In the absence of a complete collection and identification of all species of eggs and larvae which may be present in the plankton, it is not possible, of course, to state with certainty that no other species will present some or all of the same characteristics as those described above. Indeed, the early developmental stages are conspicuously devoid of any prominent characters which immediately place them in any one restricted category. The eggs, for instance, belong to a very large group of pelagic teleostean eggs which have a smooth spherical membrane, a small perivitelline space, a single oil globule, and a clear, almost featureless, yolk. They lack the characteristic elliptical shape of the anchovy (Engraulis), the cellular yolk and large perivitteline space of the pilchard (Sardinops), or the multiple oil globule of the yellow-eyed mullet (Agonostomus forsteri). The description of the larval stages could also apply with only minor modifications to a great number of genera.
Perhaps the most useful general characteristic, particularly in the case of the egg, is size. By this means the eggs of a great number of other species may be immediately eliminated from any sample. Table II gives the diameters of various spherical eggs which have been collected in the Hauraki Gulf during the snapper spawning season. With the exception of Sardinops all eggs have been taken from the female of the species named, and thus are positively identified. The pilchard eggs were taken from the plankton and identified by their close resemblance to other species of the genus in Australia and South Africa (Munro 1945, Davies 1954). All measurements have been taken from material fixed in 5 per cent, formalin. At least in the case of the snapper, shrinkage of the egg membrane is so slight that the live material is not likely to deviate appreciably from these figures.
|m = mean diameter s = average standard deviation within samples sm = standard deviation between sample means. N = number of samples, n = total number of eggs measured.|
Of this list, only one, on the basis of size, could be confused with the snapper— i.e., Agonostomus, which is readily distinguished by its multiple oil globules which may be up to ten in number. From the above statistics it is safe to say that only one in every hundred snapper eggs will fall outside the diameter range 0.81-1.02 mm. Although there may be some slight change in diameter between fertilisation and hatching, this variation is small compared with that between individual eggs. In all the 40 samples of snapper eggs the length of the parent fish was recorded (ranging from 26 to 60 cms), but there was found to be no significant correlation between size of fish and size of egg.
The diameter of the oil globule is also constant within narrow limits, not only in the egg, but also in the larva up to the second or third day. From a sample of 200 preserved eggs this was found to be 0.25 mm with a standard deviation of 0.06 mm. The position of the globule is typically at the posterior extremity of the yolk sac in the larva (cf. the tarakihi, Cheilodactylus macropterus, where it is anterior).
Owing to the relatively rapid growth rate. other dimensions change too rapidly to be more than a general guide to identification. The scale given at the bottom of Plate 26 is applicable to all figures.
Apart from size, the most obvious feature of the second day egg and of the larva up to three days is the yellow pigment, visible even to the naked eye as three distinct patches, one on the head, one in front of the anus, and one midway along the tail. Under the microscope the more detailed pattern shown in Plate 26 may be distinguished, the more prominent patches maintaining a fairly constant position in relation to myotomes and other organs.