FEEDING AND GROWTH OF YOUNG WHILE IN PATERNAL MOUTH.
At the time of hatching, i. e., bursting of the shell, the young measure about 45 mm. over all, but when they leave the mouth and begin an independent existence they are about 85 to 100 mm. long. An early egg, 20 mm. in diameter, after having been in 4 per cent formalin for 2 years and 6 months, weighed 3.5 grams. A larva with body walls which had just completely closed over the yolk, the seam of closure only being visible, after being in 4 per cent formalin for 4 years and 3 months, measured 93 mm. and weighed 9 grams. This fishlet was in all respects normal and was just in the stage when the young begin to lead a free life. To make this point clear, the reader is asked to contrast the young fish in figures 11 and 12 of plate iv with the eggs with comparatively early embryos shown in figures 8, plate in, and 9, plate iv.
This increase in length of over 45 mm. (a doubling) and in weight of 5.5 grams (almost a trebling) can not be accounted for by yolk alone—the young feed in the mouth of the parent. At some future
time it will be shown that the larvae reared in running sea-water grew faster and when fed by hand were less voracious than those kept in a diluted sea-water filtered many times, the latter feeding ravenously when given bits of oyster. The conclusion drawn in 1907, which has remained unchanged, is that the young feed while in the mouth of the father by filtering out of the respired sea-water, by means of their closely set gill-rakers, minute crustacea to satisfy their hunger. In this way only can their great increase in size and weight be accounted for.
Von Ihering (1888) is the only investigator who has given any data whatever on the growth of the young while in process of brooding. Of Arius commersonii he records that the eggs in early development stages average 2.5 grams, while at a time when the larvae are 60 mm. long they weigh 4.3 grams. The increase in weight, he argues, is due to the intake of nourishment by the embryo.