The origin and formation of hair on external valve surfaces of the tropical marine mussel Modiolus traillii (Reeve, 1857)

Abstract

Direct observations using digital videography in an aquarium showed that hairs on the valve surfaces of Modiolus traillii are not periostracal in origin, as often assumed for similar features in other mussels, but are secreted and planted individually by the foot, so they are correctly termed 'byssal hairs'. Deposition of byssal hair occurred most frequently just after dark, and the time taken to form a strand of serrated hair varied between slightly more than 1 min to almost 10 min, with a mean time of 4.4 min. We observed one individual deposit a total of 11 hairs in 3.5 h. The formation of some 25 hairs were successfully observed on video for six individuals, and about 3,000 individual hairs from 12 mussels were measured in relation to their position, size and density. While hairs varied considerably in length and width, they are consistently flattened, with one edge bearing serrations and the other edge being smooth. Long hair required more time to form compared with shorter hair, and longer hair was deposited farther away from the byssal gape towards the posterior end of the valves. However, a higher density of short hair was laid around the byssal gape. Smaller mussels tended to have an overall higher density of hair compared with larger individuals. There was no discernible pattern in the order in which long and short hairs were secreted on the periostracal surfaces of either valve. Byssal hair microstructure was generally consistent with the distal region of byssal threads, having a tough but thin outer cortex surrounding a 'spongy' honeycomb matrix. © The Author 2014.