BULLETIN OF MARINE SCIENCE, 49(3): 699-714,1991

DIFFERENCES IN INSHORE-OFFSHORE AND VERTICAL DISTRIBUTION OF PHYLLOSOMA LARVAE OF PANULIRUS, AND SCYLLARIDES IN THE FLORIDA KEYS IN MAY-JUNE, 1989

Cynthia Yeung and Michael F. McGowan

ABSTRACT As part of a multidisciplinary study of recruitment to South Florida reefs, lobster phyl- losoma larvae were sampled with a MOCNESS to describe their vertical and horizontal distribution in relation to coastal oceanography. A total of 850 phyllosomata of the Panulirus. 200 of the genus Scyllarus. and 43 of the genus Scyllarides were caught at 26 stations. The phyllosomata of Scyllarus spp. were more abundant near shore, while those of Panulirus spp. phyllosomata were distinctly more abundant at offshore stations. All stages of Scyllarus were present in comparable proportions, but 60% of Panulirus were stage-I larvae. Scyllarides phyllosomata were similar to Panu/irus in distribution and stage composition, Limited data from three additional anchor stations showed small range of diel vertical mi- gration in the upper 75 m by Panu/irus stages IV-VIII. Vertical migration crossed the ther- mocline but did not go below the 24° C isotherm. These preliminary data support the hy- pothesis that Scyllarus is locally recruited while Panu/irus and Scyllarides are recruited from upstream populations.

The phyllosoma larvae of the spiny lobsters (Palinuridae) have an extended planktonic life estimated to last 6 to 12 months (Lewis, 1951; Sims, 1966; Chit- tleborough and Thomas, 1969; Phillips et a1., 1979; Kittaka, 1988; Kittaka and Ikegami, 1988; Kittaka and Kimura, 1989). During this larval period they are susceptible to long-distance transport by local and oceanic circulation, and their survival is critically affected by physical oceanographic processes, Studying the larval distribution of these species in the oceanographic context assists in the de- duction of linkages between the variability of the physical and the biological regimes, and constitutes a key aspect of recruitment research. Although the long planktonic duration favors long-distance dispersal (Thorson, 1961), local oceanographic features such as gyres and countercurrents have been implicated in the entrainment and retention of phyllosomata in South Africa (Lazarus, 1967), California (Johnson, 1960a, 1960b, 1971a, 1974), and Australia (Phillips et a1., 1978) for different palinurid species of the genus Panulirus. Some active control over advection is believed to be exercised through diurnal vertical migration of the phyllosomata, by which they encounter currents flowing in op- posite directions at different depths and achieve a horizontal distribution con- ducive to retention (Johnson, 1971a; Rimmer and Phillips, 1979; Phillips, 1981). In the Florida Keys, near the northern and down-current limit of the range of Panulirus argus, gyres and countercurrents have also been observed (Brooks and Niiler, 1975; Lee, 1975; Lee and Mayer, 1977) which might promote the retention and, ultimately, the recruitment oflocally-spawned phyllosomata to maintain the large adult populations. Previous studies of the distribution of phyllosomata in the southern Straits of Florida (Lewis, 1951; Ingle et a1., 1963; Robinson and Dimitriou, 1963b; Sims and Ingle, 1966; Richards and Potthoff, 1980) have not treated the phyllosomata of the genera Scyllarus and Scyllarides (Scyllaridae) though they are common in the plankton in this area (Robinson and Dimitriou, 1963b; Robertson, 1968a; 1968b; Little, 1977; Lyons, 1980). As was suggested for larval fish species (Miller et al., 1988), description and comparison

699 700 BULLETINOFMARINESCIENCE,VOL.49, NO.3, 1991

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of the spatial distribution across genera might reveal how the interaction of bi- ological and hydrographic mechanisms affect the recruitment process. In this study we quantitatively describe and compare the horizonta1.distribution and abundance of phyllosomata of Scyllarus and Scyllarides, as well as Panulirus, in the Florida Keys during 26 May-5 June 1989. We present data on vertical distribution and preliminary observations of vertical migration. We describe the distributions of larvae with regard to advective loss or retention in countercurrents. Finally, we discuss the implications of the observed distributions to contrasting recruitment strategies among Panulirus and Scyllarus in the Florida Keys.

