Temporal and spatial variation in recruitment and ... - CSIRO Publishing

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Marine and Freshwater Research, 2003, 54, 117–125

Temporal and spatial variation in recruitment and growth of medusae of the jellyfish, Catostylus mosaicus (Scyphozoa : Rhizostomeae) Kylie A. PittA,B,D and Michael J. KingsfordA,C A School

of Biological Sciences, A08, The University of Sydney, NSW 2006, Australia. address: School of Environmental and Applied Sciences, Griffith University, PMB 50 Gold Coast Mail Centre, Qld 9726, Australia. C Current address: School of Marine Biology and Aquaculture, James Cook University of North Queensland, Townsville, Qld 4811, Australia. D Corresponding author; email: [email protected]

B Current

Abstract. The timing of recruitment and growth of medusae of the commercially harvested jellyfish, Catostylus mosaicus (Scyphozoa), was examined over a period of 8 years at Botany Bay and 2.5 years at Lake Illawarra in New South Wales, Australia. Recruitment events occurred sporadically during December and between March and July at Botany Bay and between February and July at Lake Illawarra. Recruitment did not occur during late winter or spring at either location, although small numbers of recruit medusae could potentially occur during any time of year. Despite anecdotal observations that recruitment sometimes occurred after periods of heavy rain, we found no correlation between the timing of recruitment and rainfall in Botany Bay over a period of 8 years. Cohort analyses indicated growth of small medusae was very rapid (max. 4.81 mm day−1 ), with growth rates decreasing as medusae grew larger. Medusae appeared to grow faster at Botany Bay than Lake Illawarra. A conservative estimate indicated medusae of C. mosaicus can live for up to 13 months. Extra keywords: longevity, physical forcing, population dynamics, rainfall.

Introduction Jellyfish are important members of the pelagic community. They are voracious predators and a source of prey for many organisms and, therefore, have a major influence on the trophic dynamics of pelagic systems (Kingsford et al. 2000). Jellyfish are also a source of structure in the pelagic environment and provide habitat for fish and invertebrates (Kingsford 1993). In recent years, jellyfish fisheries have been expanding rapidly and catches of rhizostome medusae now regularly exceed 500 000 Mt year−1 (Food and Agriculture Organization 2002). The life history of scyphozoan medusae is complex and consists of a pelagic medusoid phase and a polypoid benthic phase (scyphistoma) (e.g. Uchida 1926; Calder 1973; Pitt 2000). Larvae, which are produced sexually by medusae, settle and metamorphose into polyps that then produce juvenile medusae (ephyrae) via asexual budding (a process known as strobilation). Medusae are typically short-lived and grow rapidly (Arai 1997; Kingsford et al. 2000) and ten-fold increases in biomass of medusae have been observed over periods of weeks to months (Garcia 1990). The timing of recruitment of medusae is predictable in some places (Brewer 1989) but it is irregular elsewhere (Omori et al. 1995). © CSIRO 2003

Changes in the biomass of medusae, therefore, can occur very rapidly and often unpredictably and this could have major effects on the ecology of pelagic communities. Our understanding of patterns of recruitment and growth of medusae is largely confined to species of the order Semaeostomeae and there have been few detailed studies of recruitment and growth of rhizostome species, despite their economic importance. The few studies that have been conducted have been mostly confined to a single location and usually to only one year (but see Kikinger 1992; Pitt and Kingsford 2000a). Detailed information about temporal and spatial variability of recruitment and growth of medusae will contribute greatly to studies of pelagic ecology and is essential for developing sustainable management strategies for jellyfish fisheries (Kingsford et al. 2000). The timing and magnitude of recruitment of scyphomedusae depends on numerous factors including the abundance and rates of strobilation of polyps and the rates of survival of ephyrae. Little is known regarding survival of ephyrae, although availability of appropriate food appears to be important (Olesen et al. 1996; Bamstedt et al. 1997). More is known regarding factors regulating the rate and timing of strobilation and studies have demonstrated that strobilation 10.1071/MF02110

