Ecological consequences of supernumerary arms in eastern Pacific sea stars

Abstract

Pentameric symmetry characterizes echinoderms and is most readily observed in sea stars. However, some pentamerous sea star individuals deviate from the normal 5-arm pattern, most likely as a result of errors in regeneration, but the frequency of these deviations and their ecological consequences for the individuals are poorly understood. Here, we report the extent of deviations from pentamery in multiple populations of co-occurring sea star species and tested 3 potential advantages of supernumerary arms—increased oral surface area, increased feeding, and faster righting response—in 1 species, the bat star Patiria miniata. Using underwater surveys and behavioural experiments at 16 sites in Barkley Sound, British Columbia, Canada, we found individuals with atypical arm numbers in 5 of 9 pentamerous sea star species. There were significant differences among sites and species, with site-specific mean percentages of atypical sea stars ranging from 0.8 to 14.6% and species-specific mean percentages ranging from 0 to 10%. Bat stars had the highest proportion of individuals with atypical numbers of arms, with site-specific frequencies ranging from 0 to 25%. The probability of feeding and righting speed were similar between bat stars with and without supernumerary arms, but bat stars with supernumerary arms had slightly larger oral surface areas for a given arm length, which could confer an advantage in adherence to the substrate. Although the advantages of supernumerary arms for bat stars appear to be negligible, the lack of any clear disadvantage suggests that selection against atypical variants might be weak, leading to limited selection on the accuracy of regeneration in this species.