METHODS

This study is part of the initial phase of the multidisciplinary Southeastern Florida and Caribbean Recruitment (SEFCAR) project investigating the ecological factors affecting the recruitment of reef organisms to this area. Field sampling was conducted in the Florida Keys from the R/V CALANUS during SEFCAR cruise CA8906 26 May-5 June 1989. The study area (Fig. 1) stretched alongshore from Carysfort Reef in the northeast to Cosgrove Shoal in the southwest. Sampling stations were placed 5 to 10 km apart along transects which extended normal to the shoreline from the edge of the YEUNG AND McGOWAN: DISTRIBUTION OF PHYLLOSOMATA IN THE FLORIDA KEYS 701

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2.0 o 1.0 0.0 DAY (n=!?) NIGHT (n=9) Figure 2. Day versus night comparison ofln-transformed standardized catch ofphyllosomata (catch ·10 m-2); n = number of samples. reef tract (2-4 kIn offshore) to the approximate edge of the shelf (30 km offshore, or less). The cruise was divided into two legs. During the first leg (26-29 May) four transects running offshore from Carysfort Reef, Tennessee Reef, Looe Key, and Key West, were sampled. During the second leg (30 May-5 June) five transects were sampled: Cosgrove Shoal, Looe Key, Sombrero Reef, Davis Reef, and Carysfort Reef. Sampling was carried out continuously day and night, depending on the time the ship arrived on station (Table I). The means of the abundance of total phyllosomata (natural-log transformed) caught at day (N = 17) versus night (N = 9) stations (Fig. 2) were not significantly different (t.J.O'.24= 0.475, Pob",rved= 0.639, t-test, Sokal and Rohlf, 1981), therefore no adjustments in catch were made for day- night sampling bias. In addition to the 26 standard stations, 3 stations were made at the same location (stations 38-40) at Looe Key on 2 June (Fig. 1), consecutively sampling at 0245, 0805, and 1818 Universal Time (UT) to investigate the diet vertical migration pattern ofthe plankters. Local time was 4 h earlier than Universal Time. Due to equipment failure, samples were not collected during the entire 24-h cycle, but samples from these three samples were examined for preliminary indications of a diel vertical migration. A l_m2 MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System, Wiebe et aI., 1976) was employed in sampling. It has nine nets of 0.333-mm mesh-size which can be opened and closed sequentially at the desired depth by electronic signals sent through a conducting cable from the surface. A flowmeter and conductivity-temperature depth (CTD) sensors are attached to the net frame. Data from these devices are transmitted back up the towing wire to a shipboard computer for recording and real-time plotting. For our sampling, the first in the set of nine nets was fished obliquely from the surface down to 200 m or to the closest multiple of25 m, if the depth of water at the station was less than 200 m. Each subsequent net fished approximately 250 m3 within 25 m strata as the MOCNESS was towed to the surface at a vertical ascent rate of approximately 5 m· min-I. The depth- stratified samples of the upward tow were used in our data analysis. Plankton samples were preserved in formalin and seawater, and were later transferred to 70% ethanol. Phyllosomata were sorted from the samples, and then identified to the lowest taxonomic level possible and assigned stages using literature references (Lewis, 1951; Robertson, 1968a, 1968b, 1968c, 1969a, 1969b, 1969c, 1971, 1979; Baisre and Ruiz de Quivedo, 1982) for confirmation or elimination of the possible species present. Following Lewis (1951), 11 stages for the entire larval development were assigned to Panu/irus, 9 stages to Scyllarus based on the maximum number of stages assigned to Western Atlantic Scyllarus species by Robertson (1968b), and 11 stages to Scyllarides based on Robertson (1969c). The catch of each net was standardized to numbers per 1,000 m3 of seawater (no. of phyllosomata per 1,000 m3 = catch x 1,000 -;- volume filtered). This concentration ofphyllosomata was assigned to the mean depth that each net fished to illustrate the vertical distribution. The catch per unit volume was converted to abundance (catch under an area) by multiplying by the depth fished by the net. The 702 BULLETIN OF MARINE SCIENCE, VOL. 49, NO.3, 1991