1323-1650/03/020117

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Marine and Freshwater Research

can be influenced by numerous factors including light (Custance 1964), temperature (Loeb 1972), food abundance (Purcell et al. 1999) and presence of zooxanthellae (Sugiura 1964) or interactions of multiple factors (e.g. temperature and salinity; Rippingale and Kelly 1995; Purcell et al. 1999). Some factors implicated in regulating strobilation are associated with those that occur during terrestrial run-off following heavy rain. For example, salinity, water temperature and food abundance may all alter during run-off events. Catostylus mosaicus (Quoy and Gaimard 1824) is a rhizostome medusa that occurs commonly in estuaries and semienclosed coastal waters along the eastern and northern coasts of Australia (Kramp 1965). Although the life cycle of C. mosaicus has been described and is characteristic of other scyphomedusae (Pitt 2000), the location of the polyps in the field and factors influencing the timing and rate of strobilation of C. mosaicus are unknown. Great variation in the timing of recruitment among estuaries has been observed for medusae of C. mosaicus over a period of two years (Pitt and Kingsford 2000a). However, longer time series are required to examine temporal variation and derive possible models that may explain, and eventually predict, the timing and magnitude of recruitment. During the course of our study, recruitment of C. mosaicus was sometimes observed following periods of heavy rain, suggesting that the physical or biological changes associated with run-off may promote recruitment of medusae. The aims of the current study were to: (i) examine temporal and spatial patterns of recruitment of C. mosaicus; (ii) test the hypothesis that recruitment occurred following periods of heavy rain; and (iii) estimate growth rates and longevity of medusae. Crucial to our study is the knowledge that populations of medusae are retained within individual estuaries (Pitt and Kingsford 2000a).

K. A. Pitt and M. J. Kingsford

to contraction. Medusae were measured in the water by divers using snorkels and care was taken to avoid touching or disturbing animals to ensure that diver presence did not cause changes in shape or size of the medusae. Measurements were made while swimming in a straight line to ensure that individuals were not measured repeatedly. A minimum of 100 medusae was measured at each of two sites (0.5–5 km apart) each time a location was sampled to ensure the population was sampled representatively. However, owing to the scarcity of medusae or inclement weather, sometimes fewer than 200 measurements were obtained. Recruitment was arbitrarily defined as occurring if >10% of the population had a BD ≤50 mm because newly recruited medusae would not grow more than 50 mm between sampling events. Although a smaller arbitrary size would have provided a finer measure of recruitment, medusae that recruited shortly after sampling might grow beyond the smaller arbitrary diameter by the next sampling event and thus not be recorded in any recruitment cohort. Consequently, a recruitment event may not have been recorded. The 50 mm BD size category was deemed to be a suitable compromise between providing an accurate estimate of the timing of recruitment and providing a reasonable chance that a recruitment event would be recorded, given the length of time between successive measurements. Relationship between recruitment and rainfall Time series analyses were used to compare the timing of recruitment in Botany Bay and rainfall over the period of December 1990–October 1998. Monthly rainfall data were obtained for the east Sydney region from the Bureau of Meteorology. Size–frequency data were collected at irregular intervals, so size measurements and rainfall data were pooled into three-month intervals beginning in December 1990. Despite pooling data, no size data were collected during the period of December 1994 to February 1995. The missing value in the time series was interpolated using distance-weighted least squares. For the purpose of these analyses, recruits were defined as medusae ≤100 mm BD (cf. 50 mm BD previously). A larger size category was used here because there was a longer interval between successive size measurements (up to 3 months) and, given the rapid growth of small medusae, defining recruits as a larger size category increased the likelihood that a recruitment event would be recorded. Significant autocorrelations in data may artificially inflate correlation between variables (Chatfield 1979). Therefore, autocorrelation functions were plotted to identify any serial dependence in the data (Diggle 1990).A cross-correlation analysis was used to examine the relationship between the proportion of recruits in the population and rainfall.

