Sigmoidal growth in the brittle star Amphiura filiformis (Echinodermata: Ophiuroidea)

Abstract.

The post-larval growth pattern and dynamics of Amphiura filiformis (Echinodermata: Ophiuroidea) were investigated in 1991 at the sill of the Gullmarsfjord. A total of 201 specimens were examined in detail by video-techniques to analyse the growth patterns of disks, arms and arm segments. Maximum settling was recorded in October, with 7,100±7,400 (mean±SD) ind. m^–2, after which settlement rapidly declined. The initial size of the settlers was around 300 µm in disk diameter with one arm segment developed. Specific growth rates during August–November were 0.42% day^–1 for disk diameter and 1.76% day^–1 for mean arm length. Growth of the oral width between 1.5 and 3 mm (3.6–9.7 mm disk diameter) shows a von Bertalanffy growth pattern, and the best fit for early growth was a model in which disk size increased exponentially with time. When combining the juvenile growth curve with adult growth curves from the literature, an asymptotic sigmoidal pattern appeared, which was best described by a Gompertz curve, suggesting that adult size and sexual maturity (ca. 4 mm disk diameter) are attained around 2 years after settlement. The inflexion point in the sigmoidal growth curve occurs ca. 1 year after settlement, at an approximate disk diameter of 2 mm. The higher growth rate after the early phase of life is suggested to be attributable to the attained capability of suspension feeding at this size, which in this species may be a more efficient means of acquiring energy than other modes of feeding. Predictions of disk diameter (up to 6 mm) from the reconstructed growth curve correspond well with measurements from the North Sea and elsewhere.