DISTRIBUTION OF TIIE LARVAE OF PLEURONCODES PLANIPES IN THE CALIFORNIA CURRENT. Alan R. Longhurstl

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1 DISTRIBUTION OF TIIE LARVAE OF PLEURONCODES PLANIPES IN THE CALIFORNIA CURRENT Alan R. Longhurstl Institute of Marine Resources, Scripps Institution of Oceanography, University of California, San Diego ABSTRACT Zoeae and megalopas of Pleuroncodes planipes Stimpson were found in 483 of 2,546 plankton samples from the California Current in 1957 and Their distribution in the samples showed that 1) breeding occurred during winter, mostly in February and March; 2) while some breeding could be ascribed to offshore pelagic adults, the greatest larval concentrations were over the continental shelf, often only in the presumed prescncc of benthic adult populations; 3 ) breeding extended far to the north of the usual range in the warm years centered arouncl 1959, though it was probably not successful in the extreme north; and 4) there was clear evidence of late zoeae and megalopas being carried offshore in accordance with known geostrophic flow patterns so that at least a high proportion of megalopas had the possibility of changing to the benthic phase only after an excursion into oceanic regions. The data cannot be used to decide whether 5) any of these megalopas do, in fact, achieve a return to the coast or whether 6) any megalopas descend directly into the benthic phase after metamorphosis over the continental shelf. INT..RODUCTION Pleuroncocks planipes Stimpson is one of the few species of galatheid crabs in which the pelagic habit may be cxtcnded from larval into adult life, or may recur in adult life after a period of benthic existcnce; both in the pelagic and the benthic phase the species is extrcmcly abundant in the southern part of the California Current, where it forms the major item in the diet of commercially important tropical tunas during their summer migration into the region. The general biology of P. planipes has recently been reviewed by Boyd (1963, 1967) and by Longhurst ( 1967a, b ), The species passes through five larval stages, each comprising a single instar except the fourth, in which from four to eight instars may be passed; the larval stages have been described by Boyd (1960) and Boyd and Johnson (1963). There are, of course, other galatheids in the region and the possibility of confusion between the larvae of these and of P. planipes does exist, the more so since the larvae of the species concerned have not thcmselvcs been described; these are a species of 1 Present address: FisheryOceanography Center, Bureau of Commercial E isheries, La Jolla, California Galathea, two of Munida, and four of Munidopsis ( Schmitt 1921). Since the planktonic larvae of several species of the first two genera have been described from other regions and have proved to have characteristics generally distinct from those of P. planipes, and because the third genus is likely to have larvae rather distinct from those of the other genera, it is reasonable to make certain assumptions about the identity of planktonic larvae in the California Current. These are that any larvae referable to the Galatheidac (Gurney 1942), having a serrated rostrum in all except the first instar, not having an aciculate second antenna, and having a dorsal abdominal spine in later instars, can be considcrcd to belong to P. planipes. In fact, this is by far the most abundant galatheid in the region, and galatheid larvat having the above characteristics far outnumber all other galatheid larvae. The objects of this investigation were to identify the breeding areas and seasons of this important species, to determine the relative contributions of the adults in the benthic and pelagic phases to the larval population, and to determine to what extent the final ecdysis into the megalopa took place in water shallow enough to give

2 144 ALAN R. LONGHURST TABLE 1. Breeding season of Pleuroncodes planipes from data on timing and composition of catches of larval forms in the CalCOFI plankton samples Date 1957 Jan FCb Mar Apr May Jun JUl Aug Sep Ott Nov Dee Stage Total Stas. 0 ) I larvae Avg No./Sta. (individuals/m Sta.) I ~~5tve Max observed Larval stages <.. density (indi (%) viduals/m ) I II III IV V Megalopas J an Feb Mar Apr May Jun Jul A11g SeP Ott Nov Dee ,s 0.1 L J an Feh the megalopas the possibility of entering the benthic phase without drifting into the oceanic environment. The material comprised 2,546 plankton samples taken during the CalCOFI cruises with a standard lm net fished in a standard fashion to a depth of 140 m ( Ahlstrom 1948) ; such samples are available from a 15year pcriod, but 1957 and 1959 were selected for their good coverage and because the first was a typical cool year, the second a warm year ( Sette and Isaacs 1960). The coverage was later extended a few months into 1958 and Aliquots of the total samples were examined, and 483 of these were California Cooperative Oceanic Fisheries Investigations. found to contain zoeae or megalopas of P. pzanipes. Also available were the data on the occurrence of adults of the pelagic phase of the same species in the same plankton hauls, which had already been discussed in relation to the changing hydrographic conditions of this period (Longhurst 1967a ). I am extremely grateful to D. L. R. Scibert who was responsible for the acquisition of data, and who performed most of the examinations of the plankton samples himself. The work was supported by the Bureau oe Commercial Fisheries under contracts /221 and formed part of the Scripps Tuna Oceanography Research Program.

