Recognition of individual knobby sea stars Protoreaster nodosus (L., 1758) using aboral surface characteristics

Abstract

A photo-identification method was developed to recognise individual Protoreaster nodosus, a large sea star widely distributed in the Indo-Pacific region. Digital photographs of the aboral surface of P. nodosus individuals were obtained from intertidal populations in Singapore between 2002 and 2010. Each photograph was given a numerical code based on the colouration of arm tips, the condition of the arms and the number of tubercles, comprising a total of 17 independent characters, or 27 for developing individuals. Photographs with similar codes were grouped together using a computer and then matched by eye to confirm if they were from the same individual. Natural characteristics not included in the code, such as the arrangement of tubercles and pigmentation pattern, were also used in the recognition process. The natural markings selected were discrete characters that are easily quantified. Based on the variations observed in 4842 photographs of 1817 individuals over 9years, the coding system has the potential of identifying up to 4.24×1021 individuals. The extensive range of natural marks used in the code also ensures that individuals can be re-identified even when some natural marks have become modified due to injury or growth. The code generally remained stable for 2–3years for each individual, and lasted up to 9years in some individuals. The reliability of the photo-identification method was verified by blind studies using temporary tags. This photo-identification method allows the implementation of long-term mark–recapture studies on sea stars, which are currently lacking due to the difficulties in tagging or marking sea stars in the field.