Otolith development and daily increment formation in larvae of the Kabyabya, a Malawian cyprinid, Opsaridium tweddleorum

Abstract

Features of the sagitta, asteriscus, and lapillus of laboratory-hatched larvae of the Kabyabya, Opsaridium tweddleorum, were investigated until day 33 after hatching. The sagitta was round until day 17, subsequently becoming arrowhead-shaped with the development of anterior and posterior rostra. Increments in the sagitta increased at the rate of one per day before rostrum formation. However, they were occasionally overestimated in rostrum sections as a result of subdaily increment occurrence. Furthermore, both anterior and posterior rostra were fragile, being thin and platelike, and were often damaged during the extraction and grinding processes. The lapillus was also round until day 17, thereafter becoming fan-shaped. Increments in the lapillus were distinctive from the core to the margin, usually increasing at the rate of one per day after hatching. The asteriscus appeared in fish larger than 9.15 mm in standard length (from day 17), being oval with a somewhat ambiguous core, causing difficulty in discerning the first increment. These features indicated that the lapillus was the most appropriate otolith part for daily increment analysis in this species, although the sagitta was useful before rostrum formation.