3 DISTRIBUTION OF PLEURONCODES LARVAE 145 TABLE 2. Frequency distribution of temperatures at 10 m at stations where Pleuroncodes planipes larvae occurred in the plankton samples 1957 J an F& Temp e4611;; c Mar Apr May Jun Jul _ I: 1: :: &g 1 1 c Sep _ L (jet ;y:i:ilz,_ Nov c ( C) Dee _ 1959 Jan Feb Mar Apr May Jun JUl Aw SeP Ott Nov s s L ;:z:,, 211 Dee a _ w TEMPORAL AND SPATIAL DISTRIBUTION OF BREEDING Boyd and Johnson ( 1963) examined adult P. planipes taken in the CalCOFI net tows during I959 and I960 and found that ovigerous females occurred only from December to the end of March and compriscd more than 50% of all females taken during February; these data support their observation, but indicate that stage I larvae (which are unlikely to have been released from the females more than 10 days earlier ) occur in the samples for more than the four months in which ovigerous females were taken (Table 1). In 1957, stage I larvae were taken until the end of May, though peak abundance was in February; the same pattern was seen during 1959, although in that year the peak occurred during the March cruise. In the breeding season of , which was covered in its earlier stages better than during other years, breeding apparently began in late summer and occurred rather commonly during October, November, and December. There is a general correspondence between the level of maximum abundance in any sample and the number of stations with larvae in the same month, so that breeding is most widespread when it is most intense in any region. The timing of breeding was remarkably little altered during the warm year compared with the more usual conditions during early Since the net tows from which the samples were obtaincd integrated the plankton of the upper 140 m, there is no information available on the distribution of the larvae with depth, but they are presumed to have occurred mostly in the mixed layer. Thus, it may be valid to compare their distribu

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5 DISTRIBUTION OF PLEURONCODES LARVAE 147 tion in the samples with the distribution of temperatures at a depth of 10 m. The data in Table 2 indicate that there were considerable differences in these tcmperaturcs at stations at which larvae occurred bctwecn the two years, notably in the months of January and February when a high percentage were in stages I o,r II. There is cvidcnce that breeding occurred in colder water in 1959 than in 1957quite contrary to expectation. Fig. 1 shows that in February 1959, as compared with February 1957, there had been a shift of both breeding adults (indicated by larvae) and surface isotherms towards the north; it also shows that the crabs were displaced farther northwards in 1959 than were the isotherms. As the breeding season advanced in each of the two years, the early larvae occurred in progressively cooler water until about May when temperatures began to rise and progressively later larval stages prcdominated in the catches. Examination of the monthly plots of larval distribution in the samples shows that in the first four or five months of the year not only does breeding occur progressively farther towards the north (especially in 1959), but also the inshore regions along the coasts of southern and Baja California become more influenced by the effects of coastal upwelling, so that inshore temperatures drop. Later in the year when older zoeae and megalopas were found in water with temperatures in excess of 25C, upwelling had ceased (Reid, Roden, and Wyllie 1959) and larval populations were far offshore. The range of temperatures at which early zoeae were found corresponds rather closely to the range of tolerance established experimentally by Boyd and Johnson (1963) for Pleurancodes larvae. Only in the later stages, and particularly as mcgalopas, were larvae found at temperatures in cxccss of 20C (above which Boyd and Johnson were not able to culture larvae through their co,mplete development ). Early larvae (stages I and II) were present over a rather extended area off southern and Baja California in months in which breeding occurred, but thcrc were typically several rather restricted areas where larvae were present in numbers higher than over the wider area by a factor of 10 or even of 100. Such areas have been supposed to represent major concentrations of the adult breeding populations. In 1957 these areas were found only on the coast of Baja California and only over the continental shelf; during January a patch of early larvae appeared that was close to the coast in the bight south of the Point Eugcnia peninsula, and in the following month much of this bight was occupied by the larvae (stage I and II combined reached a maximum density of 130/m ). Farther to the south, over the shelf close in to Cape San Lazaro and Santa Margarita Island, a breeding con centration occurred in the first four months of the year; early larvae reached a maximum density of 189/m3 in February just off the edge of the shelf at Santa Margarita Island (Sta ). In 1959 the breeding concentrations extended along the coast of southern California as far north as Point Conception. In January and February, breeding occurred along the northern coast of the Santa Barbara Channel, but densities of early larvae as low as 2/m3 were encountered in this region; in March, there was a dense concentration of stage I and II larvae in Sebastian Vizcaino Bay that reached a maximum density of 9,7/m3 at Sta To the south of the Point Eugenia peninsula a high concentration occurred during February and March and occupied much of the bight between there and Cape San Lazaro in a position similar to the 1957 high concentration. In March a density of 61/m was recorded. These patches of larvae could have originated either from the stocks of benthic adult P. pkznipes known, in general terms, to occur on the upper part of the continental slope along the coast of Baja California in cool years (Boyd 1963, 1967; Longhurst 1967a, b ), or they could have originated from pelagic stocks in the same area. The distribution of adults of the pelagic phase was examined to determine

