Why habitat architecture and color are important to shrimps living in pelagic Sargassum: use of camouflage and plant-part mimicry

Camouflage and plant-part mimicry, 2 forms of resemblance of animals to their habitat, were investigated for 2 species of shrimps living in pelagic Sarqassum natans (Linnaeus) Gaillon in the Sargasso Sea.We used field collections and laboratory experiments to correlate the size, shape, color, and behavior of Latreutes fucorum (Fabricius) and Hippolyte coerulescens (Fabricius) with the architecture and color of the Sargassum habitat (i.e. the number, size, shape and arrangement of structural components of the habitat).Components of the structural architecture of Sargassum and plastic plants were measured and related to shrimp number, body size and shape.Latreutes most resembled Sargassum fronds, while Hippolyte resembled the gas-filled bladders (vesicles).Behavioral observations showed that Latreutes characteristically clings to the fronds and Hippolyte to the main stalk and vesicles of Sargassurn plants.In habitat selection experiments, the shrimps were offered Sargassum and arhficial algae with hfferent architectures and colors.Both species were found in equal densities on Sargassum and artificial algae suggesting that food value was unimportant relative to habitat architecture.When offered a modified habitat architecture of Sargassurn, Latreutes preferred a 'fronds only' to a 'vesicles only' plant while the reverse was true for Hippolyte.When plastic plants wlth small and large fronds were offered, small Latreutes showed a significant preference for small-frond plants, while large shrimp showed no preference.This was also true in the presence of a predator.Latreutes chose yellow artificial plants significantly more than brown artificial plants but Hippolyte chose both plants equally.In general, small shrimps of both species tend to be solid-colored or transparent, while large shrimps have disruptive color patterns corresponding to their habitat.It may be that there is an ontogenetic shift from use of plant-part mimicry to camouflage.As shrimps outgrow the specific plant parts they mimic, they use color patterns to camouflage themselves and generally resemble the Sargassum habitat matrix.In this system, camouflage and plant-part mimicry may depend, in large part, on the size, shape and color of the shrimps relative to elements of the Sargassum habitat, and may change in importance as the shrimps grow larger.The importance of fish predation is discussed in relation to the shrimps and fishes found in Sargassum collections from this study.