The structure of the latero-frontal cirri on the gills of certain lamellibranch molluscs and their role in suspension feeding

Photomicroscopy and scanning electron microscopy were used to determine the fine structure of the latero-frontal cirri on the gills of three species of bivalve mollusc: Mytilus edulis Linn., Barnea candida (Linn.), and Petricola pholadiformis Lam. The component cilia are seen to branch off alternately along the length of each cirrus. Each of these free, lateral lengths of the cilia are separated from the next on their side by a mean distance of 0.6 m. They extend to a mean length of 2.7 m in M. edulis, the species described in detail. It is shown that a complete straining mesh can be formed over the gill ostium during normal filtering and the aperture of this mesh correlates well with the previously recorded filtering efficiency for Mytilus.     

Particle capture in the musselMytilus edulis: The role of latero-frontal cirri

Microscope video graphs of particle paths near one-filament-thick mussel gill preparations, stimulated with a nerve transmitter (10^–6 M serotonin which restores normal ciliary activity), were used to disclose the capture of 6 m algal cells. Suspended algal cells carried with the water were stopped for a while at the entrance to the interfilament gap by the action of the latero-frontal cirri (Ifc), and transferred to the frontal side of the filament to be transported towards the marginal food groove. The event of transfer took place during approximately a time interval of 1150 to 1/25 s. To gain a better understanding of the capture mechanism and retention efficiency versus particle size, the flow through and around the Ifc was theoretically estimated. Normally beating Ifc create periodic, unsteady, three-dimensional flows at the entrance to the interfilament canal. During the active beat most of the water is deflected to flow around the branching cilia of the Ifc while some of the water is strained by these. Large particles (> 4 m) are stopped and transferred to the frontal current, whereas smaller particles either follow the flow around the Ifc and escape or they are stopped by the branching cilia.     

Ultrastructure of the gill of the hydrothermal-vent mytilid Bathymodiolus sp.

Specimens of Bathymodiolus sp. were collected at 2 620 m depth during the Biocyarise Cruise (12°5880N; 13°5660W) in March 1984, and samples of the gill were fixed for ultrastructural observations. The gill of this hydrothermal-vent mytilid constitutes the main organ in the nutritional processes. The lamellae display abundant ciliation, normally comprised of frontal cilia, compound latero-frontal cirri and lateral cilia. At the ventral margin of each demi-branch, a longitudinal, ciliated, feeding groove is present. the lamellae are composed of numerous homorhabdic filaments connected by tufts of cilia. Each filament is made of a thin wall overlying a central lumen containing amoebocytes. Ultrastructural observations revealed the filament wall to be composed of four types of cells: (1) The ciliated cells of the frontal, latero-frontal and lateral ciliation, characterized by an abundance of mitochondria. (2) Mucous cells present to some degree among the ciliated cells, but more abundant on the distal edge and containing dense droplets of mucus. (3) Cells colonized at their apical pole by numerous bacteria enclosed in membrane-delimited clear spaces and composing the major part of the filament wall. (4) Thin ciliated cells separating the bacterial cells and characterised by a dense fringe of microvilli at their apical pole. The lumen of the filament contains amoebocytes of different morphological aspects which seem to accumulate electron-dense granules, possibly related to detoxification processes.     

Fluid mechanics of the mussel gill: The lateral cilia

Quantitative data on the water currents produced by the ciliary tracts of the gill filaments are needed to understand the fluid mechanics of suspension feeding in bivalves, as well as in other ciliary suspension feeders. This paper investigates the water currents produced by the bands of lateral cilia, as studied on isolated gill filaments, gill fragments, and intact gills of the mussel Mytilus edulis L with severed adductor muscles. The metachronally beating cilia produce oscillatory currents near the oscillating enveloping surface of the ciliary bands and rectilinear currents, the interfilamentary through-currents, farther from the surface. It is suggested that the oscillatory currents play an important role in the fluid mechanical capture of suspended particles. In the intact gill the interfilamentary currents pass the bands of lateral cilia at velocities that are two or more times higher than those generated by the bands of isolated filaments. The mussel gill is compared with an optimized peristaltic pump.     

