taken from the literature, and by comparison with 

 larvae reared in the laboratory from eggs of known 

 parentage, planktonic larvae were assigned to 

 genera. Subsequent examination of accumulated 

 material revealed the presence of protozoeal, mysis, 

 and postlarval characters which remained constant 

 within each genus. These characters were, in 

 turn, used as criteria to construct a key to local 

 genera. A key based for the most part on plank- 

 tonic rather than laboratory-reared material has 

 its limitations, but the scarcity of information 

 concerning penaeid larvae from this area never- 

 theless justifies its presentation at this time. 



Despite the fact that Penaeus aztecus Ives 

 (brown shrimp); P. duorarum Burkenroad (pink 

 shrimp); Sicyonia brevirostris (Stimpson) and 

 S. dorsalis (Kingsley) (rock shrimps); Trachy- 

 peneus similis (Smith); and Xiphopeneus krfiyeri 

 (Heller) (seabob) have been reared successfully 

 through the nauplial stage under laboratory 

 conditions, the nauplii were found to be so similar 

 as to defy attempts to fit them into a key. Al- 

 though differences in setation are minor or absent, 

 the lack of a dorsal protuberance (fig. If), as well 

 as larger relative size, serves to distinguish 

 nauplii of the genus Penaeus. In genera other 

 than Penaeus, this protuberance is present on the 

 dorsal surface of the body above the insertion 

 of the second antennae. 



Within a given developmental stage (e.g., 

 Nauplius II, Protozoea I, etc.), the size ranges of 

 penaeid larvae as a whole are extremely variable, 

 although in the northern Gulf, larvae of the genus 

 Penaeus are generally larger than those belonging 

 to comparable stages of other genera. Hudinaga 

 (1942) found that the protozoeal stages of P. 

 japonicus Bate exhibited intermolt growth, the 

 occurrence of which may also be true for other 

 stages as suggested for nauplii of Xiphopeneus 

 krp'yeri by Renfro and Cook (1963). The possi- 

 bility also exists that larvae (and postlarvae) 

 of the same species grow dissimilarly at different 

 times of the year. Since the relative size at each 

 stage overlaps considerably between, as well as 

 within, the various genera, it should be used 

 with discretion for purposes of identification. 



While the number of substages in each penaeid 

 larval stage described in the literature has been 

 found to vary, the normal situation in the north- 

 western Gulf of Mexico — as ascertained from 

 material in plankton collections — seems to be 



five nauplial, three protozoeal, and three mysis 

 substages. Examples of departure from this se- 

 quence are provided by the larvae of Sicyonia 

 brevirostris which, when reared in the laboratory, 

 appeared to pass through four mysis substages, 

 and by those of Parapenaeus sp. which, as de- 

 termined from sample material, also have at 

 least four. Such apparent anomalies suggest 

 that descriptions of penaeid larvae obtained 

 either from rearing experiments or plankton 

 samples must be viewed with caution until more 

 is known of the effects of environmental factors 

 on early growth and morphology. 



Table 1, in addition to presenting the principal 

 diagnostic characters included in the following 

 key, also furnishes other valuable characters 

 for distinguishing larvae and postlarvae. 



All illustrations are intended to clarify generic 

 characteristics and do not represent particular 

 species. 



KEY TO STAGES AND SUBSTAGES OF 

 PENAEID LARVAE AND EARLY POST- 

 LARVAE 



1 Body simple, unsegmented; three pairs of ap- 



pendages arising from anterior portion of body, 

 first unbranched, second and third branched; 

 paired caudal spines arise from posterior end of 



body (Nauplius) 2 



Not as above 7 



2(1) Body pear shaped; pairs of caudal spines of equal 

 length, extending straight posteriorly; lateral 

 setae on appendages arise singly or in pairs; 

 appendages lack spines or processes such as would 

 be utilized for feeding purposes; carapace present 

 only as a close-fitting rudiment in later stages 



(Penaeid nauplius fig. 1) 3 



One or more of the following characters present: 

 body elliptical; pairs of caudal spines of unequal 

 length or extending medianly, crossing one 

 another; lateral setae on appendages arising in 

 clusters; spines or processes such as would be 

 utilized for feeding purposes present; a well- 

 developed or prominent carapace present 



Nonpenaeid nauplius 



3(2) Five setae on exopod of second antenna; one pair 

 of caudal spines; surface of body between inser- 

 tion of caudal spines convex.. Nauplius I (fig. la) 

 More than five setae on exopod of second antenna; 

 one or more pairs of caudal spines; surface of 

 body between insertion of caudal spines not 

 convex 4 



4(3) Six setae on exopod of second antenna; usually 

 one, sometimes two, pairs of caudal spines; 

 surface of body between insertion of caudal spines 

 flat Nauplius II (fig. lb) 



438 



U.S. FISH AND WILDLIFE SERVICE 



