CalCOFI Reports, Vol. 11, 1967 - California Cooperative Oceanic ...

70 CALIFORNIA COOPERATIVE OCEAINIC FISHERIES ISVESTIGATIONShad to need to develop adaptative trends in respectto this factor. The present author (Dziekonska, 1958),as well as other workers, holds the view that individualsof one species from water of a certain salinitymight reproduce in waters whose salinity is lethalfor other individuals of the same species living inwaters of lower salinity (Ivlev, 1940).INFLUENCE OF LIGHTExperiments have shown that the influence of lightis not a factor of great importance in the embryonicdevelopment of Engraulis anchoita. Eggs which werekept in complete darkness developed and hatched ina normal way, and no differences were observed inrespect to the controls.In aquaria kept under continual illumination,hatching took place earlier and a greater number ofanomalies was observed, although this latter fact ispossibly the result of a temperature increase due toillumination rather than to the effect of light itself.The relative indifference of the embryos in respectto light is probably due to the fact that the eggs ofthe anchovy are planktonic and thus they stay fairlyclose to the surface of the sea, and are exposed to therays of the sun as well as to darkness. Also, their developmentis quite rapid and, therefore, both the presenceand absence of light acts upon their developmentduring a brief period of time. In the case of somespecies of salmonids, for instance, it has been shownthat light generally has a negative influence. In thecase of this species, its development requires a longtime and takes place in natural conditions, in darkness,under sand or pebble beds (Eider, 1957; Willer,1928). The embryos of Pleuronectes, which developunder natural conditions similar to those of the anchovy,also appear to be indifferent to the effect oflight. (Johansen, Krogh, 1914).Up to the closing of the blastopore, the weight of theslide alone is sufficient to destroy the embryo. Afterthe vitellus is completely enveloped, the resistance ofthe embryo increases. Close to the hatching stage, theweight of the slide together with a weight of 40 gmis necessary for the destruction of the embryo. Thisphenomenon seems to have its explanation in the factthat the vitellus is protected by the envelopment ofembryonic tissues, and theref ore offers greater resistance.This slight susceptibility of the anchovy embryobefore hatching does not agree with the observationsof Schaperclaus (1940), upon the eggs of Esoxlucius. This author found that the resistance of theembryos of this species in respect to the pressure factordecreases sharply just before hatching.As is shown in Table 2, the resistance of the eggmembrane becomes greatest at the stage correspondingto the closing of the blastopore, and decreases sharplyimmediately prior to hatching. This phenomenon maybe attributed to the effect of the hatching enzymes,which at this stage act by lyzing the chorion.The experiments concerning the effect of shock ineggs dropped from different heights leads to conclusionsanalogous to those suggested by the pressureexperiments. The results are depicted in Figure 3. AsINFLUENCE OF MECHANICAL FACTORSAnchovy eggs which are submitted to pressure reactin different ways, according to the developmentalstage which the embryos have reached. The results ofthe experiments are shown in Table 2. As may be observed,the embryo appears to be more susceptibleto pressure during the early stages of its development.TABLE 2RESISTANCE OF THE ANCHOVY EMBRYOS AND EGG MEMBRANESUBMITTED TO THE EFFECTS OF DIFFERENT PRESSURE DURINGVARYING STAGES OF DEVELOPMENT (WEIGHT IN GMS)1Maximum w. Maximum w.Stage of developmentmembrane 1 embryoFIGURE 3. Percentage of anchovy eggs at differing stage of develop-160slideEarly gastrula .._____.._______..___________slidement killed after falls from different heights.The blastop. ring exceeds the equator of theegg ____-_..__-_._.._--.~~~~~--~~...~.~110 slideClosing of the blastoporeit was impossible to obtain suitable materials for per-.______...________200slideEmbryo-Kupffer’s vesicle ------- - --.- ----1703-5Commencement of the separation of the cau-140 10130167540Early blastula- - - - - _ _ ___ - - _ ___ __ ___ __ - - _ _dal region of the vesicle sac _ _____________Embryo occupying W of perimeter .____.._rEmbryo prior to hatching _ ________________I I I Iskgesl i z 3 4forming experiments upon the early cleavage stages,no conclusions will be drawn on the susceptibility ofthe embryos during the early stages of development.The earliest stage at which the embryos were submittedto shock was the early blastula. In this stage,