KNIGHT and OMORI: LARVAL DEVELOPMENT OF Sh'RCKSTh'S SIMIL1S 



Gurney and Lebour (1940) believed the 

 elaborate protozoeal phase of Sergestes s.l. to be 

 of particular importance, as it might "point to a 

 satisfactory subgeneric grouping of the adults." 

 They separated the second and third protozoeae 

 of thirteen species of Sergestes s.l., representative 

 of all of the nine species groups later defined by 

 Yaldwyn (1957), into three types: dohrni, 

 ortmanni, and hispida. The carapace has the 

 same number of processes in all three types but 

 the armature of the processes differs as follows: 

 dohrn i type with numerous long lateral spines on 

 supraorbital, lateral, and posterior processes; 

 ortmanni type with long spines on supraorbital 

 processes but with long spines only at the bases of 

 lateral and posterior processes on carapace; 

 h ispida type without long spines on supraorbital, 

 lateral, or posterior processes, although there 

 may be basal spines on lateral and posterior 

 processes. Gurney and Lebour observed that the 

 ortmanni armature seems to be derived from the 

 dohrni type in that it retains long lateral spines 

 on supraorbital processes. 



These larval types do correspond with three 

 divisions of species within Sergestes s.l. Of the 

 species described by Gurney and Lebour (1940), 

 the hispida type larvae all belong to the genus 

 Sergia, while the dohrni and ortmanni types 

 belong to Sergestes; S. corniculum is of the 

 ortmanni type, but all other species of Sergestes 

 identified are of the dohrni type. The zoeal stages 

 could not be separated into groups which 

 corresponded to the protozoeal types. 



Gurney and Lebour (1940) noted that the 

 dohrni type carapace was found in a number of 

 species which were not supposed to be particu- 



larly closely related and which could not be 

 grouped further by structure of the protozoeal 

 phase. The identification of S. similis larvae has 

 proved this untrue with respect to the arcticus 

 group species, but apparently it does apply to 

 species of the atlanticus group, the only other 

 species group within Sergestes, or Sergia, all of 

 whose protozoeal stages are identified. Gurney 

 and Lebour described the larvae of Sergestes 

 atlanticus and S. cornutus, the two species which 

 comprise the atlanticus group, and observed that 

 larval morphology did not corroborate the close 

 relationship implied by the morphology of adult 

 petasma. The carapace armature in protozoea II 

 and III of the arcticus and atlanticus groups is 

 compared in Table 1 to show the range of 

 variation within each group; the species groups 

 themselves are not considered to be closely re- 

 lated within the genus (Judkins 1972). Sergestes 

 arcticus and S. similis may have the same 

 armature in both protozoeal stages, while S. 

 atlanticus and S. cornutus differ in each stage; all 

 of the lateral spines of the atlanticus group have 

 smooth tips rather than the brushlike endings 

 characteristic of the arcticus group. 



The difference in larval morphology within the 

 atlanticus group is in accordance with the signi- 

 ficant difference described by Foxton (1972) 

 between S. atlanticus and S. cornutus in 

 morphology of the organs of Pesta. This dis- 

 crepancy was one of two exceptions noted by 

 Foxton to a generalization that species of 

 Sergestes that are the most similar in other adult 

 diagnostic characters usually have identical or 

 closely similar organs of Pesta; he does not note 

 any difference between the arcticus group 



Table 1.— Comparison of the number of long lateral spines which arm carapace processes in protozoea II and 

 III of two species groups of Sergestes; the lateral spines have smooth tips in the atlanticus group and branching 

 tips ("brushlike endings") in the arcticus group (descriptions of the atlanticus group and S. arcticus are taken 

 from Gurney and Lebour (1940). 



'Gurney and Lebour (1940) report eight long spines on the lateral carapace process, but their figure shows seven with brushlike 

 endings and the simple spmulose tip of process, the common armature in S. similis 



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