Validation of the flow-through chamber (FTC) and steady-state (SS) methods for clearance rate measurements in bivalves

Summary To obtain precise and reliable laboratory clearance rate (filtration rate) measurements with the ‘flow-through chamber method’ (FTC) the design must ensure that only inflow water reaches the bivalve's inhalant aperture and that exit flow is fully mixed. As earlier recommended these prerequisites can be checked by a plot of clearance rate (CR) versus increasing through-flow (Fl) to reach a plateau, which is the true CR, but we also recommend to plot percent particles cleared versus reciprocal through-flow where the plateau becomes the straight line CR/Fl, and we emphasize that the percent of particles cleared is in itself neither a criterion for valid CR measurement, nor an indicator of appropriate ‘chamber geometry’ as hitherto adapted in many studies. For the ‘steady-state method’ (SS), the design must ensure that inflow water becomes fully mixed with the bivalve's excurrent flow to establish a uniform chamber concentration prevailing at its incurrent flow and at the chamber outlet. These prerequisites can be checked by a plot of CR versus increasing Fl, which should give the true CR at all through-flows. Theoretically, the experimental uncertainty of CR for a given accuracy of concentration measurements depends on the percent reduction in particle concentration (100×P) from inlet to outlet of the ideal ‘chamber geomety’. For FTC, it decreases with increasing values of P while for SS it first decreases but then increases again, suggesting the use of an intermediate value of P. In practice, the optimal value of P may depend on the given ‘chamber geometry’. The fundamental differences between the FTC and the SS methods and practical guidelines for their use are pointed out, and new data on CR for the blue mussel, Mytilus edulis, illustrate a design and use of the SS method which may be employed in e.g. long-term growth experiments at constant algal concentrations

Growth-prediction model for blue mussels (<i>Mytilus edulis</i>) on future optimally thinned farm-ropes in Great Belt (Denmark)

A recently developed BioEnergetic Growth (BEG) model for blue mussels (Mytilus edulis), valid for juvenile mussels, has been further developed to an ‘extended model’ and an alternative ‘ad hoc BEG model’ valid for post-metamorphic mussels, where the latter accounts for changing ambient chl a concentration. It was used to predict the growth of M. edulis on optimally thinned farm-ropes in Great Belt (Denmark), from newly settled post-metamorphic mussels of an initial shell size of 0.8 mm to marketable juvenile 30–35 mm ‘mini-mussels’. Such mussels will presumably in the near future be introduced as a new Danish, smaller-sized consumer product. Field data for actual growth (from Day 0 = 14 June 2011) showed that size of ‘mini-mussel’ was reached on Day 109 (Oct 1) and length 38 mm on Day 178 (Dec 9) while the corresponding predictions using the extended model were Day 121 (Oct 13) and Day 159 (Nov 20). Similar results were obtained by use of the ad hoc BEG model which also demonstrated the sensitivity of growth prediction to levels of chl a concentration, but less to temperature. The results suggest that it is possible (when the conditions are optimal, i.e., no intraspecific competition ensured by sufficient thinning) to produce ‘mini-mussels’ in Great Belt during one season, but not the usual marketable 45-mm mussels. We suggest that the prediction model may be used as a practical instrument to evaluate to what degree the actual growth of mussels on farm ropes due to intraspecific competition may deviate from the potential (optimal) growth under specified chl a and temperature conditions, and this implies that the effect of thinning to optimize the individual growth by eliminating intraspecific competition can be rationally evaluated

