Cryptic coloration as a predator avoidance strategy in seagrass arrow shrimp colormorphs

The arrow shrimp, Tozeuma carolinense, is a highly abundant small crustacean inhabiting seagrass beds along the US southeastern and Gulf coast. These shrimp inhabit the tops of seagrass blades, feeding on epiphytic algae. Despite T. carolinense abundance, it is largely underrepresented in the diets of predatory fish, presumably due to its cryptic morphology and behavior. Here, we quantify the degree of crypsis of green and brown colormorphs of T. carolinense to the visual systems of two potential predators, dichromatic Cynoscion nebulosus and trichromatic Sciaenops ocellatus, using a visual chromatic contrast model. Contrast of shrimp colormorphs viewed against seagrass backgrounds was similar between predators but varied greatly for both colormorphs among the backgrounds. For both dichromatic and trichromatic predators, green morphs produced significantly less contrast than brown morphs on live seagrass, while brown morphs produced significantly less contrast than green morphs on dead seagrass. Each colormorph had at least one complimentary seagrass species against which it would be difficult for fish predators to discriminate. Model predictions were largely borne out in a survey of a natural population of T. carolinense in Tampa Bay, FL (USA), where shrimp were found only in monospecific patches of live Syringodium filiforme, and green morphs significantly outnumbered brown morphs. Although these shrimp do not rely solely on cryptic coloration for predator evasion, the ability of T. carolinense to match its background does minimize visual contrast and likely contributes to their lack of representation in the diets of predatory fish.

Research highlights
► We quantified crypsis of Tozeuma carolinense to fish predators using a visual contrast model.
► Green-morphs had less contrast than brown on live seagrass, with the opposite on dead seagrass.
► Model predictions were largely borne out in a survey of T. carolinense in Tampa Bay, FL (USA).
► Background matching in T. carolinense minimizes visual contrast and aids in predator avoidance.