GORE: LARVAL DEVELOPMENT OF GALATHEA ROSTRATA 



by some 20°C larvae, and thus could be thought of 

 as an intercalated stage, if the advanced stage IV 

 be considered more indicative of the developmen- 

 tal sequence. Other features shared between the 

 advanced fourth and regular fifth zoeal stages 

 (besides the presence of well-developed pleopod 

 buds noted earlier) include increased numbers of 

 antennular aesthetascs, a remarkable elongation 

 of the antennal endopodite, the appearance of a 

 mandibular palp, and slight changes in setae 

 number on maxillulae, maxillae, maxillipeds, and 

 telsonal uropods (see section on Description of the 

 Larvae). Moreover, the advanced stage IV zoeae 

 were always larger than the regular stage IV 

 zoeae. 



It will probably remain a question of semantics 

 whether the regular stage IV is considered an in- 

 tercalated stage or one that occasionally may be 

 skipped. It could just as well be asked whether the 

 advanced stage IV was an intercalated stage be- 

 cause it embodies many of the features of regular 

 stage IV, plus some seen only in stage V zoeae in 

 the developmental sequence. What is of more im- 

 portance in the development of G. rostrata is that 

 the substitution of an advanced stage IV and the 

 subsequent elimination of the regular stages IV 

 and V allows earlier postlarval metamorphosis. 

 The resultant early benthic crab stages may be 

 reached in a shorter period of time by the species, 

 thereby reducing the time spent in" the plankton. 



Discussion 



It is, of course, conjectural as to whether the 

 larvae of G. rostrata skip stages in their develop- 

 ment in the natural environment or are ever sub- 

 ject to constant low (e.g., 15°C) or intermediate 

 (20°C) seawater temperatures. The adults of the 

 species, found in deeper continental shelf waters, 

 presumably are often exposed to cool seawater 

 temperatures, as was noted, e.g., during the time 

 the adult females for this study were collected. It is 

 not unreasonable to assume that developmental 

 stages may occasionally be subjected to relatively 

 constant cool temperatures as well, either im- 

 mediately after hatching or just prior to postlarval 

 metamorphosis when the megalopae settle to the 

 sea floor. In addition, should the larvae become 

 entrained in cyclonic cold core rings of Gulf 

 Stream origin (see Richardson 1976; Wiebe 1976; 

 Wiebe et al. 1976), they would presumably be sub- 

 jected to relatively constant cold water (at least 

 17°C) for at least part of their developmental 



period. Delayed metamorphosis provides an alter- 

 native hypothesis against the more traditional 

 "stepping-stone" idea, to account for the rather 

 extensive distribution of the species along the 

 Middle and North American continental shelves. 



There is some evidence that larvae of other 

 species of Galathea may skip stages in the 

 plankton (Lebour 1930, 1931) and that other 

 galatheids may intercalate substages (e.g., Boyd 

 and Johnson 1963). For example, the larvae of four 

 of the five British galatheids described by Lebour, 

 viz. Munida rugosa (Fabricius 1775 [as M. banffica 

 = M. bamffica (Pennant 1777)]), Galathea inter- 

 media Lilljeborg 1851, G. squamifera Leach 1814, 

 and G. strigosa (Linnaeus 1767) developed 

 through four zoeal stages, whereas G. dispersa 

 Bate 1859, exhibited four or five stages. Lebour 

 ( 1930) considered five stages in the latter species 

 as "probably normal" but pointed out that the 

 megalopa could be obtained from the fourth 

 [numerical] stage, and "the normally fifth 

 [numerical] stage has been seen to emerge from 

 the third stage." She stated that the fourth or fifth 

 stage may therefore be omitted in G. dispersa, but 

 made no mention of intercalated stages or sub- 

 stages. 



The developmental situation in G. dispersa is 

 quite similar to that noted in this report for G. 

 rostrata, in which an advanced fourth stage re- 

 places the regular fourth and fifth stages, thereby 

 causing them to be omitted from the developmen- 

 tal sequence. Lebour's (1930) "fifth stage. . . from 

 third" is probably equivalent to what is termed in 

 this report the advanced fourth stage. Her state- 

 ment that long, unjointed pleopods appear in the 

 "last" stage of G. dispersa indicates that either the 

 fourth stage (or advanced) or fifth stage (or regu- 

 lar) possess these appendages, depending on 

 whichever stage is "last." It also indicates that the 

 molt to megalopa does not occur without the ap- 

 pearance of pleopods in the "last" larval stage. 

 However, North Sea species of Galathea differ 

 from G. rostrata in possessing pleopod primordia 

 "in the third stage" which are "long but unjointed 

 in the last stage" (Lebour 1930). In addition, Sars 

 (1889) had also noted and illustrated pleopod de- 

 velopment in the "last" stage of larvae attributed 

 to G. intermedia, Munida rugosa, and Munidopsis 

 [as Galathodes]tridentata (Esmark 1857). The lat- 

 ter species will be considered further below. 



Rayner (1935), using planktonic stages from 

 Argentinian waters, described the larvae he at- 

 tributed to Munida gregaria (Fabricius 1793) and 



785 



