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L .M.Z. Chıcharo, M.A. Chicharo J. Exp. Mar. Biol. Ecol. 243 2000 81 –94
Sprung, 1994. The Ria Formosa is a highly dynamic system and is characterised by ´
high larval abundance, especially of Mytilus galloprovincialis Chıcharo, 1996. The Ria Formosa has a long tradition of bivalve culture, especially Ruditapes decussatus,
contributing 90 of Portuguese production. However, the importance of diversification of cultured species is recognised. The high availability of mussel larvae in the plankton
offers an important alternative for bivalve production, if adequate research on the subject is implemented.
In situ Mytilus galloprovincialis larval growth and mortality have not received much attention, despite their ecological importance. This is probably because their life history
variables are difficult to assess due to methodological problems. The most accurate estimations of field growth and mortality result from continuously sampling the larvae in
the plankton from the same water column and detecting their variation in length and in number. However, identifying and following larval cohorts in the plankton is very
difficult Morgan, 1994, 1995. Due mainly to this limitation, estimates of mortality and growth of bivalve larvae based on larval cohorts in the plankton have only been made by
Korringa 1941, Quayle 1964, Jørgensen 1981 and You and Ryu 1985. A comparatively larger number of studies have analysed laboratory reared bivalve larvae
growth and mortality e.g., Bayne, 1965; Walne, 1966; Pechenik et al., 1990; Rumrill, 1991; Beiras et al., 1994.
Several environmental factors, such as temperature and salinity Calabrese, 1969; Kingston, 1974; Hrs-Brenko, 1978, food concentration Sprung, 1984; Pechenik et al.,
1990, physical mechanisms Morgan, 1994; Belgrano et al., 1995; Richards et al., 1995, or predation Young and Chia, 1987 may affect bivalve larval growth and
mortality rates. Whereas in laboratory experiments some of these factors can be controlled, their influence in natural habitats is impossible to ascertain.
The dynamics of larvae from planktonic populations have important implications for the life histories of marine benthic invertebrates Vance, 1973a,b; Strathmann, 1985;
Hines, 1986. Thorson 1950, 1966 highlighted the impact of larval mortality in the recruitment of benthic populations. In fact, rates of growth and natural mortality can
vary among populations of larvae, and the availability of settling larvae may influence patterns of recruitment.
Specific objectives of the present study were the estimation of abundance and growth during the planktonic life of Mytilus galloprovincialis in a coastal lagoon, Ria Formosa.
The abundance results were compared with tidal amplitude, water temperature, salinity, wind velocity and direction and a food availability indicator chlorophyll a.
2. Methods
2.1. Study site M
. galloprovincialis larvae were collected from the plankton of the Ria Formosa lagoon south Portugal, a tidal lagoon including salt marshes, creeks and tidal flats,
extending for about 55 km in length and up to 6 km in width, with an average depth of 3 ˜
m Fig. 1. Two major openings Faro–Olhao bar and the Culatra bar allow
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.M.Z. Chıcharo, M.A. Chicharo J. Exp. Mar. Biol. Ecol. 243 2000 81 –94 83
Fig. 1. Location of sampling station d. This station was sampled over the period May–August 1990.
communication with the Atlantic Ocean. The exchange of water between the Ria and the ´
adjacent ocean is about 50–75 in each tidal cycle Aguas, 1986. The water temperature varies from about 12–138C in winter to 27–288C in summer. Variations of
˜ salinity are small, ranging between 25.5 and 36.9 PSU practical salinity units Falcao
et al., 1985. 2.2. Environmental parameters
Environmental parameters were measured two to three times per week, between May 4 and August 31, 1990, at the location indicated in Fig. 1. Water temperature and salinity
were measured with a Kent Eil 5005 MC5 probe. Wind velocity and direction data were obtained weekly from the Meteorological Service of Faro Airport Fig. 1, which is
directly adjacent to the study site. Chlorophyll a was determined spectrophotometrically and corrected for phaeopigments Lorenzen and Jeffrey, 1980. Tidal amplitude was
calculated by subtracting the low tide m from the high tide level m.
2.3. Sampling strategy and laboratory procedures M
. galloprovincialis larvae were collected using a water sampler at the same location where environmental parameters were monitored. Surface samples of 40 l of water were
taken at ebb tide between high tide and low tide, two to three times per week, also between May 4 and August 31, 1990. Plankton samples were replicated and filtered
through gauze of 63 mm mesh immediately after collection and treated in 5 sodium
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L .M.Z. Chıcharo, M.A. Chicharo J. Exp. Mar. Biol. Ecol. 243 2000 81 –94
hypochlorite solution. Bivalve larvae were identified according Le Pennec, 1978, counted and measured to the nearest micrometer at 400–1000 magnification, using a
Zeiss IM35 inverted microscope, after subsampling using a Stempel pipette. Shell growth lines were counted on photographs.
2.4. Growth models Length–frequency distributions were analysed by the Bhattacharya 1967 method,
and calculations were performed with the Complete Elefan package Gayanilo et al., 1988. This method assumes that the components are normally distributed. These
underlying distributions can be identified as a series of two or more points defining a regression line with negative slope when the logarithms of the ratios of successive
frequencies are plotted against the corresponding midpoints. Two criteria were used to identify the modes: a the separation index, a ratio based on the difference between the
means of the components and their standard deviations — components showing a separation index greater or equal to 2 were considered meaningfully separated; and b
the confidence interval of the correlation coefficients of the regression lines referred to above. The observed and expected distributions were compared by the Chi-square
method Sokal and Rohlf, 1981.
The age of each modal class was attributed considering that age-0 corresponds to the length of the smallest larvae caught, with 0–1 growth lines ‘D’ veligers, according to
Le Pennec, 1978. The modal length of different larval cohorts was plotted separately to follow their progression over the sampling period. Series of estimated mean lengths-at-
age, considered to represent growth in time, were used to estimate the daily growth rate. The age attribution was compared with the number of fine growth lines present in the
Mytilus galloprovincialis larval shell.
Following previous literature Bayne, 1976; Zweifel and Lasker, 1976; Cerrato, 1990; Campana and Jones, 1992 larval growth was examined using several models: the von
Bertalanffy 1938 model:
2K t 2t
c
L 5 L [1 2 e ]
t `
the Laird–Gompertz model Gompertz, 1825; Laird et al., 1965:
2G t
[G G 12e ]
L 5 L exp
t
the linear model: L 5 a 1 K t
t c
and the exponential model:
K t
c
L 5 b e
t
where L refers to the shell length at time t in days from the larva appearing in the
t
plankton, L is the maximum or asymptotic length, L the theoretical length corre-
`
sponding to age-0, G the specific growth rate at t 5 0, G the rate of exponential decay, a and b are scaling factors and K is the growth rate coefficient.
c
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.M.Z. Chıcharo, M.A. Chicharo J. Exp. Mar. Biol. Ecol. 243 2000 81 –94 85
By solving the Laird–Gompertz equation it is possible to calculate the length, corresponding to the inflexion point of the sigmoid curve, at which the growth rate is
maximal L :
i
L 5 L e
i `
as well as the maximal growth rate G :
CM
G 5 G L
CM i
and the maximum length for the species L :
` K
c
L 5 L e
`
3. Results