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7 DISTRIBUTION OF PLEURONCODES LARVAE 1 ADULTS l 1019 l a u P 0 0 L 0 l4 I FIG. 3. Distribution of adult and of stage I Pleuroncodes in CalCOFI plankton samples in January Units and locations as Figs. 1 and 2.

8 150 ALAN R. LONGHURST loo l ' FIG. 4. Distribution of adult and of stage I Pleuroncodes ruary Units and locations as Figs. 1 and 2. in the CalCOFI plankton samples in Feb

9 DISTRIIXJTION OF PLEURONCODES LARVAE 151 ) AWL13 I 00 ) LHKVHE, l \ oo FIG. 5. Distribution of adult and of stage I Pleuroncodes in the CalCOFI plankton samples in uary Units and locations as Figs. 1 and 2. an

10 152 ALAN R. LONGHURST 3 ADULTS /..O I Ill l / FIG. 6. Distribution of adult and of stage I Pleuroncodes in the CalCOFI plankton samples in February 190. Units and locations as Figs. 1 and 2.

11 DISTRIBUTION OF PLEURONCODES LARVAE 153 if such stocks were present in the required areas in these two years at the periods of peak spawning: The evidence was conflicting. In February 1957 ( Fig. 2)) the general occurrence in the region of Baja California of pelagic adults and stage I larvae corresponded, but the detailed distribution showed many anomalies. In particular, the highest numbers of each occurred at different locations. At Sta off Santa Margarita Island there were 151 larvae/m3, but no adults; these were most abundant between SO and 130 km to the southwest at Stas and It is supposed, therefore, that this patch of larvae had an origin in benthic rather than pelagic phase adults. In January 1958 ( Fig. 3)) pelagic adults extended farther offshore and to the southwest than the larvae, while these extended farther north along the coast than the pelagic adults. There was good evidence to the west of Cape San Lazaro of a larval patch derived from pelagic adults, as is evident in the figure. The same is seen, but less clearly, to the west and northwest of Cape Colnett. In February 1959 ( Fig. 4)) the correlation between adults and early larvae was not clear, for larvae were distributed rather evenly along the whole surveyed coastline with unusually low variation in larval densities between regions; however, in each case where larvae were observed away from the coast, they were in relatively close proximity to collections of pelagic adults. Finally, in 1960 ( Figs, 5 and 6) the same pattern is seen again; there are offshore patches of stage I larvae, much too far from the coast to have drifted off during the first instar, which correspond with patches of pelagic adults. There are also inshore patches, again predominantly on the Baja California coast, where large numbers of larvae occur with few or no adults, and which seem to indicate breed ing by the benthic stocks. As in the two previous years, both warm seasons, the larvae occurring sparsely along the coast of southern California do not appear to bc correlated with the presence of pelagic adults. The correspondence between the distribution of offshore adults and larvae, where neither are present in high conccntrations, cannot be expected to be precise: A zooplankton net is a far more efficient sampler of larvae (having relatively large numbers per unit volume and little avoidance ability) than of adults (relatively sparse and relatively efficient avoiders of nets). It is assumed that samples of offshore larvae, in rather low concentrations and apparently in the absence of adults, were actually the result of spawning by nearby pelagic adults that were missed by the sampling technique used. The evidence can bc summarized thus: offshore concentrations of early larvae always appear to be correlated with the presence of pelagic adults, while the converse is not necessarily trueeven during the breeding season there are patches of adults offshore unaccompanied by larvae. The greatest concentrations of early larvae are always, notwithstanding the above statement, over or close to the continental shelf and frequently in areas where few or no pelagic crabs accompany them in the samples. Apparently, the pelagic adults that were swept northwards along the coast of southern California from late 1957 to mid1959 by the unusual conditions of flow in the California Current in this period (Boyd 1967; Glynn 1961; Longhurst 1967a) were able to breed in regions far to the north of their usual breeding range. SUBSEQUENT HISTORY OF LARVAL PATCHES The duration of the larval stages varies considerably ( Boyd and Johnson 1963); stage IV, comprising from four to eight instars, occupies about 5060% of the total duration of larval development, while the other stages comprise only a single instar. This is reflected in the total catch composition of larvae in a year s samples, expressed in per cent: Year hi II III IV V Meg&pa