Feeding Behaviour of the Mussel, Mytilus edulis

Under optimal conditions, bivalves tend to filter the ambient water at a maximum rate but under suboptimal environmental conditions, including low or very high algal concentrations, the filtration rate is reduced. The upper algal concentration at which the blue mussel, Mytilus edulis, exploits its filtration capacity over an extended period of time was identified by stepwise raising the algal (Rhodomonas salina) concentration in steady-state experiments above the threshold for continuous high filtration rate. The duration time before incipient saturation reduction decreased with increasing algal concentration, and the threshold concentration for incipient saturation reduction of filtration activity was found to be between about 5,000 and 8,000 cells mL−1, equivalent to 6.3 and 10.0 μg chl a L−1, respectively. Reduced filtration rate was related to total number of algal cells ingested previous to incipient saturation and found to be 11.4±1.7×106 cells. Video-microscope recordings of pseudofaeces production revealed that the trigger threshold concentration for formation of pseudofaeces was about 12,000 cells mL−1. Faeces produced by saturated mussels consisted of closely packed undigested algal cells, indicating severe overloading of the digestive system caused by high algal concentrations which mussels are not evolutionary adapted to cope with

Comportamiento alimentício de la hidromedusa Aequorea vitrina

The prey-capture mechanism of the hydromedusa Aequorea vitrina was studied by means of laboratory video-microscope observations. In stagnant water A. vitrina remains stationary with its very long (about 4x bell diameter) marginal tentacles motionless hanging down in the water, ready for ambush capture of prey organisms that collide with the tentacles. A. vitrina was found to be efficient at capturing brine shrimps (Artemia salina), less efficient at capturing rotifers (Brachionus plicatilis), and very inefficient at capturing copepods (Acartia tonsa). The initial hauling up of an extended marginal tentacle with an adhering prey is fast (>10 mm s-1). Both the bell margin and the mouth move towards each other so that the captured prey can be transferred from the tentacle to the elongated mouth-lips to be further transported into the mouth and stomach. It takes about 20 s from when an Artemia prey organism encounters a tentacle until it is transferred to the mouth-lips. The subsequent digestion in the stomach takes about 30 min. When A. vitrina encounters a jellyfish-prey (a small medusa of Aurelia aurita), it starts to swim in order to adhere the relatively big prey to its mouth-lips. Then A. vitrina opens its mouth wide to swallow the captured medusa, a process which takes about 15 to 20 min. The subsequent digestion takes 2 to 3 h. Field observations of undisturbed A. vitrina made by snorkelling in the Limfjord (Denmark) revealed that the feeding behaviour was similar to that observed in the laboratory in stagnant water. It is concluded that A. vitrina is an ambush-predator, and not a cruising-predatory medusa as previously suggested.El mecanismo de la hidromedusa Aequorea vitrina para la captura de presas fue estudiado por medio de observaciones microscópicas y registros de video en el laboratorio. Aequora vitrina permanece inmóvil en aguas tranquilas con sus largos tentáculos (aprox. 4 veces el diámetro de la umbrela) colgando inermes y a la espera de la captura de presas que colisionen con ellos. A. vitrina se mostró eficiente en la captura de Artemia salina, aunque menos en la captura de rotíferos (Brachionus plicatilis), y muy ineficiente sobre copépodos (Acartia tonsa). La contracción inicial de un tentáculo marginal previamente extendido y que lleve una presa adherida es rápida (>10 mm s-1). Tanto la campana como la boca se mueven una hacia la otra por lo que la presa capturada puede ser transferida desde el tentáculo a los alargados labios orales para ser transportados luego al estómago. Pasan unos 20 segundos desde que una Artemia queda adherida a un tentáculo hasta que es transferida a los labios orales. La subsiguiente digestión en el estómago dura unos 30 minutos. Cuando A. vitrina encuentra una presa gelatinosa (una pequeña medusa Aurelia aurita), empieza a nadar para que la presa relativamente grande se adhiera a sus labios orales. Entonces A. vitrina abre su boca ampliamente para ingerir la medusa capturada, un proceso que dura entre 15 y 20 minutos. La siguiente digestión dura 2-3 horas. Observación in situ de ejemplares no alterados de A. vitrina en Limfjord (Dinamarca) revelaron que el comportamiento trófico fue similar al observado en el laboratorio en aguas estancadas. Se concluye que A. vitrina es un depredador pasivo y no un nadador activo como se había sugerido previamente