IX. 12 



REPRODUCTION 



265 



the Weberian ossicles, p. 217) may be associated with pressure recep- 

 tors that assist in the control of the bladder. Loaches are famous as 

 fish barometers, whose behaviour can be used to predict weather 

 changes. 



12. Special reproductive mechanisms in teleosts 



The teleosts show great variation in breeding habits, the eggs being 

 sometimes left to develop entirely by themselves, in other cases looked 

 after by one or both parents, while in a few species they develop 



Fig. 159. Deep-sea fish Photocorynus with parasite male attached. (From Norman.) 



viviparously within the mother. Hermaphrodite individuals are not 

 uncommon and in some species of Sparidae and Serranidae are 

 invariably monoecius and self-fertilizing. The method of association 

 of the sexes is correspondingly varied and there are numerous devices 

 for bringing sexes together, such as colour differences, sound produc- 

 tion, and the liberation of stimulating substances into the water. In 

 some deep-sea fishes the male is much smaller than the female, to 

 which it remains permanently attached (Fig. 159). Breeding is often 

 preceded by a migration of the fishes to suitable situations and the 

 association into large shoals. 



The eggs may be classified as either pelagic, if they float, or demersal, 

 if they sink to the bottom. In the former case they are sometimes pro- 

 vided with an oil globule and are exceedingly numerous. Thus a 

 single female turbot has been calculated to contain nearly 10 million 

 eggs, a cod 7 million, and a ling 28 million, whereas the herring, 

 whose eggs sink to the bottom, probably does not lay more than 



