JACKSON ET AL.: KEY TO GENERA OF PENAEID LARVAE 



While the larvae have many similarities, there 

 were two important characters missing from the 

 M. palmensis described here, which Paulinose 

 claims to be diagnostic for the genus: a dentate 

 postero-inferior carapace margin, and serrated 

 abdominal pleura. Given the large number of 

 Metapenaeopsis species, these characters may 

 vary within the genus. 



Morphological Variation 



The larval descriptions given in this study 

 were based on a number of individuals sampled, 

 where possible, from a number of spawnings. 

 However, previous studies have snown that with 

 the degree of intraspecific variation in morphol- 

 ogy, hundreds of larvae from many spawnings 

 should be examined, and each substage should 

 be sampled several times to account for intra- 

 molt growth (Rothlisberg et al. 1983a; Jack- 

 son 1986). In this study, there was insufficient 

 material for such exhaustive examination, and 

 so the full range of variations may not be 

 described. 



Some morphological variation can be induced 

 by environmental factors such as salinity and 

 temperature (Jackson 1986), and the special 

 environment of the laboratory may do the same. 

 In an unsuitable laboratory environment, 

 obvious deformities can occur (Rao and 

 Kathirval 1973), and less extreme environ- 

 mental conditions may result in more subtle 

 morphological effects. Therefore, while in this 

 study much emphasis has been placed on labora- 

 tory rearing as a means of ensuring correct 

 larval identification, more work is needed to 

 discover to what degree laboratory-reared 

 larvae differ from those in the natural environ- 

 ment. Differences between field-caught and 

 laboratory-reared larvae have been found for 

 the carid shrimp Pandalus jordani (Rothlisberg 

 1980), both in morphology and in the number of 

 substages. 



While many workers have searched for tax- 

 onomic characters to separat'e species of penaeid 

 larvae within genera, they have generally been 

 unsuccessful (Cook 1966b; Cook and Murphy 

 1971; Courties 1976). The only reliable way of 

 distinguishing species of larval penaeids is to 

 make a discriminant analysis of a number of 

 morphological characters (Rothlisberg et al. 

 1983a; Jackson 1986). Discriminant analysis is 

 less successful for postlarvae, and a technique 

 based on electrophoresis is being developed 

 (Lavery and Staples in press). The descriptions 



presented in this study are therefore of Umited 

 value in species identification. 



Keys to the Genera of Larval Penaeids 



The genera Penaeus, Metapenaeus, Parape- 

 naeopsis, Parapenaeus, Track ypenaeus, Meta- 

 penaeopsis, Atypopenaeus, Penaeopsis, 

 Funchalia, Trachypenaeopsis, Sicyonica, Aris- 

 taeomorpha, and Solenocera are included in 

 Paulinose's (1982) key to penaeid larvae. Recon- 

 structions of larval series from plankton collec- 

 tions were used as reference material for most of 

 the genera. The protozoea key uses the length of 

 the rostrum, the distribution of dorsal abdominal 

 spines and the number of telson setae without 

 qualifying these characters according to the sub- 

 stage. Most protozoeae would not be identified 

 correctly because protozoea I does not have a 

 rostrum; only protozoeae III have dorsal ab- 

 dominal spines; and telson spines in Penaeus, 

 Trachypenaeus, Parapenaeopsis, Macro- 

 petasma, and Parapenaeus vary in number be- 

 tween protozoea II and protozoea III. In the 

 mysis key, Atypopenaeus are identified by a 

 telson setal formula of 7 + 1 + 7, although the 

 present study shows A. formosus has 7+7. 

 Metapenaeopsis mysis larvae are said to have 

 the pleura of the first five abdominal segments 

 serrated ventrally, a characteristic not found in 

 M. palmensis in the present study. 



This is the first generic key for the Indo-west 

 Pacific penaeid larvae that relies on descriptions 

 of laboratory-reared larvae. Relying on labora- 

 tory roarings restricted the number of descrip- 

 tions and species available for reference, but we 

 feel this was justified by the improved precision 

 obtained. In the present study, it was not possi- 

 ble to include Funchalia, Heteropenaeus, 

 Penaeopsis, or Trachypenaeopsis, as no labora- 

 tory-reared larvae of these genera have been 

 described. Owing to their rarity, the omission of 

 three of these will have Httle effect on the prac- 

 tical application of the key. The fact that Pe- 

 naeopsis is not included is more unfortunate 

 since this genus is relatively abundant (Dall et 

 al. in press). The key will be enhanced when 

 larvae from the above genera are reared, as well 

 as more species of Atypopenaeus, Metape- 

 naeopsis, Parapenaeopsis, Parapenaeus, and 

 Trachypenaeus. We have reared several species 

 of Trachypenaeus and are preparing descrip- 

 tions of T. fulvus larvae to compare with other 

 Trachypenaeus species in our reference collec- 

 tion. 



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