Functional morphology and feeding behavior of Paraprionospio pinnata (Polychaeta: Spionidae)

The functional morphology and feeding behavior of Paraprionospio pinnata (Ehlers) were studied. The tentacular feeding palps of P. pinnata possess four groups of cilia: laterals, latero-frontal cirri, frontals and basal transverse rows. The lateral cilia beat in metachronal waves creating in current that flows toward the frontal surface of the palp. The latero-frontal cirri deflect suspended particles onto the frontal surface and potentially resuspend deposited particles. The frontal cilia line the groove of the palp and transport particles to the ciliated pharynx. The basal transverse cilia also beat in metachronal waves and together with the frontal cilia of the branchiae produce a U-shaped current that removes particles rejected by the pharynx from the burrow. The significance of particle selection at the site of the pharynx in spionid polychaetes is hypothesized to be constrained by the interaction of specific morphologies and behaviors that reduce the effects of epifaunal predation and browsing. The relationship between sediment permeability and type of respiratory or ventilation current is hypothesized to be a potentially important factor in understanding the distribution of spionid polychaetes.     

Effects of exposure to petroleum hydrocarbons on the gill functions and ciliary activities of a marine bivalve

The effects of long-term exposure to low levels of water-accommodated fractions of Kuwait Crude oil, or to shortterm exposure to chemically dispersed oil, on the gill performance of the clam Venus verrucosa were investigated. Reduced pumping activities of the lateral cilia as well as interference with the normal beating activities of the eulaterofrontal cirri led to reduced clearance rates and retention efficiencies of food particles less than 6 m in diameter. On the other hand, frontal ciliary activities were significantly accelerated, while any retained oil droplets were conducted to the mouth region as food particles. The activities of terminal and sensory cilia were also enhanced and mucus production increased. The significance of these responses to the clam's energy budget is discussed.     

Particle capture in the crown of the ciliary suspension feeding polychaete Sabella penicillus: videotape recordings and interpretations

An experimental setup was designed for in situ videotape recording of the particle capture process in the crown of the polychaete Sabella penicillus. Intact individuals of S. penicillus (collected in the Gullmornfjord, Sweden in 1992) were exposed to either 6 m Latex spheres or Rhodomonas sp. flagellate cells (6 m). The capture of the added particles was recorded on video. From frameby-frame analyses particle velocities were estimated and the shape of the three-dimensional particle paths was inferred. The mean velocity of particles approaching the crown was estimated to be ca. 1 mm s^-1, increasing to ca. 1.7 mm s^-1 in the interpinnule channel. At the moment of capture the particles were seen to follow a curved, near circular path close to the tips of the latero-frontal cilia. The transport velocities on the frontal side of the pinnules and filaments were estimated to be up to 0.15 and 0.5 mm s^-1, respectively. Counting captured particles relative to particles arriving within the area of the pinnules gave a rough, direct estimate of nearly 100% retention rate when the polychaete was feeding undisturbed. Together with results from clearance measurements in the literature this implies that the worm is able to capture particles down to 3 m entering the interpinnule channel almost 100% effectively. In view of the 80-m wide interpinnule channel and 40-m spacing between the tips of the latero-frontal cilia on both sides of the channel, this result cannot be explained by mechanisms based solely on direct mechanical contact between cilia and particles but must involve fluid mechanical mechanisms. The present work is the experimental basis for ongoing numerical simulations of the particle motion in the interpinnule channel.     

Gill function and particle transport in Placopecten magellanicus (Mollusca: Bivalvia) as revealed using video endoscopy