Table I. Station data and catch ofpalinurid and scyllarid phyllosomata on cruise CA8906, 26 May- 5 June 1989 (Catch from the 24h anchor stations 38, 39, 40 are excluded from the total)

Latitude Longitude Number of phyllosomala caught Distance N W offshore Time Depth Panu- Scy/· Scy/· Station (km) (decimal degree) Date (UT) (m) lirus /arus /arides I 2.87 25.20 80.19 5-26 2322 day 75 152 I I 2 7.96 25.15 80.15 5-27 0341 night 139 183 0 0 3 4.10 24.71 80.77 5-27 1326 day 75 II 25 2 5 13.16 24.62 80.71 5-28 0107 night 200 91 6 4 7 22.65 24.54 80.64 5-29 0529 night 207 1 2 0 II 1.73 24.53 81.40 5-28 1608 day 40 10 9 I 13 10.62 24.45 81.36 5-28 1933 day 189 91 2 3 15 21.02 24.36 81.32 5-28 2345 day 213 5 0 I 16 30.59 24.28 81.28 5-29 0325 night 262 I 0 0 18 19.86 24.31 81.79 5-29 1158 day 225 3 I 0 20 12.64 24.37 81.80 5-29 1554 day 185 6 0 0 22 3.07 24.44 81.80 5-29 1843 day 54 5 23 0 23 5.56 24.42 82.20 5-30 1831 day 77 0 74 0 25 14.20 24.36 82.20 5-30 2136 day 181 0 4 0 27 22.21 24.29 82.20 5-31 0308 night 237 53 4 0 30 1.83 24.53 81.40 6-1 1747 day 42 0 17 0 33 11.90 24.44 81.36 6-1 1336 day 190 12 I I 35 20.81 24.36 81.32 6-1 1825 day 207 5 I I 38 24h 15.55 24.41 81.35 6-2 0245 night 185 12 0 0 39 24h 15.84 24.41 81.35 6-2 0805 night 185 8 I 0 40 24h 15.30 24.41 81.34 6-2 1818 day 185 I 0 I 42 2.01 24.61 81.09 6-3 1219 day 48 3 10 0 44 10.45 24.53 81.05 6-3 1631 day 195 4 II 0 46 17.93 24.46 81.01 6-3 1737 day 201 4 0 0 48 24.44 24.41 80.93 6-4 0121 night 222 20 0 0 49 13.97 24.81 80.38 6-4 0918 day 195 13 0 I 50 11.96 24.82 80.41 6-4 0354 night 166 6 0 I 51 3.29 24.90 80.51 6-4 0648 night 40 3 5 3 52 2.33 25.20 80.20 6-4 1234 day 68 168 4 24 Total 850 200 43

abundance was standardized to numbers under 10m2 sea surface for each net and this was summed for all nets at a station to give the total abundance under 10 m2 sea surface at each station. The horizontal distribution is displayed by plotting the standardized abundance of phyllosomata at each station (Fig. 3). The sum of the standardized abundances over all stations was also used as the basis for comparing percentage frequencies of stages. The relative nearshore-offshore distribution of one genus versus another was tested statistically by comparing the mean distance offshore for each genus collected on each transect. The mean distance offshore was treated as a binomial variable with the two possibilities at each transect being (I) phyl- losomata of genus A were further offshore, or (2) phyllosomata of genus B were further offshore. Exact binomial probabilities of the observed outcomes were calculated using the formula (Sokal and Rohlf, 1981): P(r) = [nl/ {r!(n - r)!}]p'qn-, where n = number of transects with> I station and the co-occurrence of both genera, p = probability of event (I), q = probability of event (2) and r = number of outcomes of event (I). The stage compositions of the total standardized catch (n '10'm-2) for each genus are described and compared using percentage frequency histograms. The horizontal and vertical distribution of each genus are also tabulated by larval stage.

