A comparative study of the otoliths and associated labyrinthine apparatus of the New Zealand flatfish (order Pleuronectiformes)

Abstract

A comparative otolith and labyrinth study was conducted on 12 species of New Zealand Flatfish (Order Pleuronectiformes). The otic capsule is taxonomically useful at the generic and on occasions specific level. Three distinct levels of labyrinth partitioning were encountered. The 'right-eyed flounders (family Pleuronectidae, subfamily Rhombosoleinae) possess a relatively simple level of partitioning. The 'southern flounders' (Family Achiropsettidae) have an intermediate stage of partitioning. The witch flounder, Arnoglossus scapha (Family Bothidae) has an advanced stage of partitioning, more akin to active teleost species. An ultrastructural study, utilising the scanning electron microscope, was conducted on the labyrinth of six species of flatfish. The nonsensory regions of the sacculus were found to consist of squamous epithelial cells, while the sensory macula sacculus is composed of a network of sensory hair cells topped with ciliary bundles, and supporting cells. Morphologically the ciliary bundles are divisible into three non-discrete types. A unique ciliary sheath occurred occasionally, which may provide a coupling mechanism between the bundle and the otolithic membrane. Aragonitic otoconia of the crenelated spherulitic habit are found in close association with the macula, often forming dense aggregations. A comparative study was conducted on the otoliths (primarily the sagitta) of 12 species of flatfish. The sagittae of the 'southern flounders' (family Achiropsettidae) are unique, and they do not appear to be related to the Bothidae, with which they are often grouped. The sagittae of Arnoglossus scapha (family Bothidae) are remarkably similar to those of the 'right-eyed flounders' (family Pleuronectidae), perhaps indicating a common ancestor. The sagittae of the 'right-eyed flounders' are diverse and varied. The evolutionary relationships of this subfamily are proposed based on external sagitta morphology. A key is provided to facilitate otolith (sagitta) identification. The otolith (sagitta) microstructure of two species of Rhombosolea: R. leporina and R. retiaria was investigated, based on population sampling and tag returns. A small scale flounder tagging study was conducted in the Ruamahanga River to investigate the periodicity of 'daily' growth increments. Daily increment counts indicated that the nucleus is formed during the first growing season of the fish. The finest growth increments are found to correlate with daily periodicity.