12 154 ALAN R. LONGHURST It would be unrealistic to use thcsc figures to make estimates of mortality rates, because it is not known whether all stages involved were equally well sampled. There are reasons to suspect that they were not: early larval patches are small and dense, later larvae are more widespread; older larvae undoubtedly avoid nets better than early stages; there may well be a differential in the relative depth distribution of early and late stages. That the percentage catch composition follows approximately the pattern expected from the known relative duration of each stage (Boyd and Johnson 1963) indicates that no gross sampling differential occurred and that for relative distribution purposes the data are probably adequate. Larvae of P. planipes occur in a general area of overall offshore drift of water from the coast away to the southwest as the California Current turns in this direction. Johnson (1960) has discussed this in rclation to the coastal spiny lobster Panulirus interruptus; the situation poses certain problems for the retention of enough larvae during a long larval life so that a sufficient number can finally settle onto the continental shelf after metamorphosis. There is usually a large population of late megalopas and small subadult P. planipes in the southwest corner of the CalCOFI station pattern in late summer. This population stretches at least 320 km to the southwest of Cape San Lucas and oceanic populations comprised mostly small subadult individuals occur up to 1,690 km offshore (Longhurst 1967a). The data were cxamincd for cvidencc of the offshore movement of late larvae and megalopas or of the direct settlcmcnt of megalopas into the benthic phase without a prior offshore advection. The subsequent history of the larval patches representing major centers of breeding were traced up to six months, with the following results. The February 1967 patch in the bight south of Point Eugenia was traced to a patch of stage III and IV larvae in the central bight in March and to stage V larvae and megalopas at the continental edge in the same area in April; in June a patch of megalopas was found at Sta and again in July at about 130 km beyond the edge of the continental shelf. The geostrophic flow pattern for July ( Wyllie 1966) shows complex offshore eddies in this region capable of producing the observed offshore transport, The patch observed in January 1957 south of Cape San Lazaro grew in size and in density of early larvae until April, and during this time changes in the relative distribution of the various stages were observed. The following were numbers per 1113 of the early larval stages observed at Sta just clear of the shelf off Santa Margarita Island: 1957 I II III Feb Mar Apr In March there were also 29 stage IV/m3 at this coastal station, but by April the concentration of later larvae had moved to the southwest; at Sta about 190 km from the coast there were 17 stage IV, 33 stage V, and 10 megalopas per m3. There was no cruise in May, and by June a patch of megalopas at Sta (another 65 km southwards) represented the remnants of the original breeding patch. This movement is in general agreement with the observed geostrophic flow. In 1959 the larval patch that developed in the region of the Santa Barbara Channel in January and February gave rise to a patch of stage IV larvae in March outside the Channel Islands and stage V larvae in the same area in April. No megalopas were seen in this region, and the patch was not traced any further. The breeding that occurred in Sebastian Vizcaino Bay in March 1959 was not traced beyond the initial month; before the cruise in April, the bay was flushed clear of larvae. Their fate could not be traced because of the large larval patches of local origin south of Point Eugenia with which they are presumed to have mingled.