RESULTS Out of 26 stations on 9 transects (excluding 24-h stations), 850 Panulirus, 200 Scyllarus, and 43 Scyllarides phyllosomata were caught (Table I). These three YEUNG AND McGOWAN: DISTRIBUTION OF PHYLLOSOMATA IN THE FLORIDA KEYS 703

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LONGITUDE LONGITUDE Figure 3. Horizontal distribution and relative abundance of the phyllosomata of each genus at the sampling stations during SEFCAR cruise CA8906 Leg I (26-29 May 1989) and Leg 2 (30 May-5 June 1989). The symbols are proportioned in size to the abundance range (catch· 10 m-2) and centered at the sampling stations; O-Panu/irus. ~-Scyllarus. O-Scyllarides. *-no catch. a) Panu/irus Leg I, abundance range 1-165 (per 10 m2); b) Scyllarus Leg I, 1-17; c) Scyl/arides Leg I, 1-5; d) Panu/irus Leg 2, 1-155; e) Scyl/arus Leg 2, I-50; f) Scyllarides Leg 2, 1-22. genera comprised the entire catch of 1,095 phyllosomata except for two stage I phyllosomata identified as Parribacus antarcticus. The Panu/irus phyllosomata we obtained during this cruise do not show separate morphs to enable identifi- cation at the species level. Four Scyllarus species were obtained: 77.5% of S. chacei, 14% of S. americanus, 7.5% S. depressus, and 1% of an unidentified species, probably S.jaxoni, the only other known Scyllarus species to occur in these waters which larvae have not yet been described (Robertson, 1968b; Lyons, 1970; Ham- mer, 1974). Only two of the five Scyllarides species known in the west Atlantic are common in the Florida and Caribbean region-the first three stages of S. nodifer and S. aequinoctia/is are difficult to separate (Robertson, 1969c), but the few mid-stages collected (IV, VI, VII) were all identified as S. aequinoctialis. The distribution of phyllosomata differed among the three genera in terms of relative nearshore-offshore abundance. Both Panu/irus and Scyllarus were caught in seven of the eight transects which have more than one station, and the peak abundance of Panu/irus occurred further offshore than Scyllarus on all seven (Fig. 3). This statistically significant (P(r) = 0.008) contrast in spatial distribution between the two genera is clearly illustrated in the cross-section of the Cosgrove Shoal Transect (Fig. 4). Scyllarides co-occurred with Panu/irus on five transects with more than one station. The mean abundance of Panulirus was distributed further offshore in two of them, which essentially shows no statistical difference in their relative distance from shore (P(r) = 0.3125). On the four transects with both Scyllarides and Scyllarus, Scyllarides was further offshore in all of them (P(r) = 0.0625). In our data Scylla rides occurred further offshore than Scyllarus, with 704 BULLETIN OF MARINE SCIENCE, VOL. 49, NO.3, 1991