The technique of endoscopic video observation was used to study feeding processes of Placopecten magellanicus (Gmelin), collected from Bull Arm, Newfoundland in August 1991 and 1992, under near-natural feeding conditions. The fate of captured particles depended on the extent of ingestive or handling capacity saturation. Under low (1 to 10 particles l^-1) to medium (10 to 20 particles l^-1) particle concentrations, most particles were incorporated in continuous anteriorly directed slurries in the dorsal ciliated tracts of the gill arch and dorsal bends. As particle concentration or exposure time to the lower particle concentrations increased, four endogenous mechanisms of ingestion volume control were increasingly observed: (1) rejection of dense mucus-particle masses from the principal filament troughs onto the ventrally beating cilia and associated currents of the ordinary filament plicae, counter to and below the incoming pallial current maintained by the principal filament cilia; (2) intermittent stopping of the anteriorward flow in the dorsal ciliated tracts; (3) reduction or cessation of input from the principal filaments to the dorsal ciliated tracts; (4) detachment of the dorsal bends from the mantle to establish a shunt from the infrabranchial to the suprabranchial cavity. Chemical and histochemical tests of purified fluid withdrawn from the dorsal ciliated tracts indicate that mucus is present at all particle concentrations. Mucus therefore participates both in normal feeding and in ingestion volume control on the bivalve gill, although different mechanisms, and types of mucus, effect transport of material in the dorsal (feeding) and ventral (cleaning) ciliated tracts.     

A hydromechanical principle for particle retention in Mytilus edulis and other ciliary suspension feeders

It is generally thought that the laterofrontal cirri of the bivalve gill act as filters that retain suspended particles in the through current and transfer the particles onto the frontal surface of the gill filaments. In Mytilus edulis calculations indicated that if water passed between the branching cilia of the cirri that are assumed to constitute the filter the pressure drop needed would amount to about 10 times the actual pressure drop across the whole gill. Thus, instead of acting as filters the laterofrontal cirri seem to move water. Presumably, the cirri together with the frontal cilia produce the water currents along the frontal surface of the gill filaments. Particle retention in the bivalve gill implies the transfer of suspended particles from the current of water about to enter an interfilamentar space into a neighbouring frontal surface current. The complex three-dimensional pattern of flow that arises where the 2 systems of current meet is characterized by steep velocity gradients. Particles that enter such steep, steady velocity gradients become exposed to transverse forces that cause the particles to migrate perpendicularly to the direction of flow. Whether particles enter the surface current, i.e. are retained, or they stay within the through current andescape, depends primarily upon particle size, and upon the steepness and height of the gradients within the boundary zone between the surface current and through current. Further studies are needed to evaluate the capacities and relative importance of this hydromechanical particle-trapping mechanism in suspension feeding bivalves. It is suggested that in downstream particle-retaining systems, e.g. on the tentacles of polychaetes and entoprocts, velocity gradients between through currents and surface currents also act as the particle-collecting mechanism.     

Lysosomal responses to experimentally injected anthracene in the digestive cells of Mytilus edulis

Experimentally injected anthracene (10 to 100 g mussel^-1) has been shown to induce dose-dependent lysosomal destabilisation and release of hexosaminidase in the digestive cells of Mytilus edulis after 24 h. This destabilisation was accompanied by cytological evidence of cytolysis of the digestive cells. The destabilising effect of 100 g of injected anthracene persisted for 96 h with a return to the control condition by 168 h. These results are discussed in the context of environmental contamination by polycyclic aromatic hydrocarbons.     

Development of the pericalymma larva of Solemya reidi (Bivalvia: Cryptodonta: Solemyidae) as revealed by light and electron microscopy

Solemya reidi Bernard 1980 is a gutless protobranch bivalve known to possess intracellular chemoautotrophic bacterial symbionts in its gill. A light and electron microscope study on the embryology and larval development of S. reidi provides data for the bivalve Subclass Cryptodonta. S. reidi spontaneously spawned large eggs (271 m in diameter), which developed within individual gelatious egg capsules. The first several cleavages were equal and a distinct molluscan cross was formed at the animal pole of the embryo, features previously unreported in bivalve development. Lecithotrophic pericalymma larvae (similar to the larvae of paleotaxodont protobranch bivalves and aplacophoran molluscs) hatched at 18 to 24 h and remained in the water column for a further 5 d at 10°C. At hatching, larvae measured from 360 to 440 m in length and from 225 to 265 m in cross-sectional diameter. Definitive adult structures developed within an epithelial locomotory test entirely covered with compound cilia. The test histolysed at metamorphosis and was ingested throught the mouth into the perivisceral cavity. Length and height of the shell following metamorphosis was 433 m (±42 m, n=16) and 282 m (± 29 m, n=13), respectively. Primary data and data from the literature show that the type of larval development in both paleotaxodont and cryptodont bivalves cannot be reliably estimated from egg or prodissoconch sizes.     

