Setal descriptions, locations, and abundances: Their impacts on decorating behaviors in the spider crab Libinia (Crustacea: Epialtidae)

Abstract

The setal abundances and distributions of the spider crab Libinia are examined, using two species. These spider crabs decorate their bodies with environmental materials and camouflage from predators. These decorations attach to hooked setae, but other types setae are prevalent in body regions where decorations occur. Adult crabs undergo a terminal molt and lose the ability to regrow setae. Little is known about setal changes over the lifetime in these crabs and if hooked setae abundance decreases with age, resulting in a loss of camouflage strategy in larger individuals. The objective of this study is to determine the prevalence and abundance of hooked and non‐hooked setae in the decorated body regions and if these setal patterns vary by species and body size. Scanning electron microscopy is used along with computer software to describe, count, and measure the setal coverage of hooked and non‐hooked setae in body regions. Small individuals have more hooked setae than larger individuals and the two species have different setal abundances of hooked and non‐hooked setae. Non‐hooked setae cover much surface areas of crabs, attach decorations, and remain on the crabs even when hooked setae are damaged and broken. Setal morphologies and fouling are described among different sized crabs and the two species, with setal types and morphologies being similar but fouling being different. Large individuals likely employ a different camouflage strategy than small individuals due to losing hooked setae but retaining non‐hooked setae.

Role of Body Size and Shape in Animal Camouflage

“…In addition to the inherent disruptive coloration in animals, another form of camouflage strategy also helps achieve disruptive effects through decoration. Wortham (2012Wortham ( , 2021 discovered that two spider crab species, Libinia dubia and L. emarginata , could utilize available materials in their environment to adorn their carapaces with hooked bristles, creating false edges disrupting the body outline. This behavior was more prevalent in smaller spider crabs, and as individuals matured, the number of hooked-end bristles decreased in larger spider crabs, prompting the adoption of alternative defense strategies.…”