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Figure 4. Cross-section of the Cosgrove Shoal transect (stations 23, 25, 27) showing the vertical distribution and relative concentration of the phyllosomata of Panulirus (0) and Scyllarus (6) in the water column (*-no catch), with temperature contours. The symbols are proportioned in size to the concentration range (catch' 1,000 m-3) and centered in the mean depth of each 25-m sampling stratum. The dashed line approximates the front between eastward Florida Current flow on the right and westward countercurrent flow on the left. The solid line delineates the sea bottom. a horizontal distribution pattern more akin to that of Panulirus. However, the small sample size of Scyllarides makes conclusions about it tentative. The relative abundance of different larval stages also differs among the genera (Fig. 5). Approximately 95% each of the total standardized catch (=sum of the standardized abundances in n· 10, m-2 over all stations) of Panulirus and Scyl- larides is between stages I-III. In contrast, each Scyllarus stage between and including II and VII makes up 15-20% of the total standardized catch, leaving approximately 5% each for stage I and for combined stages VIII and IX in the tails of the distribution. Overall, Panulirus and Scyllarides were more likely to be early stages than Scyllarus and they were more likely to be further offshore. Stages V and above of Panulirus (Table 2) were caught in four out of nine transects (Key West, Looe Key 2, Sombrero, Davis) at those stations which are furthest, or next furthest, offshore on a transect. On no occasion were they caught at the station closest to shore on a transect. The horizontal distribution of stages I-IV was more variable than that of the later stages. They were caught at almost all stations, but the highest abundance tends to be at stations midway on transects. Over 50% of the stages I-IV were caught at Carysfort, where the shelf is narrow (200 m deep less than 10 km offshore). The width of the shelf here is less than one-half the average breadth of the Pourtales Terrace. The offshore extent of the two transects made at this location is therefore limited: one has two stations (Carysfort 1) 5 km apart and the other has only one station (Carysfort 2). Phyllosomata of each of the stages I to IV are more abundant at the outer station than at the inner one at Carysfort 1. Except for Carysfort 1 where the standardized catch consists of only one Scyl- larus phyllosoma (stage VII), the remaining eight of nine transects contain Scyl- larus phyllosomata of early stages I-IV and of later stages V and above (Table 2). All stages of Scyllarus are more abundant at stations closer to shore, and abundance decreases with increasing distance offshore. At the Looe Key 1, Som- brero, and Davis transects, no Scyllarus were caught in the outermost two stations. YEUNG AND McGOWAN: DISTRIBUTION OF PHYLLOSOMATA IN THE FLORIDA KEYS 705

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The sample size of Scylla rides is too small to portray a definitive horizontal distribution pattern of the larval stages (Table 2). Moreover, 60% occurred at the one-station Carysfort 2 transect. Later stages were very rare: only about 6% were stages V and above ([n'10'm-2]: VI[I]; VII[I]), and stage IV made up only 3% (I. 10· m-2). None of the later stages were caught at the inshore-most station. The overall vertical distribution by genus and stage ofphyllosomata is described using the average concentration (n' 1,000' m-3) of all stations at each sampling stratum (Table 3). The highest average concentration of each genus occurred in the top 50 m of the water column, and none was caught from deeper than 175 m. However, for all three genera, some early (I-IV) and late (V and above) stage phyllosomata were caught from below 50 m to as deep as 175 m. The average concentration of Panulirus phyllosomata in 25-50 m was 1.5 x higher than in the 0-25 m stratum and more than 40 x higher than in any deeper stratum. Stage I Panulirus were found in all depths, down to 175 m, and late stages do not show any strikingly different pattern of vertical distribution from early stages. Scyllarus early and late stages also inhabited a wide vertical range, but stage I Scyllarus phyllosomata occurred only in the top 25 m. The average concentrations of total Scyllarus phyllosomata in the 0-25 m and 25-50 m strata were about equal. Nearly all Scylla rides were of stage I-III. Like Panulirus, the highest concentration was at 25-50 m, and stage I was found as deep as the 150-175 m stratum. The three stations made at the same location over 24 h (38,39,40) were sampled at approximately 2 h after sunset (0245 UT), 2 h before sunrise (0805 UT), and mid-day (1818 UT). Equipment failure prevented us from sampling near midnight local time. The vertical distributions (Fig. 6) are similar for the two dark periods. Phyllosomata were concentrated at the surface stratum (0-25 m) and none was found below 50 m. At mid-day, no phyllosomata were caught in the surface 25 706 BULLETIN OF MARINE SCIENCE, VOL. 49, NO.3, 1991

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Table 3. Average standardized concentration (N '1,000 m-3) of the larval stages of each genus at each depth stratum. The number of samples for the depth stratum foIlows in parentheses. Oblique tow samples from stations I and 2, and samples from vertical migration stations 38, 39, 40 are omitted

Panu/irus Slage

Deplh(m) II III IV V VI Vll VIll IX X Xl 0-25 (23) 23.8 6.2 0.6 1.0 0.1 1.0 1.4 0.3 0.1 0.2 25-50 (19) 47.3 2.7 0.6 0.8 1.0 0.2 0.2 50-75(15) 0.3 0.2 0.5 0.3 75-100 (15) 0.2 0.3 0.3 0.2 100-125 (15) 0.2 125-150 (13) 0.7 150-175 (10) 0.5 0.2 175-200 (3)