Materials and methods

Estimates of densities of recruits

Temporal patterns of recruitment

A potential problem associated with defining a recruitment event by the proportion of recruits within a population is that the proportion of recruits can be influenced by fluctuations in numbers of adults. For example, selective mortality of large numbers of adults would increase the proportion of recruits in the population, even if few recruits were actually present. Numbers of recruit medusae (≤50 mm BD) in Botany Bay were counted at approximately monthly intervals between June 1996 and June 1998 to determine if a recruitment event, measured as the proportion of recruits in the population, coincided with the occurrence of large numbers of recruit medusae. Numbers of recruit medusae were counted from the bow of a boat along transects that were 500 m long, 3 m wide and 1 m deep (sensu Pitt and Kingsford 2000a). Each time, six transects were sampled at each of six sites. Very small medusae are translucent and can be difficult to count from a moving boat, so estimates of abundance were considered to be conservative.

Measures of the size distribution of medusae were used to determine the presence and size of cohorts of new recruits at two locations: Botany Bay (151◦ 15 S 32◦ 17 E), near Sydney; and Lake Illawarra (150◦ 50 S 34◦ 30 E), a coastal lagoon approximately 80 km south of Botany Bay. In Botany Bay, Catostylus mosaicus occurs most commonly in the upper reaches of the bay and is rarely found near the entrance or in the adjacent coastal waters, suggesting that a single population of medusae is retained within the bay (Pitt and Kingsford 2000a). Lake Illawarra is an intermittently open coastal lagoon that has a shallow entrance to the ocean. The limited exchange of water between the lake and the ocean suggests that the lagoon also contains a single population of medusae. At Botany Bay, the size–frequency distribution of medusae was measured 70 times, at approximately 2–12 week intervals, between December 1990 and October 1998. The size–frequency distribution was measured at Lake Illawarra on 19 occasions at approximately 4–12 week intervals between February 1996 and May 1998. The bell diameter (BD) was measured to the nearest 10 mm using a ruler and all measurements were made while the bell was expanded to its maximum size prior

Growth Growth rates of medusae were estimated by tracking the increase in size of cohorts through time. Growth was estimated for three periods

Recruitment and growth of Catostylus mosaicus medusae

at Botany Bay (March–June 1992, March–November 1994 and December 1997–February 1998) and for two periods at Lake Illawarra (March 1996–February 1997 and March–September 1997), with the periods examined being determined by when a cohort of new recruits appeared. The mean size, standard deviation and proportion of the population within each cohort were determined using mixture analysis and the program ‘MIX’ (Macdonald and Pitcher 1979). This technique uses maximum-likelihood methods to fit a series of normal distributions to individual size–frequency histograms, with the number of normal distributions equating to the number of cohorts present. The number of cohorts was estimated from visual inspection of the frequency distribution and, if the number of cohorts was not clear, several different numbers of cohorts were tried for each distribution to determine which number gave the optimal fit of the data. For each number of cohorts, the best fit was obtained by manipulating the means, standard deviations and the proportion of the population within each cohort. Each series of normal distributions produced an expected count of individuals within each size class. The fit of the model to the data was tested using the chisquared statistic, with the number of categories determined by the number of size classes in the distribution. The chi-squared statistic is overestimated if the expected frequency in each category is small (generally less than 5; Zar 1984). In all cases, some size categories contained fewer than five observations. The chi-squared statistic and resulting P -values should, therefore, be interpreted cautiously. On two occasions at Lake Illawarra (29 March 1997 and 15 April 1997), distinct bimodal size distributions occurred. Separate mixture analyses were conducted on each mode to reduce the number of categories that would have contained zero, or values less than five. Multiple cohorts were present during each sampling period and it was sometimes difficult to follow the progression of individual cohorts through successive samples, particularly when multiple cohorts recruited over a short period of time. Cohorts varied greatly in size, so to simplify our analysis, growth rates were only estimated for cohorts that contributed more than 15% to the total population. The criterion of 15% was selected because it was considered that cohorts of this magnitude represented a significant proportion of the population. The mean sizes of all cohorts exceeding 15% of the population were plotted for each sampling period. For the 1992 and 1994 periods in Botany Bay and Lake Illawarra, individual cohorts were identified in successive samples by assuming that growth was always positive (but see the results for Botany Bay in 1994) and that younger cohorts never grew larger than older cohorts over the same period of time (i.e. that growth curves for different cohorts did not intersect). This was considered a reasonable assumption because different cohorts of medusae co-occurred within the same general location and, therefore, probably experienced similar physical and biological environments. The shape of the size distribution of cohorts and the distance between successive cohorts also aided in identifying the same cohorts in successive samples. Data for the 1997/1998 sampling period in Botany Bay were more complex and it was not easy to identify individual cohorts. It was assumed, therefore, that the same cohort produced the largest mode in each sample and a growth curve was derived by tracking the increase in size of the largest mode through time. Conservative estimates of daily growth rates were derived by assuming that growth between successive sampling periods was linear (sensu Worthington et al. 1992). Longevity A conservative estimate of maximum longevity was made by following the progression of cohorts of medusae in Botany Bay and Lake Illawarra. Cohorts were followed from the time recruit medusae first appeared until the cohort became indistinguishable from more recently recruited cohorts.