Effect of heavy metals (Zn,Hg, Cu,Cd, Pb,Ni) on the length growth of Mytilus edulis

The shortterm (10–22 d) effect of Zn, Hg, Cu, Cd, Pb, and Ni on the length growth of Mytilus edulis is studied. Significant reductions of growth rate was found at 0.3 g Hgl^-1, 3 g Cul^-1, 10 g Znl^-1, and 10 g Cdl^-1 added to the local sea water, while concentrations of up to 200 gl^-1 of Pb and Ni had no effect on the growth. With exposure to Cu and Zn, there was a linear reduction in growth rate with increasing metal concentration up to about 6 g Cul^-1 and 100 g Znl^-1. Above these levels, growth stopped with Cu, while with Zn it was stabilized at about 20% of control growth. When Hg and Cd were added, a curvilinear relationship between growth and metal concentration is indicated. With Hg, growth rate is nearly zero above 3–4 g Hgl^-1, while the growth rate was 50% of control after 10 d of exposure to 100 g Cdl^-1. At 2 g Cdl^-1 there was a significant stimulation of length increase. Observed EC₅₀-values for growth were 0.3–0.4 g Hgl^-1, 3–4 g Cul^-1, 60 g Znl^-1, and 100 g Cdl^-1.     

Energetic costs and efficiencies of ciliary filter feeding

The relationship between activity of the lateral cilia and oxygen consumption was studied in fragments of the gills of Mytilus edulis L., and the effects of stimulation with the nerve transmitters serotonin and dopamine were measured. Beat frequency, f, and oxygen consumption, M_{O 2}, increased with increasing doses of the transmitters. Serotonin increased f up to 26 Hz and the relationship with M_{O 2} could be described by M_{O 2} (l cm^-2 gill area h^-1)=0.44+0.35 f (r=0.99). The relationship after dopamine stimulation did not differ from that established by serotonin, but the maximum value of f was only 19 Hz. At the frequency of 10 Hz, characteristic of undisturbed pumping, the cells carrying the lateral cilia were estimated to account for one third of the total oxygen consumed, equivalent to 1.05×10^-10 mol ATP cm^-2 s^-1. This rate of synthesis of ATP was compared with the rate at which the active cilia hydrolyse ATP. ATP hydrolysis ranged from 58 to 87% of synthesis as the assumed ratio between number of ATP molecules hydrolysed per beat and number of dynein molecules ranged from 2 to 3. The rate at which ATP is hydrolysed by the active cilia was compared with the work done in pumping water. At two estimates available of the pumping pressure, 1.4 and 2.5 mm H₂O, the work amounted to 1.4×10^-6 or 2.5×10^-6 joule cm^-2 s^-1, equivalent to 2.9×10^-11 and 5.2×10^-11 mol ATP cm^-2 s^-1. The efficiencies of work thus corresponded to 48 or 86% at the ATP/dynein ratio of 2. The overall efficiencies of the mussel pump amounted to 18 or 32%.     

Petroleum hydrocarbons in the marine bivalve Venus verrucosa: accumulation and cellular responses

Cellular and subcellular responses of the marine burrowing bivalve Venus verrucosa collected from the north-eastern coastline of Malta from January to June 1985, after exposure to petroleum hydrocarbons (PHC) were investigated. After long-term exposure to 100 gl^-1 of water-accomodated fractions (WAF) of crude oil, PHC were found to accumulate most rapidly in the digestive gland and then in the gills, with saturation levels being reached within 100 d of exposure in both cases. PHC accumulation, both in the mantle and muscle tissues, was more gradual and consistent throughout the whole exposure period. After 150 d of exposure, the digestive cells of the digestive gland were significantly reduced in height (atrophy) and exhibited reduced lysosomal membrane stability. After 144 h of exposure to higher concentration of PHC (820 and 420 gl^-1), several cytological effects were recorded, including an increase in cell volume and activity of gill mucocytes as well as in the number of haemocytes in gill blood sinuses. There was also evidence of damage to the epithelial lining of the foot, stomach and style sac and marked atrophy of the digestive cells of the digestive gland. The significance of such responses is discussed.     