Scyllarus slage 0-25 (23) 1.1 2.3 1.5 1.8 2.7 1.9 1.0 0.1 25-50 (19) 1.8 1.7 3.6 1.5 1.4 0.8 0.4 50-75 (I 5) 0.5 0.5 75-100 (15) 0.3 0.3 0.6 0.2 100-125 (15) 0.1 0.1 0.1 125-150 (13) 0.4 150-175 (10) 0.7 175-200 (3)

Scyllarides Slage

0-25 (23) 1.2 0.4 0.1 25-50 (19) 4.5 0.2 0.2 50-75 (15) 0.3 75-100 (15) 0.2 100-125 (15) 125-150 (13) 150-175 (10) 0.4 175-200 (3)

m nor below 75 m, but the highest concentration shifted down to between 25- 75 m. The diel vertical migration interpreted from the small and virtually mono- specific catch data has a narrow range limited within the upper 75 m. All except two of the phyllosomata caught during these three stations were Panulirus (Table 4), and late stages predominated ([number of specimens]: IV[4]; V[2]; VI[12]; VII[I]; VIII[I]). The other two were a Scylla rides stage VII and a Scyllarus stage VIII. Therefore the vertical migration patterns we observed primarily reflect the behavior of Panulirus late stage phyllosomata.

DISCUSSION We collected more than four times as many Panulirus than Scyllarus phyllo- somata, and only about 4% of the total catch was Scyllarides. Panulirus phyllo- somata would be expected to dominate in the plankton since the adults are known to be much more abundant than any ofthe Scyllaridae (Opresko et aI., 1973), but Robinson and Dimitriou (1963) found mostly Scyllarus phyllosomata in the Florida Keys and the Tortugas Shelf. This could be attributed to the coastal nature of their sampling, in waters 15-50 m deep. It has been observed in other areas, as in our results, that Scyllarus dominate in coastal shelf waters (Johnson, 1971c; Baisre, 1976; Maigret, 1978; Phillips et aI., 1981; Sekiguchi, 1986a, 1986b; Phillips YEUNG AND McGOWAN: DISTRIBUTION OF PHYLLOSOMATA IN THE FLORIDA KEYS 709

0805 1818 1919

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Figure 6. The vertical distribution of phyllosomata recorded at the anchor stations (38, 39, 40), at 2 h after sunset (0245 UT), 2 h before sunrise (0805 UT), and mid-day (1818 UT). The depth of the mixed surface layer (MSL) and the 24·C isotherm were taken from CTD data at the station before (2316 UT) and after (1919 UT) the series of sampling. The standardized catch (catch' I,000 m-') at each 25-m sampling stratum is placed at the mean depth of the stratum, so the bottom and top points indicate the first depth of zero catch.

and McWilliam, 1989), while palinurids are more oceanic and dominate in mid- ocean samples (prasad and Tampi, 1965; Saisho, 1966; George, 1974; Phillips and McWilliam, 1986, 1989). Near the Florida Keys offshore versus nearshore must be defined in terms of water mass properties and current flow direction because the front between the Florida Current and coastal water is dynamic. Thus, although the Panulirus larvae may not have been beyond the continental shelf defined by the 200 m isobath, they were far enough from shore to be in the Florida Current which would advect them away from the Keys. This seems likely for our study because analysis of the direction of the currents (Thomas N. Lee, pers. comm.) at the Cosgrove Shoal transect during the period of sampling indicates a countercurrent (westward) flow inshore where Scyllarus were concentrated, and eastward flow in the western edge of the Florida Current at the outermost station where Panulirus were concentrated (Fig. 4). This difference in distribution might reflect an intergeneric difference in the recruitment process involving the interaction of physical oceanographic and bi- ological mechanisms. A difference in vertical migration pattern, perhaps arising from different behavioral responses to light (Baisre, 1976; Rimmer and Phillips, 1979), can subject phyllosomata to different vectors of current transport. Repro- ductive migration is known for some Panulirus species, but not for scyllarids. P. argus, for example, migrate to deeper waters to spawn, the adaptive value is presumably in releasing larvae near dispersive ocean currents (Hermkind, 1980). The length of planktonic life can also have important implications on larval transport and recruitment. Larval durations of Scyllarus species range from 25 to 125 days (Robertson, 1968b). The larval duration of Scyllarides is estimated 710 BULLETIN OF MARINE SCIENCE, VOL. 49, NO.3, 1991

