280 ^"^ ANIMAL KINGDOM 



reaching maturity during the second week will not breed unless the 

 night is cloudy, and usually are forced to wait some time. In the third 

 week, alter the full moon, a period of darkness separates sunset and 

 moonrise, and nightly during this period of darkness large numbers of 

 nereids swarm to the surface to breed. By compressing the shedding 

 of gametes into this hour or so in the third week of each lunar month, 

 enough worms breed at the same time to guarantee fertilization of the 

 eggs. Other nereids have different lunar cycles. 



Other worms may use the same external rhythms, but respond differ- 

 ently to them and thus have different behavioral rhythms. A remarkable 

 example of periodic reproduction is found in the Palolo worms, a species 

 of polychaete living on coral reefs in the south Pacific. Over 90 per 

 cent of the population breeds within a single two-hour period of the 

 entire year. The seasonal rhythm limits the reproductive period to about 

 a month, the lunar rhythm to a day, and the diurnal rhythm to a couple 

 of hours after complete darkness. The major swarm occurs in November 

 during the last quarter of the moon when the low tide is unusually low. 

 This is the spring rainy season in this region. A smaller swarm usually 

 occurs four weeks earlier, at the previous neap tide, and a different 

 species of annelid always swarms the night before the Palolo. 



The posterior half of the Palolo worm not only becomes different 

 from the anterior half, but actually breaks off. On the night of breeding 

 individuals back out of their holes and the posterior half twists counter- 

 clockwise until it breaks free. It then swims backward to the surface. 

 Each segment has a pair of eyes beneath the parapodia, so that broken 

 pieces will still swim appropriately. After swimming at the surface for a 

 few minutes they burst, shedding eggs or sperm and leaving a rapidly 

 disintegrating body. 



These posterior halves packed with gametes are frantically collected 

 in dip nets by the island natives during the brief period when they are 

 available. They are made into a thick soup said to taste like spinach. 

 The natives have learned to predict when the Palolo will swarm and 

 lookouts camp on the shores at the right season to watch the water daily. 

 When the water is suddenly full of spume and debris, apparently be- 

 cause extreme tides produce severe wave action on the reefs, swarming 

 will follow in two days. 



Reproductive periodicities are found in many other animals. The 

 oysters described earlier are also coordinated by the integration of sea- 

 sonal and lunar rhythms, and several arthropods and fishes follow tidal 

 cycles in their behavior. 



127. Earthworms and the Soil 



Although earthworms usually forage on the surface from temporary 

 burrows, they also dig extensively, as much as one or two feet beneath 

 the surface. Much of the dirt is eaten and later deposited on the surface 

 as castings. They also pick up debris while foraging and carry it below 

 the ground, and at dawn may pull sticks and leaves into their burrows 

 for concealment. 