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Results Populations usually consisted of multiple cohorts. Recruitment of multiple cohorts over a short period of time was observed at Botany Bay during 1992. The size distribution of medusae was sampled on three occasions over a period of five weeks (19 March 1992–24 April 1992; Fig. 1a). On each occasion medusae less than 30 mm BD were observed, suggesting that recruitment occurred almost continuously over the five-week period. Multiple cohorts were also clearly visible during other sampling periods. For example, at Botany Bay on 18 February 1998, three cohorts were clearly distinguishable (Fig. 1c). When only large size classes were present, the distributions were approximately uni-modal, indicating that cohorts of different ages had grown into a single, large size class (e.g. Botany Bay, 3 November 1994; Fig. 1b). Recruitment Frequent sampling over a period of eight years in Botany Bay indicated that recruitment commonly occurred between March and July and during December (Table 1). During these months there was a greater than 1 in 3 chance that recruitment would be observed. Although recruitment events were observed during limited periods of the year, recruit medusae (i.e. ≤50 mm BD) could be potentially found during any month (Table 1). However, they were never abundant outside the major recruitment periods and were rarely seen during the period of August–November. At Lake Illawarra, recruitment was observed between February and June but because only 2.5 years were sampled, there were fewer replicates of each month (and none in January). Relationship between recruitment and rainfall There was great variation in recruitment of medusae in Botany Bay and rainfall in the east Sydney metropolitan region over a period of eight years (Fig. 2a, b). Autocorrelation functions indicated there was no serial dependence, or other trends, in either the rainfall or recruitment data that would confound our interpretation of the cross correlation. Cross correlation of recruitment and rainfall indicated there was no relationship between the two variables and there were peaks in recruitment that shared little or no relationship with rainfall (e.g. 1998; Fig. 2). However, recruitment did occur shortly after the two rainfall events that exceeded 200 mm (1991 and 1992). Between June 1996 and October 1998, two recruitment events occurred, as determined by the large proportion of recruits in the population (Fig. 2a). Both recruitment events coincided with the occurrence of large numbers of recruit medusae, as counted along transects (Fig. 2a, c). Thus, the recruitment events were caused by recruitment of small medusae rather than mortality of large numbers of adults in the population.


(b) 16 Mar. 1994 n = 248

50

16 Dec. 1997 n = 212

30

40 30 20 10 60 9 Apr. 1992 n = 212

30

20 Apr. 1994 n = 280

50 40 20

10

10

30

24 Apr. 1992 n = 197

20

60 50

30

60 50 40

20

10

10

21 Jan. 1998 n = 208

4 Feb. 1998 n = 199

30

16 Jun. 1992 n = 201

30

30

28 Jun. 1996 n = 200

30

20

20

10

10

50

28 Aug. 1996 n = 254

40

10

10

40

30 Sep. 1994 n = 108

30

5

10

40

18 Feb. 1998 n = 204

30

40 30

40

27 Nov. 1996 n = 202

30

3 Nov. 1994 n = 299

50 40

350

300

250

60

200

10

150

10 50

10 100

10

40 30

22 Aug. 1997 n = 270

150

18 Feb. 1997 n = 199

30 Sep. 1997 n = 726

120 90

20

30 20

60

10

Bell diameter (mm)