Table 4. Catch and standardized catch (N'1,000m-3, rounded to the nearest integer) ofphyllosomata at the Looe Key anchor stations. Samples were collected 2 June 1989 at 0245 UT, 0805 UT, and 1818 UT. Local time is 4 h earlier than Universal Time

Station Depth (m) Genus Stage Catch Catch-I,OOO m ' 38 25-50 Panulirus VII I 4 38 0-25 Panulirus IV 4 17 38 0-25 Panulirus V 1 4 38 0-25 Panulirus VI 6 26 39 25-50 Scyllarus VIII 1 3 39 0-25 Panulirus V 1 3 39 0-25 Panulirus VI 5 14 39 0-25 Panulirus VII 1 3 39 0-25 Panulirus VIII 1 3 40 50-75 Scyllarides VII 1 4 40 25-50 Panulirus VI I 5

to be 8-9 months, almost as long as that of the palinurids (Robertson, 1969c; Johnson, 1971b; Phillips and Sastry, 1980; Phillips and McWilliam, 1989). A possible recruitment strategy for phyllosomata of short planktonic life is to be retained locally by coastal eddies and countercurrents close to the natal area until they eventually settle. The Pourtales Gyre could provide such a mechanism in the middle and lower Keys for those Scyllarus species with planktonic durations of 1 to 2 months (Lee et al., submitted).l It is unlikely for the long-lived Panulirus and Scyllarides phyllosomata to be retained for the entire pre-settlement period in an area with a hydrography as dynamic as the Straits of Florida. They are probably dispersed by oceanic currents downstream to an unknown fate (Little, 1977). The abundance and predominance of early stages reflect the May-June spawning peak of Panulirus (P. argus, Lyons et al., 1981; P. guttatus, Caillouet et al., 1971), and the start of the spawning season for Scyllarides (Lyons, 1970). All stages of Scyllarus were relatively well-represented, indicative of year-round spawning (Ly- ons, 1970). The nearshore distribution of the whole range of stages also supports that the entire larval development of Scyllarus species takes place in coastal waters (Robertson, 1968b). Early stages (I-IV) of Panulirus tend to be offshore of the innermost station, perhaps as a result of offshore spawning, or offshore transport of inshore-spawned larvae (Sims and Ingle, 1966). It is also possible that some early stages were brought in by the Florida Current from an upstream source. In the laboratory, P. argus and P. guttatus can take approximately 15 days to pass through the first phyllosoma stage (Yeung, unpubl.). Taking the upper limit to be the maximum surface speed at 100 cm· S-1 of the Loop Current in the eastern Gulf of Mexico, and the lower limit to be an estimated low at 20 cm· S-1 of the Florida Current in the southern Straits of Florida, 15 days could translate into a distance of approximately 100 to over 1,000 km. This means a stage I phyllosoma could have come from the Yucatan, the Gulf of Mexico, Cuba, or anywhere in the Caribbean as currents and distance allow. From their distinctly offshore dis- tribution placing them in or near the Florida Current, it is likely that the late stage Panulirus phyllosomata (V and above) were foreign arrivals. Their absence from nearshore stations suggests that they were not circulated in the gyre or counter- current which passes close inshore.

