M. Alexandra Chícharo

Nutritional condition and starvation in Sardina pilchardus (L.) larvae off southern Portugal compared with some environmental factors

The aim of this study was to determine the condition and the incidence of starvation among field-caught Sardina pilchardus larvae, using RNA/DNA ratios and to compare them with various abiotic and biotic factors. S. pilchardus larvae were collected off southern Portugal, during April and May 1992. A highly sensitive fluorometric method for quantifying larvae nucleic acid was applied. Sardine larvae were generally in good condition. The means of the RNA/DNA ratios were relatively high and low percentages of starving larvae (RNA/DNA ratio <1.3) were recorded, ranging from 0 to 12.5%. These two results were associated with high prey availability, which contributes to enhanced condition, and with a high density of potential predators, which may result in fewer larvae in poor condition.

Effects of environmental conditions on planktonic abundances, benthic recruitment and growth rates of the bivalve mollusc Ruditapes decussatus in a Portuguese coastal lagoon

Abstract Controlling environmental parameters for the early stages of marine invertebrates have received little attention, particularly in field studies. This study involves measurement of abundances and growth rates of the bivalve Ruditapes decussatus Linnaeus 1758 during its planktonic larval and early benthic life stages in a coastal lagoon: Ria Formosa, Portugal. Measured abundances were compared with tidal amplitude, water temperature, salinity, wind velocity and direction, and a food availability indicator (chlorophyll a). Data were obtained on abundance and prodissoconch length of the larvae, measured two to three times per week, and of post-larval stages individuals that had settled in artificial collectors over 10 months. Larval and juvenile cohorts were identified using size-frequency distributions and larval ages estimated by larval shell growth lines. Data were fitted to linear, exponential, and Laird–Gompertz growth models. Larval growth rates best matched a linear model: 3.78±0.55 per day, r 2 =0.75, p r 2 =0.98, p

The distribution of estuarine fish larvae : Nutritional condition and co-occurrence with predators and prey

Fish larvae were collected monthly between March and September 1997 in the Mira and Guadiana estuaries (southern Portugal). Hydrological parameters were registered and zooplankton samples were obtained simultaneously. Densities of fish larvae (ind·100 m -3 ) were calculated from 211 samples and larval nutritional condition measured as RNA/DNA ratios were obtained for 346 individuals, using a fluorimetric method for nucleic acid quantification. Correlating variables were further studied using multiple regression analysis in order to assess the relative importance of abiotic and biotic factors affecting within-year trends in abundance and nutritional condition of estuarine fish larvae. Results indicated that: 1) the abundance of fish larvae seems conditioned by temperature and predation; and 2) their nutritional condition is dependent on temperature and prey availability. Temperature is an important variable structuring estuaries and therefore conditions the behaviour and physiology of fish larvae. Furthermore, the co-occurrence of predators and larvae might be related to similar feeding patterns or comensalism. Whenever feeding conditions are suitable, they usually determine enhanced growth and nutritional condition. However, predation seems to control this latter relationship through its effect on larval mortality.

Does the nutritional condition limit survival potential of sardine Sardina pilchardus (Walbaum, 1792) larvae off the north coast of Spain? RNA/DNA ratios and their variability

The aim of this study was to determine the nutritional condition of Sardina pilchardus larvae and the percentage of larvae that were starving at the time of capture. The survey was conducted, during the spawning season, in April and May of 1991 and 1992, off the northern coast of Spain. An accepted fluorimetric technique was used to determine the concentrations of both RNA and DNA and to calculate RNA/DNA ratio for each larva. RNA/DNA ratios were related to the zooplankton biomass (>53 μm). Low percentages of starving larvae (RNA/DNA ratio less than 1.3) were registered, ranging from 0% to 2.5%. Results based on the mean and variance of individual larval growth rates showed that predation pressure was not increasing from 1991 to 1992. Based on the presence of sardine larvae in good condition together with low predation, a high recruitment was expected. However, the 1993 recruitment, from 1992 spring spawning, was very poor. This was associated mainly with unfavourable advection from the nursery area. Moreover, even low levels of starvation, such as registered, operating over long time periods could have considerable consequences for larval mortality.

Estimation of the life history parameters of Mytilus galloprovincialis (Lamarck) larvae in a coastal lagoon (Ria Formosa south Portugal)

Abstract The life history parameters of early stages of marine invertebrates, particularly field estimations, have received relatively little attention. The aim of this research was to estimate in situ abundance and growth of Mytilus galloprovincialis planktonic larvae. Plankton samples were filtered through gauze of 63 μm mesh and identified, counted and measured using an inverted microscope. Short-term fluctuations in Mytilus galloprovincialis larvae abundance and environmental parameters (water temperature, salinity, wind velocity direction, tidal amplitude and chlorophyll a ) were monitored (two to three times a week). Larval cohorts were identified using size–frequency distributions and age estimates compared with larval shell growth lines. Data were fitted to the linear, exponential, von Bertalanffy and Laird–Gompertz growth models. Larval growth adjusted better to the Laird–Gompertz model (0.525±0.073 μm d −1 ; r 2 =0.768; P

A juvenile recruitment prediction model for Ruditapes decussatus (L.) (Bivalvia: Mollusca)

Despite the importance of larval abundance in determining the recruitment of benthic marine invertebrates and as a major factor in marine benthic community structure, relating planktonic larval abundance with post-settlement post-larvae and juveniles in the benthos is difficult. It is hampered by several methodological difficulties, including sampling frequency, ability to follow larval and post-larval or juvenile cohorts, and ability to calculate growth and mortality rates. In our work, an intensive sampling strategy was used. Larvae in the plankton were collected at weekly intervals, while post-larvae that settled into collectors were analysed fortnightly. Planktonic larval and benthic post-larval/juvenile cohorts were determined, and growth and mortality rates calculated. Integration of all equations allowed the development of a theoretical formulation that, based on the abundance and planktonic larval duration, permits an estimation of the future abundance of post-larvae/juveniles during the first year of benthic life. The model can be applied to a sample in which it was necessary only to measure larval length.