BIOLOGY AND NATURAL HISTORY 197 



The creature that hatches from a shrimp egg is a tiny, pyriform (pear- 

 shaped) individual having three pairs of appendages. This stage in the life 

 history is known as a nauplius and is typical of all Crustacea, although 

 usually passed in the egg among higher forms. These nauplii measure from 

 0.30 to 0.56 mm. (1/80 to 1/45 inch) in length. They lack eyes but have a 

 simple, light-receptive organ which will disappear in later development. 

 They swim feebly, thrashing upward enough to offset sinking. They do not 

 feed, but live on stored yolk material from the egg. There are five naupliar 

 stages, each successive moult being accompanied by growth and develop- 

 ment. The entire naupliar period is passed within 36 hours (Pearson 1939). 



The fifth nauplius moults to yield the first protozoea, the next stage in 

 development. There are three protozoeal stages measuring from 0.80 to 2.6 

 mm. (1/30 to i/io inch) in length. The protozoeae have eyes and are active 

 swimmers. They feed by filtering smaller organisms from the plankton. 

 At the third protozoeal stage, the larva has 8 pairs of appendages. 



The third protozoea moults to yield the first mysis. There are two develop- 

 ment stages listed under this heading. The second mysis has the adult 

 number of 19 appendages. These forms are strong swimmers and continue 

 to feed by filtering. They measure from 3.2 to 4.4 mm. (1/8 to 1/6 inch) in 

 length. 



The second mysis moults to the first post-larva or subadult. The post- 

 larvae generally resemble mature shrimp, although further changes, chiefly 

 in proportions, will accompany maturity. Usually the second post-larva 

 enters an estuary and begins a demersal or bottom existence, specializations 

 in structure of appendages having made it impossible for the post-larva to 

 continue the larval mode of life. At this time, the post-larva is about 7 mm. 

 long (}i inch). 



On the basis of observed growth rates, Pearson has estimated that the 

 duration of larval life for P. setiferus is from two to three weeks, and the 

 post-larvae enter the brackish inside waters about that length of time after 

 the eggs from which they developed were spawned. 



Very little is known of the ecology of larval shrimp. They are members 

 of the ocean plankton and, after the nauplius stages, probably feed largely 

 on diatoms, one-celled floating plants. Presumably their enemies include the 

 herrings and other filter-feeding organisms of the sea as well as smaller 

 animals which may prey on individual members of the plankton. Quantitative 

 analyses of these relationships have not been attempted and, indeed, much 

 further groundwork is necessary before such can be tried. Pearson (1939) 

 has observed a "positive, though probably complicated" phototrophism 

 (attraction to light) of nauplii and protozoeae of P. setiferus. Nothing is 

 known of the mechanism of transfer of the larvae from their oceanic habitat 

 to the brackish estuaries which are the habitat of the post-larvae and young 