13 DISTRIBUTION OF PLEURONCODES LARVAE 155 The patch originating in February 1959 south of Point Eugenia could be traced more satisfactorily. In accordance with a general southerly and uneddicd flow of the California Current, this patch had moved southwards by March and contained stage IV larvae up to 93/m ; it had reached the center of the bight between Point Eugcnia and Cape San Lazaro by April, maintaining about the same composition. In May, there was derived from this a rather complex offshore distribution of patches comprising stages IV and V (prcdominantly the latter) and postlarvae to a density of 15 megalopas/m. This situation remained similar in June and July when there was an indication of offshore geostrophic flow from the bight, and megalopas were centered at Sta about 110 km west of Cape San Lazaro. Finally, in July the geostrophic flow in this region indicated that an anticyclonic eddy gave a northwest transport, which was in fact confirmed by the distribution of megalopas that were now concentrated about 225 km southwest of Point Eugenia. There is, thus, sufficient evidence to demonstrate the offshore transport of late larvae and of megalopas having their origin in coastal breeding centers. It is clear that at least a significant percentage of these larvae have this fate if the normal processes of mortality allow them to reach the megalopa stage; it is also clear from such data as those for 1957, when the megalopas were last detected far to the west of Cape San Lucas, that this transport is sometimes continued into oceanic regions with no likelihood of immediate return to the region of the coast by eddying, The question of whether any megalopas achieve the benthic phase directly over the continental shelf has hardly been resolved by these data. To settle this question, a search for small subadults shoul,d be directed to the deeper parts of the shelf south of Point Eugenia and to the south of Santa Margarita Island in the months of May and June. REFEIXENCES hilstrom, E. H A record OF pilchard eggs and larvae collected during the surveys made in U.S. Fish Wildlife Serv., Spec. Sci. Rept p. BOYD, C. M The larval stages of Pleuroncodes planipes Stimpson. Biol. Bull., 118: * Distribution, growth, trophic relationships and respiration of a marine decapod crustacean Pleuroncodes planipes Stimpson (Galatheidae). Ph.D. Thesis, Univ. of California, San Diego. 67 p Benthic and pelagic habitats of the red crab. Pacific Sci., 21: AND M. W. JOHNSON Variations in the larval stage of a decapod crustacean, Pleuroncodes planipes Stimpson ( Galatheidae). Biol. Bull., 1241: GLYNN, P. W The first recorded mass stranding of pelagic red crabs, Neuroncodes planipes, at Monterey Bay, California, since 1859, with notes on their biology. Calif. Fish Game, 47: GURNEY, R Larvae of decapod crustacea. Ray Sot., London. 306 p. JOHNSON, M. W Production and distribution of larvae of the spiny lobster Panulirus interruptus (Randall) with records on P. grac&s Streets. Bull. Scripps Inst. Oceanog., Univ. Calif., 7: LONGHURST, A. R. 1967a. The pelagic phase of Pleuroncodes planipes in the California current. Rept. Calif. Coop. Ocean. Invest., (in press ) b. The biology of mass occurrences of galatheid crustaceans and their utilization as a fisheries resource. Proc. FAO World Conf. Biol. Cult. Shrimps, (in press ). REID, J. L., JR., G. I. RODEN, AND J. G. WYLLIE Studies of the California current system. Rept. Calif. Coop. Ocean. Fish. Invest , p SETTE, 0. E., AND J, D. ISAACS The changing Pacific Ocean in 1957 and Rept. Calif. Coop, Ocean. Fish. Invest,, 7: SCHMITT, W The marine decapod crustacea of California. Univ. Calif. Publs. Zool., 23 : WYLLIE, J. G. 196G. Geostrophic flow of the California current at the surface and at 203 meters. Calif. Coop. Ocean. Fish. Invest. Atlas 4, p. viixiii + l288,