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. Ibarrola et al. J. Exp. Mar. Biol. Ecol. 252 2000 199 –219
nature of the response of the digestive gland. With low quality diets, a time-dependent capability of the digestive gland for intensifying lysosomal proteolytic production explains the increase of
food absorption rates that result from higher filtration and ingestion rates. With high quality food, digestive acclimation differed with food particle concentration: with low rations, in spite of
constant morphometrical and stereological parameters, the significant changes in the absorptive balance of biochemical components suggests the existence of an increased production of
lysosomes that promotes an accelerated turn-over rate of the digestive epithelia. With high food concentrations, this response was coupled with increased activities of cellulase and laminarinase
enzymes, probably as a consequence of higher rates of enzyme secretions from basophilic cells.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Basophilic cells; Bivalves; Cockles; Digestive cells; Digestive enzymes; Lysosomes
1. Introduction
The digestive gland of bivalves consists of blind-ending tubules connected with the stomach through a branched system of primary and secondary ducts. The tubules
comprise two kind of cells: digestive and basophilic. Intracellular digestion and absorption of food particles entering into the digestive gland takes place within the
lysosomes of the digestive cells. Four steps in the intracellular digestive process have been described see Owen 1970, 1974 and Morton 1983 for review: a production
of primary lysosomes, b fusion of primary lysosomes with pinocytic vesicles to form heterolysosomes where intracellular digestion and absorption of food occurs, c
accumulation of indigestible, electrodense residual bodies, and d release of the residual bodies to form ‘digestive spherules’ apical part of the digestive cells filled with residual
bodies, which are excreted via stomach and intestine and finally evacuated through the faeces. Such endogenous organic matter might account for part of the so-called
metabolic faecal losses Hawkins et al., 1983; Hawkins and Bayne, 1984, a negative element of the digestive process which has been recognized to limit energy gain in
mussels Hawkins and Bayne, 1985; Hawkins et al., 1990 and cockles Navarro and Iglesias, 1993; Navarro et al., 1994.
On the other hand, basophilic cells appear as darkly stained pyramidal cells lying in regular groups. In early studies, these cells were considered immature cells capable of
renovating digestive epithelia at the reconstituting phase see Morton 1983 for review. However, since mitosis has been rarely observed in the digestive gland, this role is
unclear. Conversely, histochemical Mathers, 1973; Palmer, 1979 and ultrastructural Owen, 1970 studies showing highly developed Golgi complexes suggested the
possibility that basophilic cells are involved in the synthesis and secretion of extracellu- lar digestive enzymes.
Early studies on bivalve digestion were focused on the effects promoted by discontinuous food presence in the morphological appearance of the digestive structures
Wilson and La Touche, 1978; Robinson and Langton, 1980; Robinson et al., 1981; Morton, 1983; Robinson, 1983. The results were discussed in the frame of the
hypothesis of Morton 1973 which assumes that digestive cycle in bivalves is organized
I . Ibarrola et al. J. Exp. Mar. Biol. Ecol. 252 2000 199 –219
201
in alternated and endogenously synchronized phases of i food filtration, ii extracellu- lar digestion in the stomach and, iii intracellular digestion within the gland. These
studies brought great information regarding anatomy, morphology and histochemistry of the digestive system but they fail to provide information on the physiological control of
the digestive function since none of them analyzed the changes of the parameters characterizing intra- or extracellular digestive activity in bivalves fed different foods.
The aim of this study was to identify the cellular mechanisms involved in the acute and acclimated changes of the functional gut capacity of cockles treated to different food
qualities and quantities. We performed a histological study quantifying the volume density of lysosomes, digestive and basophilic cells, the thickness of digestive epithelia
and the radium of digestive tubules together with the determination of the digestive enzyme activities of cockles fed four diets two different qualities supplied at two
different rations for 3 and 11 days. Details on the characteristics of the diets and recorded feeding rates, absorption efficiencies and absorption rates of total organic
matter and biochemical components have been reported in the preceding publication I. Feeding and absorption.
2. Materials and methods
