The structure of the eye of Sardinops caerulea, Engraulis mordax, and four other pelagic marine teleosts

It is clear from the extensive review of Walls ('42) and from the work of many other investigators that there is considerable variation in the structure of the teleost eye, and that many of the specializations are obvious adaptations to particular environmental situations. The present study was undertaken to find out what the differences are in the eyes of certain pelagic marine teleosts. A knowledge of such differences will be useful in fonnulating hypotheses to be tested in experimental studies of their behavior. The fishes reported on here are the Pacific sardine (Sardinqps caerulea), the American shad (Alosa sapidissima), the northern anchovy ( Engraulis mordax), the deepbodied anchovy (Anchoa compressa), the jack mackerel (Trachurus symmetricus), and the Pacific mackerel (Pneumatophorus diego). ' This work was undertaken as a part of the research program of the California Cooperative Oceanic Fisheries Investigations. The interest of this program for several years has been centered on understanding the causes of fluctuations in the abundance and availability of the Pacific sardine and certain of its ecological associates in the California Current System. The northern anchovy, the jack mackerel and the Pacific mackerel are ecological associates of primary interest. The American shad and the deep-bodied anchovy are included in this report to add perspective to the differences between the Pacific sardine and northern anchovy. Dissections of the eye and the ocular region of the head disclosed anatomical differences among these six species that are suggestive of differences in the way the eye is used. The anatomical differences are in eye covering, ocular circulation, oculomotor musculature and lens suspension. Oculomotor musculature and lens suspension are of particular interest because they indicate the degree of eye mobility and the direction of the visual axis respectively. These features will be described briefly for each species. Histological preparations of the retina revealed considerable differences among these species. The major differences are in the distribution and abundance of photoreceptor cells, the distribution and the abundance of certain secondary neurons, and the structural management of pigment epithelium cells. These three features of the retina will be described in detail for each species and then compared in the discussion. Differences in these features are the basis for a number of generalizations and much speculation on the differences in visual capacity among vertebrates. The accepted principles, and some of the current conjecture, are summarized and documented by Walls ('42) and Polyak ('57). Some of the major concepts, in so far as they pertain to the present study, may be summarized as follows: Visual acuity and sensitivity are known to vary with the spacing and arrangement of cones and rods, and abundance of these structures are frequently compared to indicate differences in visual capacity. The abundance of secondary neurons, such as horizontals, bipolars and ganglion cells, has also been observed to vary with visual