GORE: LARVAL DEVELOPMENT OF GMATHEA ROSTRATA 



is conservatively estimated to take well over 60 

 days (Figure 1). 



At 20°C either four or five morphologically dis- 

 tinct zoeal stages occurred. The minimum time 

 required to complete larval development and 

 reach megalopa was 18 days. Most larvae re- 

 mained in each zoeal stage from 3 to 6 days and as 

 megalopae from 12 to 13 days. The total duration 

 of development from hatching to first crab stage 

 spanned a minimum of 30 days at 20°C, if only four 

 zoeal stages were required, but took at least 37 

 days with five zoeal stages (Figure 1, inset). 



The larvae generally fared well at both culture 

 temperatures. Although the larvae at 15°C took 

 longer to complete their development, they ini- 

 tially appeared to survive better than their coun- 

 terparts at 20°C (Figure 1). At 15°C, at least 50^7^ 

 larval survival occurred through stage IV, before a 

 rapid decrease occurred in stage V and megalopa. 

 Larvae reared at 20°C exhibited a steep decline 

 after stage 1, to about 35'??^ survival, and showed a 

 continual decline thereafter. The precipitous de- 

 cline in larval survival at this temperature from 

 stage I to stage II was the result of an almost 

 complete mortality in one culture tray, for un- 

 known reasons. 



At 15°C ecdysis in the earlier zoeal stages (I-III) 

 generally was a less critical period than at 20°C, 

 although the larvae at the latter temperature 

 were still able to complete most molts. The larvae 

 at 20°C attained subsequent stages more rapidly 

 than did those at 15°C, and some were able to 

 complete zoeal development, although overall lar- 

 val mortality was relatively higher. On the other 

 hand, at 15°C larval survival may have been en- 

 hanced by lower temperature, but the major 

 difficulty then seemed to be the attainment of 

 stage V and megalopa. Only two megalopae were 

 obtained in the 15°C program and neither was able 

 to molt to the succeeding first crab stage. In con- 

 trast, four megalopae survived at 20°C, and 

 molted to crab stage I; three of these specimens 

 were maintained in the laboratory to crab stages 

 XII and XIV. 



Ecdysial and Sequential Variation 

 in Galathea rostrata 



Two modes of developmental variation were 

 noted in G. rostrata at 20°C. In one mode, some 

 zoeae III molted to an instar which, for purposes of 

 discussion, is labelled "regular" stage IV. This 

 stage was characterized, among other features, by 



a reduced number of antennular aesthetascs and 

 was always without well-developed pleopod buds 

 on the abdominal somites. Zoeae remained in this 

 stage for 3-4 days before molting to stage V, an 

 instar possessing distinct, well-developed, pleopod 

 buds and an increased number of antennular aes- 

 thetascs. The duration of stage V lasted 5-6 days 

 and was followed by the molt to megalopa. One of 

 these postlarvae subsequently molted to first crab 

 stage. 



In the second mode of variation, some zoeae III 

 molted to an "advanced" stage IV, with some, but 

 not all, of the features as noted above for stage V. 

 Zoeae remained longer in the advanced stage (5-9 

 days) before molting directly to megalopa. Three 

 of these megalopae went on to attain first crab 

 stage. The two types of development are compared 

 in the inset of Figure 1. 



Two other stage III zoeae, which remained in 

 stage III 7-8 days (instead of the usual 4-5), molted 

 to what appeared to be an intermediate stage IV. 

 These zoeae exhibited some stage V zoeal features 

 in size, maxillipedal setae numbers, and in posses- 

 sing pleopod buds, although the latter were only 

 rudimentaiy. A reduced number of antennular 

 aesthetascs similar to that of regular stage IV 

 zoeae was also seen. The two specimens survived 

 only 4-5 days in this stage before dying. This mode 

 of variation was not considered as important as 

 the previous two modes and will not be discussed 

 further. 



Remarks 



The regular and advanced fourth stages cannot 

 be equated to an early and late fourth stage, nor to 

 substages IVa and IVb, because no molt occurred 

 from one fourth stage or substage to another. If the 

 molt from stage III was to regular stage IV, this 

 was invariably followed by an ecdysis to stage V, 

 and then a subsequent molt to megalopa. If the 

 molt from stage III produced an advanced stage 

 IV, this in turn molted directly to megalopa, skip- 

 ping stage V altogether. At 15°C the regular stage 

 IV and stage V appear to be necessary plateaus in 

 larval development, whereas at 20°C development 

 may proceed in some zoeae without resorting to 

 either of these instars. 



The regular fourth zoeal stage (as defined 

 above), therefore, appears to be a true sequential 

 stage of development, inasmuch as it was seen in 

 larvae at both 15°C and 20°C programs. However, 

 it is also a stage which can occasionally be skipped 



783 



