Pike larval development
The larval period is a negligible part of the pike's entire life. However, in just one month, extremely intense structural and biological changes take place, leading to the formation of the body, similar in structure and behavior to adults. Larva coming out of the egg (Lynx. 23—A) it does not resemble this form in any way.
Lynx. 23. Pike larval development - description in the text (by Gihr)
There is no mouth or anal opening, it lacks a gill apparatus and fins. The dorsal and ventral parts of the body are uniformly circulated, transparent skin fold, being a primitive motor organ. Head, inclined at an angle of approx. 80° in relation to the longitudinal axis of the body, it is adjacent to the large yolk sac, which at this time is not only the only source of food, but - in the absence of gills - it also mediates the breathing process. First, a wide venous sinus is visible in the wall of the pouch, later breaking up into a network of smaller blood vessels (Lynx. 24), through which gas exchange takes place between the larvae's organism and the water rinsing the bag.
Lynx. 24. Respiratory vessels on the yolk sac of pike larvae: A - venous sinus (1 the day after hatching), B - venous network (3 days after hatching) (by Zięba).
The glands below the eyes play a very important role in the first stage of development, secreting a sticky substance. Young larvae can no longer remain in the water depth and, after hatching from the egg, quickly sink to the leaves lining the bottom. They are forced to change their position due to the necessity to look for places with a higher concentration of oxygen. Energetic, with seemingly uncoordinated curves of the torso they soar upwards and, with the help of the abovementioned clinging organs, they stick to the plants, twigs or other sunken objects. Seeking to change his position every now and then, they are able to move within very limited sections of space, and not finding a suitable place for the trailer, they go down, to rise again after a short rest.
This type of behavior is characterized by the first phase of development, which lasts 4-5 days at 15-17 ° (Lindroth, 1946), and in the lower one it may last 7-8 days (Samardina, 1957). At this time, the head assumes an axial position, a mouth is formed, the lower jaw begins to move forward, and in places, where the odd fins will be made, condensed clusters of cells can be seen (Lynx. 23—B—D). The body length increases on average with 7 do 10 mm.
The transition from the sticking phase to the free swimming phase is accompanied by other important transformations. The mouth cavity and gill openings make contact with the environment. The network of blood vessels in the yolk sac is lost, and the respiratory function is taken over by the gills. As soon as the esophagus and anus are opened, the gastrointestinal tract is open along its entire length. Now comes the moment of filling the swim bladder with air taken mostly directly from the water surface. Effort to overcome surface tension resistance, and forcing gas bubbles through the conduit connecting the esophagus to the bladder, that this activity is undertaken several times and interrupted by periods of rest, during which the larva sinks to the bottom (Gihr, 1957). A filled swim bladder allows the larvae to overcome their own weight enough, that they can float easily in the water. They stay in the surface layer while maintaining the horizontal position of the body thanks to the movements performed with the beginnings of the pectoral fins. Frightened by a strong ripple of water, they run towards the bottom. The no longer useful attachment organs will gradually disappear. Soon after the swim bladder is full, the larvae begin to eat food caught from the outside, even though the yolk has not yet been exhausted.