CalCOFI Reports, Vol. 11, 1967 - California Cooperative Oceanic ...

REPORTS VOLUME XI, 1 JULY 1963 TO 30 JUNE 1966 151While the oceanographic data are not complete andtheir correlation with the distribution of Pleuroncodesnot devoid of subjectivity the correspondence certainlysuggests that the circulation as indicated bycharts of geostrophic flow is sufficient explanation ofthe major between-years variation in the distributionof Pleuroncodes during this period.ECOLOGY OF THE PELAGIC PHASEIn the foregoing discussions Plezaroncodes has beenconsidered as if it were a normal planktonic species,but it is also a very abundant member of the benthiccommunity at 75-300 m along the continental edge ofthe west of Baja California, within the Gulf of Californiaand on the west coast of Mexico south to theIslas Tres Marias (Boyd, 1963 ; Parker, 1963 ; Perkins,pers. comm.) ; the known distribution of the benthicphase is shown in Figure 6. If individuals have thecapacity to alternate from benthic to pelagic phaseand vice versa, a mechanism exists which could explainthe irreguIarity of occurrence of the pelagic25'h115"HfA BOYD'J JEROE0 PARKER/ 0 PERKINS5/65 V 0 8/61FIGURE 6. Distribution of the benthic records of Pleuroncodes; closedcircles indicate the use of gear, such as a grab, in which there canhave been no possibility of the crabs entering the gear in midwater.phase in the plankton recotd through the individualssettling on, or leaving, the deposits.The evidence to support this hypothesis is ratherslight, however, and comprises mainly the demonstrationby Boyd (1963) that the benthic stock in the areasouth of Punta San Eugenio examined by him overlappedin length frequency distribution with thepelagic stocks; only at the deepest station at whichred crabs were taken by him was the modal lengthof the stock larger than had been recorded for individualsin the pelagic phase, so that by this criteriononly at the deepest station was the population composedof individuals which must have finally settledinto the benthic environment. Additionally, the hypothesisof alternation between the two phases issupported by the lack of morphological differentiationbetween pelagic and benthic individuals : benthiccrabs retain the natatory setae fringing the appendages,and pelagic crabs retain the generalized formof the second maxilliped necessary for benthic existence,and only achieved in Munida gregaria after thepelagic Grirnothea stage settles finally into thebenthos.While this is not, of course, direct evidence thatalternation between the two phases occurs, it impliesthat such alternation is not impossible on morphologicalgrounds, and that the pelagic phase is in someway comparable with the Grimothea of Munida thusbeing a post-larval extension or recurrence of theplanktonic habit of the larvae, which in Munidagregaria is variable in duration, and hence in sizeattained, within rather wide limits (Harrison Matthews,1932). It is postulated that in Pleuroncodesplanipes the pelagic phase is comparable with theGrimothea stage, and that the lack of morphologicalspecialization enables an individual, when the environmentpermits, to alternate between pelagic and benthicphases.It is critical to a discussion of the ecology of thepelagic phase that its duration in the life of an individualshould be established. Boyd (1963) inferredfrom his data on size distribution of benthic andpelagic stocks that the former contains individualsolder than 2 years, while the pelagic individuals measuredby him were only from 6 to 18 months old.From these data he inferred that it is only at carapacelength of 25-28 mm, or about 2 years old. thatthe pelagic phase is finally abandoned.A pelagic existence lasting 2 years renders it unlikelythat an individual would be maintainedthroughout this period within the inshore area withoutbeing flushed seawards on the southwesterly flowof the California Current, and this could perhaps onlybe avoided if a considerable part of the period wasspent temporarily in the benthic community. The distributionof pelagic crabs in the offshore and oceanicarea (Figure 3) indicates that some flushing of individualsfrom coastal areas does occur.Although relatively few individuals from the offshoreand oceanic areas are available for study thesesuggest that such populations are mainly of small