350

300

250

200

150

30 50

350

300

250

200

150

50

100

10

100

350

10 300

20

250

20

200

20

150

2 Jul. 1997 n = 115

15

20

50

8 Jun. 1997 n = 241

20

20

100

15 Apr. 1997 n = 1909

20

10

40

300

100

30

20

20

10

17 Apr. 1996 n = 255

30

20

40

25 Jul. 1994 n = 298

30 20

40

8 Jan. 1998 n = 203

30

10

10

29 May 1992 n = 211

40

50 40 30 20 10

20

20

40

10

30

30

50

10

40

16 Jun. 1994 n = 104

40

10

20

200

30

20

20

29 Mar. 1997 n = 207

350

40

22 Mar. 1996 n = 218

30

70 60

300

10

(e)

40

250

20

Frequency

(d)

40

200

19 Mar. 1992 n = 204

30

(c)

60

150

40

50

(a)


100

120

Bell diameter (mm)

Fig. 1. Catostylus mosaicus. Size–frequency distribution of medusae at Botany Bay (a–c) and Lake Illawarra (d). Histograms are superimposed with the optimal models generated from mixture analysis. (a, b, d, e) The symbols ◆, ★, ▲, ●, ■, ✖ identify the same cohorts in successive samples. (c) ✚ Indicates largest mode in distribution; ● indicates all other modes that exceed 15% of population; n, number of measurements. There was no difference between the modelled and raw data for any distribution at α = 0.05.

Growth Models that most accurately described size–frequency histograms were generally composed of six or seven cohorts (Fig. 1a–d). However, some of those cohorts accounted for only a very small percentage of the population. Growth rates were typically rapid for small medusae (maximum estimate of 4.81 mm day−1 during 1992 in Botany Bay; Figs 1a, 3a), with the rate decreasing as medusae grew larger. During 1994 at Botany Bay, medusae reduced in size at a rate of 0.82 mm day−1 , with the trend being consistent among all three cohorts (Figs 1b, 3b).

Growth rates were mostly similar among cohorts that recruited within short periods of time. An exception was the cohort in Botany Bay that recruited on 9 April 1992, 20 days after the first two cohorts were observed on 19 March 1992 (Figs 1a, 3a). The latter cohort appeared to grow at a much slower rate than cohorts that had recruited prior to the first day sampled (0.72 mm day−1 cf. 4.81 and 3.60 mm day−1 ). The growth rate of cohorts appeared to vary between Botany Bay and Lake Illawarra, with recruit medusae at Lake Illawarra taking approximately 3 months to grow from sizes of approximately 20 mm BD to around 150 mm BD



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Table 1. Seasonality in the recruitment of Catostylus mosaicus medusae in samples collected between December 1990 and October 1998 in Botany Bay and between February 1996 and May 1998 in Lake Illawarra Two measures are given: number of times greater than 10% of the population made up of medusae ≤50 mm BD; number of times ≥10 medusae ≤50 mm BD present. The number of months sampled is added across years. If multiple measurements were made within any month of a given year, then data from all samples were combined

Apr.

May.

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.

Dec.

Botany Bay No. of times sampled >10% ≤50 mm ≤ 50 mm present

3 0 2

6 0 6

6 2 4

7 4 6

3 2 2

7 3 5

5 2 3

5 0 2

6 0 1

5 0 2

5 0 3

6 2 5

Lake Illawarra No. of times sampled >10% ≤50 mm ≤50 mm present

0 – –

3 1 2

3 3 3

2 1 2

1 1 1

3 2 2

1 0 1

2 0 1

1 0 0

1 0 1

1 0 0

1 0 1

1992

1993

(a)

% Population ⱕ100 mm BD

Mar.

100 80 60 40 20

(b)

Rainfall (mm)

Feb.

1991

Month Jan.

400 300 200

1997

1998 1998

1996

1997

(c)

Number of medusae ⱕ50mm BD

1995

1994

1990

100

1200 900 600 300

Fig. 2. Catostylus mosaicus. (a) Proportion of recruit medusae in Botany Bay (note that for the time-series analyses, recruits were redefined as medusae ≤100 mm BD). (b) Actual monthly rainfall in the east Sydney region between December 1990 and October 1999. (c) Numbers of recruit medusae (≤50 mm BD) counted along transects in Botany Bay between June 1996 and June 1998.