I Lee, T. N., C. Rooth and E. Williams. Influence of wind and Horida Current on circulation near the Horida Keys Caral Reefs, J. Cont. Shelf Res. YEUNG AND MccGOWAN: DISTRIBUTION OF PHYLLOSOMA TA IN THE FLORIDA KEYS 711

The small number of Scylla rides phyllosomata captured have an offshore dis- tribution and stage frequency distribution similar to Panulirus. Scylla rides might then be expected to also undergo long-distance dispersal. However, for S. nodifer, the adult distribution is much more restricted than P. argus. They are found from Cuba up to North Carolina and in the Gulf of Mexico. The phyllosomata have only been captured in the Gulf and in that half of the Florida Current nearest to Florida, where they are the dominant species of the genus in the plankton (Rob- ertson, 1969c). The Florida Current and the Gulf Stream probably curb their southward extent. The distribution pattern suggests that S. nodifer undergoes complete larval development in the Gulf of Mexico, where the adult stocks are concentrated. Some larvae from these stocks probably leak out to supply recruits to the Florida Keys and southeastern United States, as has been reported for bluefin tuna spawned in the Gulf (McGowan and Richards, 1989), but mostly they are recruited back into the local stock at the end of their long planktonic life (Lyons, 1981, 1986). This presents a paradox if offshore distributed Scyllarides are retained but similarly distributed Panulirus are swept away. More extensive information is needed to resolve the question. The pattern of vertical migration and distribution of phyllosomata is virtually unknown in southeastern Florida. Austin (1972) made several observations about the vertical distribution of Panulirus spp. in the Gulf of Mexico. Basically, our observations agree with his that phyllosomata are mostly in the surface layer. All genera were concentrated in the surface 0-50 m both day and night, and there was no difference in the depth distribution of early versus late stages. The higher concentration at the subsurface 25-50 m rather than at the surface 0-25 m might be a bias from the greater number of day (N = 17) versus night (N = 9) stations, since die1 vertical migration occurs over a small range, shifting the concentration peak from 0-25 m at night down to 25-75 m during the day. Contrary to Austin (1972), the thermocline did not impede migration, although it deepened about 10m at the time the phyllosomata migrated downwards (Fig. 6). Also, some phyllosomata were present below the thermocline to as deep as 175 m. These may have been "strays" caused by downwelling or mixing of the water masses. During the first leg of our cruise the Pourtales Gyre was present between Key West and Tennessee Light in the middle and lower Keys (Lee et al., submitted).l Larval reef fish and zooplankton were relatively abundant in this region (M. McGowan et al., 1990), which generally corresponds to the area of high abundance of Scyllarus phyllosomata (Fig. 3). On the second leg of the cruise the gyre had broken down and shifted to the west. The peak abundance of Scyllarus also shifted westward and maximum abundance at a station increased three-fold. Over 50% of Panulirus stages I-IV, but none of the later stages, were caught at Carysfort (leg 1 and 2) where the Florida Current was closest to shore with a strong and persistent northerly mean flow (Lee et al., submitted). I The abundance did not decrease with the breakdown of the gyre in the second leg. This supports our conclusion that Panulirus phyllosomata in our study area are associated with the Florida Current and not with the coastal water mass. There are probably regional differences in the extent oflocal retention of spiny lobster larvae. Recent surveys found many P. argus phyllosomata, including late stages, in the coastal waters of southwest Cuba where a gyre circulation persists (1.Alfonse-Hernandez, pers. comm.). This gyre is believed to retain phyllosomata and to enhance the return of pueruli to southwestern Cuba, resulting in probably the densest population of P. argus in the Caribbean. On the northwest coast of Cuba there is no gyre but instead there is a rapid north-flowing current, and larval abundance is low, suggesting that retention is low here. Off the Florida Keys the 712 BULLETIN OF MARINE SCIENCE, VOL. 49, NO.3, 1991

Pourtales Gyre appears to be too small and of too short persistence to retain phyllosomata of Panulirus and Scylla rides for their entire larval development. It does seem to be of suitable temporal and spatial scale for local recruitment of Scyllarus and other species with similar planktonic distribution and duration.

ACKNOWLEDGMENT

This work was supported by National Oceanic and Atmospheric Administration Cooperative Agree- ment with the University of Miami #NA85-WCH-06134. This is Contribution No. I from the South- eastern Florida and Caribbean Recruitment (SEFCAR) project.

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DATEACCEPTED: January 7, 1991.

ADDRESS: Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149-1098.