160 CARNEGIE INSTITUTION OF WASHINGTON. 



annual swarm, which as usual fell within 4 days of the last quarter of the 

 July moon. As is well known, the posterior sexual segments of the mature 

 worms are cut off from the anterior vegetative portion, rise to the surface in 

 one piece in the early dawn, and after a short time burst, freeing the eggs and 

 sperm-cells. But before the discharge of the sex-cells takes place, the worms 

 occur at Tortugas with an average frequency of perhaps two or three to the 

 square foot of surface over an area of many square miles. 



Only less impressive than their teeming multitudes is the tax levied upon 

 them by the predaceous fishes of the region. Foremost among these is the 

 gray snapper (Neomoenis griseus). The snappers are for the most part 

 nocturnal bottom feeders; they cease searching for prey and return to their 

 schooling places shortly after daylight. But on the morning of the palolo 

 swarm they remain longer afield and may sometimes be seen feeding upon 

 the worms at the very surface of the water. 14 snappers, 10 to 11 inches 

 long and taken with dynamite about sunrise on "palolo day," contained 

 from 8 to 28 c.c. of worms each. One 12 inches long contained 55 c.c. and 

 two 14-inch fishes contained 45 c.c. each. These 17 fish therefore contained 

 364 c.c. of the worms, which is equivalent to saying that they had eaten 

 about that number, or roughly 20 each. How many more, if any, they 

 might have eaten, if left to themselves, it is impossible to say. Food of other 

 kind than the worms appeared in their stomachs in less abundance than 

 on other days. But the total quantity of food present far surpassed that 

 to be found on ordinary occasions. 



Snappers plainly secure much less food upon the average than they are able 

 to eat. It is also clear that their diet depends more largely upon the avail- 

 ability than on the abundance of the organisms fed upon, for these exceedingly 

 common and acceptable annelids are rarely found in their stomachs ordinarily. 

 Their reaction on this occasion demonstrates again, as Howard has demon- 

 strated in the case of secondary and tertiary insect parasites, and as Forbes 

 has demonstrated in the case of certain orchard pests, that natural agencies 

 operate at once to reduce the numbers of any species which undergoes sudden 

 increase. 



Fish such as the yellow goat-fish (Upeneus maculatus) and the various 

 species of grunts, in the dietary of which annelids constitute a constant 

 element, are gorged with them on the day of the great swarm, not mingled 

 with sand, as is usually the case when they are taken from the bottom, but 

 wholly unmixed with foreign materials. Some species, such as the black 

 angel-fish (Pomacanthus arcuatus), which feeds commonly upon blue-green, 

 green, and brown algae, as well as upon sponges, bryozoa, etc., join in the 

 attack upon the worms. But specimens of Lactophrys iriqueter, Sparisonia 

 viride, and Microspaihodon chrysurus had eaten none. Neither had a large 

 Spanish angel-fish (Angelidhys isabelita) done so, although Linton has re- 

 ported annelids in its stomach contents at Bermuda. 



Very striking is the contrast between the results in the case of the gray 

 snapper and the related species, the schoolmaster (Neonicenis apodus). 

 Of this species, 16 specimens, ranging from 9.5 to 11.5 inches in length, and 

 feeding commonly upon the same general sort of food that the gray snappers 

 do, contained only from 1 to 15 c.c. of the palolo worms, 92 c.c. in all, or 

 less than 6 c.c. upon the average. Observations of this sort, when repeated, 

 completely disprove the idea that even closely related species, apparently 

 living much the same lives, have the same habits. 



