supplementary Table 1). All individuals were stationary and had their tails tucked towards their bodies when the bands were present. In exposed sites, individuals would exhibit the same behavior sequence while continuing to undulate their pectoral fins (Supplementary Table 1). The fish gained or lost the banding in less than 15 s (Supplementary Table 1).

It is hypothesized that freshwater fishes with dark bands achieve crypsis through disruptive coloration (Armbruster and Page 1996). Specifically, the light spaces between the bands may resemble rocks and the dark bands may resemble shadows or gaps between rocks. Similarly, it was observed in this marine fish; the blanched bands might resemble corals, or sand in the substrate where the dark areas of the body may resemble shadows, or rocks. The bands on many freshwater fishes have conspicuous outlines analogous to the smooth rocks in their environment, whereas the bands of the Arothron meleagris specimens observed in this study are variable sizes with blotchy outlines (Supplementary Fig. 1b). This variation may be advantageous given the fact that their natural habitat is dominated by irregularly shaped corals with ample areas of interspersed shading (Supplementary Video 3). Therefore, this irregular banding pattern may be an example of cryptic coloration used to reduce the risk of detection (Stevens and Merilaita 2009). Future studies would be needed to verify whether this disruptive coloration is in fact aiding in crypsis and whether it also occurs in other Tetraodontidae species.