Population size structure, asexual reproduction, and somatic growth estimates of the non-indigenous brittle star Ophiothela mirabilis (Echinodermata: Ophiuroidea) on the southeastern coast of Brazil

Abstract

The ophiuroid Ophiothela mirabilis Verrill, 1867 is a potentially invasive non-indigenous species on the Brazilian coast. Here, we describe the population size structure and the asexual reproduction of this species and estimate its somatic growth in two localities within a distance of 300 km from each other: Angra dos Reis and Armacao dos Buzios. Specimens associated with two different host sponges were collected monthly from both sites over 18 months and classified into three categories: individuals with whole discs, regenerating discs, and split discs. Of all specimens collected, 59.6% had whole discs, 23.7% had regenerating discs, and 16.8% had split discs. No evidence of recruitment through larval settlement was found and fission occurred throughout the year. Therefore, asexual reproduction seems to be the main mechanism for population maintenance and growth. The highest frequency of smaller specimens (region delimited by adoral shields < 0.8 mm) was associated with the highest ophiuroid density (1.78 ± 0.63 ind ml−1 of sponge tissue), which was recorded in population from Angra dos Reis. The asymptotic size was similar in the two populations: 1.184 mm in Angra dos Reis and 1.200 mm in Armacao dos Buzios. The growth rates were estimated using length-frequency analysis. For the Angra dos Reis population, the growth rate estimate was 1.71 mm year−1, while for the Armacao dos Buzios population was 2.21 mm year−1.