(compared with 1–2 months for medusae in Botany Bay; Figs 1a–d, 3a–d). However, care must be taken in this interpretation because estimates of growth at each location were made at different times. There appeared to be small variation in growth rates among periods within each location, although direct comparisons are difficult owing to differences in initial sizes of recruit medusae and the different time intervals between successive samples.

and the inevitable merging of multiple age classes into a single large size class. At Botany Bay, the longest period a single cohort was tracked for was 6 1/2 months (16 March 1994–30 September 1994; Figs 1b, 3b). At Lake Illawarra, a cohort of recruits that was first observed on 22 March 1996 persisted until at least 29 March 1997 (Figs 1d, 3d). Thus, it would appear that a single cohort of medusae can persist for at least 13 months.

Longevity

Discussion

The duration for which an individual cohort could be tracked was largely determined by recruitment of subsequent cohorts

Recruitment of medusae of Catostylus mosaicus was sporadic but patterns were similar at both Botany Bay and Lake

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(a) 1992 200 1.32 160

0.27

120

2.50

4.81

80 0.39

3.60 0.71

40

0.72

19 Mar.

9 Apr. 24 Apr.

29 May 16 Jun.

(b) 1994 250 ⫺0.82

0.45

200 1.94

150 100

2.56

50 16 Mar. 20 Apr. 16 Jun. 25 Jul. 30 Sep. 3 Nov.

Bell diameter (mm)

(c) 1997/1998 250 1.58

200 1.44 2.46

150 3.91

100 50

1 Jan. 21 Jan.

16 Dec.

4 Feb. 18 Feb.

(d) 1996 250 200

0.46

150 1.26

0.37

0.10

0.93

100 50

22 Mar. 17 Apr. 28 Jun. 28 Aug.

27 Nov.

18 Feb.

(e) 1997 250 200 1.34 0.47

150 1.31

100

Illawarra. No recruitment was observed during late winter or spring at either location, suggesting that strobilation is minimal during this time or that few ephyrae survive. However, small numbers of recruit medusae were sometimes observed during winter and spring, indicating that polyps probably survived over this period, but that the rate of strobilation was greatly reduced. Many species of jellyfish have perennial polyps that survive winter, sometimes in an encysted state (e.g. Chrysaora quinequecirrha; Cargo and Schulz 1966; Kingsford et al. 2000). Cessation of strobilation in winter is usually attributed to low water temperatures, which can impair the ability of scyphistomae to feed (Fitt and Costley 1998) or increase the time required for polyps to strobilate (Purcell et al. 1999). Other factors, such as predation on polyps, may reduce the production of ephyrae (Hernroth and Gröndahl 1985) but the spatial scale of this type of effect and its impact on the population-wide production of ephyrae is unknown. Variability in the timing of recruitment during other times of the year indicates that strobilation and survival and growth of ephyrae may be determined by factors that vary on a stochastic, rather than a seasonal basis, or may depend on an interaction of several factors. Cues such as day length and water temperature may influence the broad seasonal trend of low to zero recruitment during winter and spring. However, these cues are likely to be less important in regulating production of ephyrae during the December to June recruitment season. Other variables, such as reduced salinity, increases in nutrients (through run-off) or food abundance may provide a cue for strobilation. Although salinity and food abundance have been shown to be important in regulating the timing of strobilation (e.g. Chrysaora quinequecirrha, Purcell et al. 1999), there was no apparent relationship between rainfall and the timing of recruitment of Catostylus mosaicus in Botany Bay. Irregular sampling of medusae necessitated pooling of both recruitment and rainfall data into three-month blocks. This reduced the resolution of the data and could have made any correlation more difficult to detect. On two occasions (1991 and 1992), recruitment occurred immediately after periods of excessive rain (>200 mm). Although this may have occurred by chance, it could indicate that rainfall must exceed a particular threshold to promote production of medusae of C. mosaicus. Polyps of C. mosaicus have only been studied in the laboratory (Pitt 2000) and a combination of manipulative field- and laboratory-based experiments are required to determine what factors affect the timing and magnitude of strobilation and, subsequently, recruitment.

