46 Papers from the Department of Marine Biology. 



THE PURPOSE OF BUCCAL INCUBATION IN THE GAFF-TOPSAIL. 



This may be summed up in one word protection. Let it be 

 recalled that these eggs are of enormous size (the average diameter 

 of 327 eggs being 19 . 5 mm.) and that when in middle embryonic 

 stages they are very attractive to the eye because of their blood-red 

 vascular yolk investment. For these reasons, if laid as other fish 

 eggs are, they could hardly be expected to escape the eyes of maraud- 

 ing fishes, but if any were so fortunate they would almost certainly 

 be eaten by crabs, those scavengers from which practically nothing 

 escapes. The result would be the inevitable extinction of the species. 

 Moreover, there is yet another danger to which the ova are exposed. 

 These catfish spawn and spend the hatching season on mud flats. If the 

 eggs were discharged on such bottoms they would (because of their 

 great weight, averaging 3.5 grams) sink into the mud and be smothered. 

 To avoid these various dangers, these fish have to do one of two 

 things to insure their perpetuity, i. e., to practice mouth gestation 

 or to lay their eggs in nests which are guarded by one or both the 

 parents. Some fresh- water catfishes have adopted the latter habit; 

 the gaff-topsail has chosen the former. 



The whole matter, barring his ignorance of the habit of buccal 

 incubation, has been admirably put by Gurley (1902). 



"It is almost impossible that a mud bottom should be a successful spawn- 

 ing-ground, as the eggs will almost inevitably be asphyxiated. Wherefore, 

 fishes experiencing an impulse to spawn on such bottom will leave few de- 

 scendants to inherit their delicately sensitive mucous membrane, while those 

 having an impulse to seek harder bottom will transmit to a larger progeny 

 their more roborant mucous membrane. Further, the exception sustains 

 the rule, the only species spawning on mud bottoms being certain catfishes, 

 the males of which excavate nests, and attend to (probably aerate) the eggs, 

 and care for the fry. " 



THE ORIGIN OF THE HABIT OF ORAL GESTATION. 



The causes leading to the practice of buccal incubation are plainly 

 set forth in the preceding section; the eggs must be guarded in a 

 nest or carried in the mouth, or else the fish would soon become extinct. 

 The gaff-topsail has chosen the latter means to maintain itself. But 

 how has the habit been developed? The answer to this is largely 

 conjectural, but there is a fair amount of data to be adduced upon 

 which to base our conjecture. 



It has long been known that some fishes pick up their eggs or young 

 in their mouths for transport from place to place much as a eat does 

 her kittens or a fox her cubs. The writer believes that in this habit 

 are to be found the beginnings of oral gestation. The fishes referred to 

 are members of the families Gasterosteidse and Osphromenidae, together 

 with one or two other isolated forms. 