The methane mussel: roles of symbiont and host in the metabolic utilization of methane

Methane mussels (Bathymodiolus sp., undescribed; personal communication by R. Turner to CRF) were collected in September 1989 and April 1990 from offshore Louisiana in the Gulf of Mexico. These mussels contain endosymbiotic methane-oxidizing bacteria and are capable of utilizing environmental methane as a source of energy and carbon. Oxygen consumption, methane consumption, and carbon dioxide production were measured in mussels with intact symbionts, functionally aposymbiotic mussels, and separated symbiont preparations under controlled oxygen and methane conditions, in order to study the roles of the symbionts and the hosts in methane utilization. The association was found to be very efficient in fixing methane carbon (only 30% of CH₄ consumed is released as CO₂), and to be capable of maximal rates of net carbon uptake of nearly 5 mol g^-1 h^-1. Rates of oxygen and methane consumption were dependent upon oxygen and methane concentrations. Maximal consumption rates were measured at 250 to 300 M O₂ and 200 to 300 M CH₄, under which conditions, oxygen consumption by the gill tissues (containing symbionts) had increased more than 50-fold over rates measured in the absence of methane. A model is proposed for the functioning of the intact association in situ, which shows the symbiosis to be capable of achieving growth rates (net carbon assimilation) in the range of 0.003 to 0.50% per day depending upon oxygen and methane concentrations. Under the conditions measured in the seep environment (200 M O₂, 60 M CH₄), a mussel consuming methane at rates found to be typical (4 to 5 mol g^-1 h^-1) should have a net carbon assimilation rate of about 0.1% per day. We suggest that the effectiveness of this symbiosis arises through integration of the morphological and physiological characteristics inherent to each of the symbiotic partners, rather than from extensive specialization exhibited by other deep-sea chemotrophic associations.     

Effects of Hg2+ and Cu2+ on the cytosolic Ca2+ level in molluscan blood cells evaluated by confocal microscopy and spectrofluorimetry

In the present work the effect of Hg²⁺ and Cu²⁺ on the level of cytosolic Ca²⁺ in mussel (Mytilus edulis L.) haemolymph cells were investigated by confocal microscopy and spectrofluorimetry utilizing the fluorescent dye Fluo3. In the blood cells of marine molluscs, exposure to Cu²⁺ and Hg²⁺ in the nanomolar and micromolar range causes a time-and concentration-dependent increase in the cytosolic Ca²⁺ level. Both the presence of a low-calcium containing medium and pretreatment of the cells with the channel blocker Verapamil greatly reduced the effects of higher (50 M) Hg²⁺ concentrations, this indicating that Hg²⁺ enhances the influx of extracellular Ca²⁺ partly through activation of voltage-dependent Ca²⁺ channels. Low concentrations of Hg²⁺ (1 M) and also of Cu²⁺ (0.5 M), an essential element, were able to induce a sustained increase in cytosolic Ca²⁺, which was not affected either by Verapamil pretreatment or by lowering the extracellular calcium concentration. These data indicate that in mussel haemocytes heavy metal cations impair Ca²⁺ homeostasis not only by affecting Ca²⁺ channels, but also by interfering with other mechanisms of calcium transport across cellular membranes, such as the Ca²⁺-ATPases. The resulting increase in cytosolic Ca²⁺ could activate Ca-dependent processes which may be involved in many of the biochemical and physiological alterations observed in the cells of metal-exposed mussels. Specimens used in these experiments were collected from the river Linker near Plymouth, U.K. in June 1991.     