1.73 50

2.90 1.07

29 Mar. 15 Apr.

8 Jun. 2 Jul.

22 Aug.

Date of observation

30 Sep.

Fig. 3. Catostylus mosaicus. Mean size of all cohorts exceeding 15% of the population (a, b, d) and dominant mode of population (c) in Botany Bay (a–c) and Lake Illawarra (d, e). Data and symbols are derived from Fig. 1. Numbers above lines indicate growth rates (mm day−1 ).


The cue that triggers strobilation may be complex and could consist of an interaction of multiple factors. Any relationships between recruitment and physical or biological variables could also be further complicated by hydrography, topographic complexity and the location of polyps. Fresh water is less dense than salt water and tends to occur as a wedge at the sea’s surface (Kingsford and Suthers 1996). During periods of heavy rainfall, the wedge of fresh water may extend deeper than usual but the salinity at depth may remain relatively constant. For polyps to be influenced by reduced salinity they would have to occur within a depth range that was susceptible to fluctuations in salinity. Polyps of Phyllorhiza punctata in the Swan–Canning estuary were thought to survive periods of low salinity during winter by occurring at depths deeper than the influence of the freshwater wedge (Rippingale and Kelly 1995). Further information on the areas and depths where larvae settle is needed to thoroughly examine the importance of physical forcing in determining the timing and magnitude of recruitment. Selective mortality of large numbers of adult medusae may have increased the proportion of recruits within the population, even though actual numbers of recruits remained small. This may have caused a recruitment event to be recorded despite few recruits being present. A comparison of actual counts of recruit medusae with measures of the proportion of recruits in the population over a two-year period showed that both methods recorded recruitment events occurring at the same time. Hence, although fluctuations in abundances of adult medusae may have influenced our interpretation of the timing of recruitment events, we consider this unlikely. Typically, recruitment events consisted of the appearance of enormous aggregations of recruits and numbers of recruits greatly exceeded numbers of adults. Hence, even large changes in numbers of adult medusae during a recruitment event would have had a relatively minor influence on the proportion of recruit medusae in the population. Growth Medusae of Catostylus mosaicus grew very rapidly, although growth rates varied. Maximum estimates indicated that C. mosaicus could grow at rates of 4.81 mm day−1 . Rapid growth rates are characteristic of many medusae and comparable growth rates have been observed for the rhizostomes Cotylorhiza tuberculata (∼5 mm day−1 , Kikinger 1992) and Rhopilema nomadica (ephyrae grow to medusae 170 mm diameter in 3 months; Lotan et al. 1994). The cohort of C. mosaicus that was first observed on 9 April 1992 in Botany Bay, appeared to grow much slower than cohorts that had recruited one month earlier. Growth rates of medusae in Lake Illawarra also appeared to be slower than those in Botany Bay. Variation in water temperature and competition for food can affect growth rates of Aurelia aurita (Schneider and Behrends 1994; Hansson 1997). Growth rates may also be reduced if resources are reallocated from somatic growth


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to reproduction as medusae grow larger, or enter a reproductive period (Lucas and Lawes 1998). Botany Bay (an open embayment) and Lake Illawarra (a shallow estuarine lake) differ greatly with respect to salinity, tidal flushing, turbidity, seasonal temperature fluctuations and biological factors such as the species composition and concentration of plankton (unpublished data). Multifactorial experiments would help to identify the factors regulating growth and explain differences in growth rates among times and between locations. Scyphozoan medusae have few anatomical structures suitable for age classification (but see Miyake et al. 1997), so estimates of growth are usually derived from length-based methods. Cohort analysis was considered the most suitable method because large numbers of jellyfish could usually be measured relatively easily. Although medusae have been tagged successfully (Kikinger 1992), tagging was considered unsuitable for the current study because population densities were typically very large and only a very small fraction could be tagged. Thus, the likelihood of recaptures would be very small. Although it was relatively easy to track the growth of some cohorts through successive sampling periods, some caution must be exercised because growth curves were determined qualitatively. A technique is available that simultaneously analyses multiple length–frequency data sets (Fournier et al. 1990). This technique uses maximumlikelihood methods to fit von Bertalanffy growth curves to multiple sets of size–frequency data. Although we applied this method to our data, the parameter estimates derived were not biologically sensible. Medusae of Catostylus mosaicus are usually sexually mature at sizes exceeding 130 mm BD (Pitt and Kingsford 2000b). Based on estimates derived from our study, recruit medusae could grow to sexual maturity within 1–2 months. Larvae of C. mosaicus settle in the laboratory within four days of collection from the adults and the resulting polyps can produce ephyrae within 15 days of settlement (Pitt 2000). Depending on the growth rate of ephyrae, the entire life cycle of C. mosaicus could potentially be completed within 2–3 months, suggesting that multiple generations of medusae could be produced within a single year. Longevity Estimates of longevity have been derived largely from individuals kept in captivity and, although some species may live for multiple years (e.g. Cassiopiea sp., 4 years;Aurelia aurita, 2 years; Zahn 1981) such estimates are unlikely to reflect the maximum age medusae attain in the field because in captivity they are isolated from natural processes. Medusae of many species live for less than one year and have a seasonal pattern of occurrence (van der Veer and Oorthuysen 1985; Brewer 1989; Rippingale and Kelly 1995). A single cohort of Aurelia aurita persisted for at least 11 months in Urazoko Bay, Japan and, based on knowledge of the seasonal appearance of planulae and reports of other studies, Yasuda (1971) suggested that

