Population and reproductive ecology of Turbo smaragdus in the Kaikoura region

Turbo smaragdus is a herbivorous gastropod of the intertidal and sub-littoral zones, which is distributed widely along the coastline of New Zealand. This study sought to investigate the population and reproductive ecology of Turbo on the Kaikoura Peninsula. This involved the determination of: spatial and temporal distribution patterns, associations with other herbivorous molluscs and algal species, recruitment of juveniles, morphometric relationships, population length-frequency structures, growth rates, spawning seasons, trends exhibited in a reproductive cycle, sex ratio, size at sexual maturation and fecundity. Turbo have a broad distribution that varies significantly both within and between sites. Transect surveys determined that Turbo have an aggregated distribution, and were present from the upper eulittoral into the sub-littoral to depths of 3m with largest numbers generally occurring in the mid-eulittoral. Juveniles (shell lengths ≤ 15mm) occurred widely on the vertical profiles of the shores sampled, but larger individuals were more common in the lower eulittoral. Sub-littoral populations were composed of larger individuals, with shell lengths ≥ 35mm. As Turbo grow, shell and operculum height, width and length increase linearly and shell, operculum, body tissue and gonad weight exponentially. Tag-recapture data indicate that growth rates decrease with increasing size of individuals. At the sites sampled, individuals can grow to a shell length of 40mm at which the von Bertalanffy growth curve predicts they will be approximately ten years of age. Growth rates of Turbo vary seasonally, with greatest rates occurring between November-March, and the slowest between August-November. Spatial variation was also shown to occur between two populations, with greater growth rates occurring in the area of slightly greater exposure to wave action. The absence of age classes in length-frequency histograms suggests that recruitment failure or high mortality rates may occur during some years. Sexual maturation of both male and female Turbo generally occurred between shell lengths of 20-25mm, although a few individuals were observed to mature at both smaller and larger sizes. Energetic investment in reproductive effort, which is indicated by increasing gonad size and fecundity, increases with increasing shell lengths. The populations sampled exhibited a 1:1 sex ratio. Turbo are broadcast spawners and have a distinct annual reproductive cycle. A major spawning event occurred on the Kaikoura Peninsula during February-March 1991 and a minor event in January 1992. Immediately succeeding the 1991 spawning period, the gonads of both sexes were reduced in volume and relatively depleted of mature gametes. Gametogenesis occurred within several months of spawning, with the immature gametes developing slowly throughout the winter and accelerating in growth in the spring to produce gonads that were densely packed with mature gametes several months prior to the January 1992 spawning event