Nutrient enrichment and the ultrastructure of zooxanthellae from the giant clam Tridacna maxima

The separate and combined effects of ammonium (10M) and phosphate (2M) on the ultrastructure of zooxanthellae (Symbiodinium sp.) from giant clams, Tridacna maxima, were examined in the field. Nitrogen addition significantly changed the ultrastructure of the zooxanthellae inhabiting the clams. After 9 mo exposure, the cross-sectional area of zooxanthellae from N-treated clams was significantly lower than that from other treatments [N=39.3 m²; C=47.9 m²; P=43.2m²; N+P=44.5 m²; (P=0.001)]. There was also a significant decrease in the size of starch bodies, especially around the pyrenoid of the zooxanthellae from N and N+P treatments [N=1.2 m²; C=2.0 m²; P=1.8 m²; N+P=1.2 m²; (P=2.08E-11)]. This presumably occurs as a result of the mobilization of organic carbon stores in response to stimulated amino acid synthesis under enriched nutrient conditions. These data strongly suggest that the symbiotic zooxanthellae of clams are limited to some extent by the availability of inorganic nitrogen, and that relatively minor changes to the nutrient loading of the water column can have substantial effects on the biochemistry of symbioses such as that which exists between clams and zooxanthellae.     

Effects of light intensity on nitrate and nitrite uptake and excretion by Chaetoceros curvisetus

Michaelis-Menten uptake kinetics were observed at all light intensities. With constant illumination, the V_max and K₁ in nitrate uptake over the natural light intensity range of 0 to 2000 E were 0.343 g-at NO₃–N(g)^-1 at protein-N h^-1 and 26 E, respectively. Nitrate uptake was inhibited at higher light intensities. The K_s for nitrate uptake did not vary as a function of light intensity remaining relatively constant at 0.62 g-at NO₃–N 1^-1. With intermittent illumination, the V_mzx for light intensity in nitrate uptake over a light intensity range of 0 to 5000 E was 0.341 g-at NO₃–N(g)^-1-at protein-N h^-1. No inhibition of nitrate uptake was observed at higher than natural light intensities. Chaetoceros curvisetus will probably never experience light inhibition of nitrate uptake under natural conditions.     

Cadmium accumulation,LC50 and oxygen consumption in the tropical marine amphipod Elasmopus rapax

Adult Elasmopus rapax, collected from the eastern coast of Venezuela in 1990, were exposed to seawater containing various CdCl₂ concentrations ranging from 0.25 to 5.5 mol l^-1. The 48-h and 96-h LC₅₀ values obtained were 4.0 and 1.6 mol Cd l^-1, respectively. In amphipods exposed to 1 mol Cd l^-1 for up to 240 h, the apparent rate of cadmium uptake was higher in dead animals (most of which had molted during the preceding 24 to 48 h) than in those which survived throughout the treatments without molting. Thus, whole-body cadmium content reached 1.74 mol g^-1 dry weight (dw) in the former and only 0.85 mol g^-1 dw in the latter; the higher body Cd-load may have caused the increased mortality observed in molters. On exposure to cadmium levels above 0.5 mol l^-1 the oxygen consumption rate of non-molters decreased from 2.2 to about 1.5 ml O₂ g^-1 dw h^-1 over the first 24 h, remaining unchanged thereafter. The results place E. rapax among the most sensitive marine organisms yet studied concerning cadmium toxicity, and emphasize the usefulness of the Amphipoda as bioindicators and research tools for bioassays.     

Background levels of soil beryllium in several countries

Beryllium and aluminium contents in uncontaminated soils from six countries are reported. The means and ranges of beryllium in the surface soils were as follows: 1.43(0.20–5.50)g g^–1 in Thailand (n=28), 0.7 (0.31–1.03) g g^–1 in Indonesia (n=12), 0.99(0.82–1.32) g g^–1 in New Zealand (n=3), 0.58(0.08-1.68)g g^–1 in Brazil (n=16), 3.52(2.49–4.97)g g^–1 in the former Yugoslavia (n=10), and 1.56(1.01–2.73) g g^–1 in the former USSR (n=8). The mean and range of beryllium contents of the surface soils in Japan (1.17(0.27–1.95)g g^–1 n=27) are situated within the values of the soils from these countries except for the Yugoslav soils derived from limestones. The mean of the mean beryllium contents of the surface soils in all these countries is 1.42 g g^–1 which will be used as a tentative average content of beryllium in uncontaminated surface soils, except for the soils derived from parent materials high in beryllium content. The beryllium contents of the subsoils were higher than those of the surface soils in New Zealand and Yugoslavia as is the case with Japan. The correlation coefficient between the contents of beryllium and aluminium in all the soil samples (n=113) including surface soils and subsoils was 0.505 (p < 0.001).