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individuals could persist from 1 to 2 years. However, fieldbased estimates of longevity become complicated if medusae are present throughout the year and cohorts of different ages occur within a single size class. Consequently there are no direct field-based estimates of age for any species that live longer than one year. The conservative estimate of 13 months for the persistence of cohort of Catostylus mosaicus is one of the longest field-based estimates of age for any scyphozoan species. In conclusion, medusae of Catostylus mosaicus were found to recruit sporadically during all times of year except late winter and spring. There was no relationship between the timing of recruitment and rainfall and manipulative experiments are required to determine what factors regulate strobilation of polyps, survival of ephyrae and, ultimately, recruitment of medusae. Under appropriate conditions, medusae can grow to maturity within 1–2 months and cohorts can persist for longer than one year. Acknowledgments We would like to thank Dennis Reid for assistance in analysing growth data and M. Finn, M. Galetto, T. Glasby, V. Gleeson and numerous volunteers for assistance in the field. We would also like to thank B. Gillanders and two anonymous referees for commenting on drafts of this manuscript. This project was funded by Environment Australia. References Arai, M. N. (1997). ‘A Functional Biology of Scyphozoa.’ (Chapman and Hall: London.) Bamstedt, U., Ishii, H., and Martinussen, M. B. (1997). Is the scyphomedusae Cyanea capillata (L.) dependent on gelatinous prey for its early development? Sarsia 82, 269–73. Brewer, R. H. (1989). The annual pattern of feeding, growth and sexual reproduction in Cyanea (Cnidaria: Scyphozoa) in the Niantic River Estuary, Connecticut. Biological Bulletin 176, 272–81. Calder, D. R. (1973). Laboratory observations on the life history of Rhopilema verrilli (Scyphozoa: Rhizostomeae). Marine Biology 21, 109–14. Cargo, D. G., and Schultz, L. P. (1966). Notes on the biology of the sea nettle, Chrysaora quinquecirrha, in Chesapeake Bay. Chesapeake Science 7, 95–100. Chatfield, C. (1979). ‘The Analysis of Time Series: an Introduction.’ (Chapman and Hall: London.) Custance, D. R. N. (1964). Light as an inhibitor of strobilation in Aurelia aurita. Nature 204, 1219–20. Diggle, P. J. (1990). ‘Time Series: a Biostatistical Introduction.’ (Oxford University Press: New York.) Fitt, W. K., and Costley, K. (1998). The role of temperature in survival of the polyp stage of the tropical rhizostome jellyfish Cassiopea xamachana. Journal of Experimental Marine Biology and Ecology 222, 79–97. Food and Agriculture Organization (2002). ‘Capture Production 2000. FAO Yearbook of Fisheries Statistics.’ Volume 90/1. (Food and Agriculture Organization: Rome, Italy.) Fournier, D. A., Siberg, J. R., Majkowski, J., and Hampton, J. (1990). MULTIFAN: a likelihood-based method for estimating growth parameters and age composition from multiple length frequency


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Manuscript received 26 August 2002; revised and accepted 4 March 2003.

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