Flow sensing in the deep sea: Morphology of the lateral line system in Stomiiform fishes
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 distribution on the head and/or body in all species. Thus, it appears that there is a proliferation of SNs in many stomiiform genera, that the initial description of just a few SNs in Argyropelecus is incorrect, and that the LL is more important to these deep-sea fishes than has been previously suggested. ^ The discovery of a proliferation of SNs in these fishes made it necessary to be able to distinguish them from the numerous bioluminescent photophores and other structures found in in the skin of these fishes. In Chapter 2, specimens were imaged under different wavelengths of light as a new tool to distinguish SNs from both complex and simple photophores. A total of 34 stomiiform species (in four families) and representatives of two other deep-sea taxa (Myctophidae, Melamphaidae) were examined under brightfield illumination, and at three different excitation wavelengths (390, 470, 545 nm). Complex photophores were visible under all wavelengths, while SNs and simple photophores were only visible at 390 and 470 nm. SNs and simple photophores were brighter under different wavelengths, allowing for SNs to be differentiated from photophores when illuminated under different wavelengths of light. This technique also revealed variation in the size, distribution, density, and orientation (direction of light emission) of numerous, minute, complex photophores on the head and trunk of stomiids. Additionally, the fragile, gelatinous coating found in some stomiid fishes and the simple photophores found within it were described in Chauliodus sloani and Stomias boa ferox.^
Ashley N Marranzino,
"Flow sensing in the deep sea: Morphology of the lateral line system in Stomiiform fishes"
Dissertations and Master's Theses (Campus Access).