Flow Sensing in the Deep Sea: Morphology of the Lateral Line System in Stomiiform Fishes

Abstract

The deep sea is characterized by extreme environmental conditions including limited light availability, which makes non-visual sensory capabilities quite important. In addition to the visual, auditory, olfactory and gustatory systems found in all vertebrates, all fishes have a mechanosensory lateral line system. This system is composed of neuromast receptor organs on the skin and in bony canals on the head and trunk and is sensitive to unidirectional water flows and low frequency vibrations. Our knowledge about the lateral line system (LL) in deep-sea fishes is limited. Of the taxa in which the LL has been described, there appears to be two morphologies: widened LL canals with large canal neuromasts, and a reduced LL canal system with a proliferation of superficial neuromasts (SNs). However, the one published description of the LL in a species of the prominent midwater order Stomiiformes, suggests that there is a third LL morphology defined by a reduction in canals on the head accompanied by just a few, small SNs. The goal of Chapter 1 was to use traditional and modern morphological methods to provide the first detailed description of the LL system in two groups of stomiiform fishes (Argyropelecus [Family Sternoptychidae] and Cyclothone [Family Gonostomatidae]) as well as other fishes in the Families Gonostomatidae, Phocichthyidae, and Stomiidae in order to test the hypothesis that reduced canals and a reduced number of SNs is a strategy in the evolution of the LL in stomiiform fishes. A total of 27 species in 17 genera in four families were studied using one or more morphological approaches (including histology and micro computed tomography, or µCT). In depth analysis of four Argyropelecus species (Sternoptychidae) revealed the presence of several incompletely ossified LL canals on the head, including the supraorbital (SO), mandibular (MD) and preopercular (PO) canals. Examination of whole preserved specimens of other taxa also revealed the presence of SO, MD, PO, and IO (infraorbital) canals with varying degrees of ossification. Few species had all canals typically found in bony fishes, but all taxa, with the exception Cyclothone species, had some cranial LL canals. A proliferation of SNs was revealed using histological material and scanning electron microscopy in fishes in the families Sternoptychidae, Gonostomatidae, and Stomiidae. These domed, circular structures (= “white dots”) stood out against darkly pigmented skin and were visible under low magnification in very well preserved specimens. They appeared to be innervated, and had a similar morphology and