Ultrastructure of the renopericardial complex in Hypselodoris tricolor (Gastropoda, Nudibranchia)

The histology and ultrastructure of the renopericardial complex of Hypselodoris tricolor (Gastropoda, Nudibranchia, Doridoidea) have been investigated by means of semithin serial sections and transmission electron microscopy (TEM). The examinations revealed a functional metanephridial system comprising a monotocardian heart with ventricle and auricle in a spacious pericardium that is linked with the single, large kidney by a renopericardial duct with prominent ciliation towards its opening. Podocytes as the site of ultrafiltration were not only detected in the auricular epicardium, but also line the entire outer pericardial epithelium. The cuboidal, highly vacuolated excretory cells of the kidney epithelium with extensive basal infoldings and an apical microvillous border indicate secretory and reabsorptive activity. Solitary rhogocytes (pore cells) of the connective tissue and haemocoel represent additional loci of ultrafiltration with a fine structure identical to that of the podocytes (slits between cytoplasmic processes, bridged by fine diaphragms and covered by extracellular matrix). The presence of podocytes situated in the epicardial wall of the auricle is regarded as plesiomorphic for the Mollusca and is confirmed for the Nudibranchia. An additional, extensive and separate ultrafiltration site in the outer pericardial wall is not known from any other taxon of the Mollusca and strongly suggests a significantly increased ultrafiltration activity in H. tricolor.     

Involvement of actin and Na+-K+ ATPase in urine formation of the freshwater pulmonate Helisoma

The site and process of urine formation in the renopericardial system of Helisoma have been investigated. Osmotic pressure and protein content of hemolymph from the heart, pericardial fluid from the pericardial cavity, prourine from the kidney sac, and urine from the ureter have been determined. Osmotic pressure is equal in hemolymph, pericardial fluid, and prourine, but less in urine. Protein content is similar in hemolymph and pericardial fluid, but much less in prourine and urine. Hemoglobin molecules are present in hemolymph and pericardial fluid but not in prourine. It is suggested that in Helisoma the kidney sac is the site of prourine formation, and prourine is an ultrafiltrate of hemolymph. The kidney epithelial cells contain 6- to 7-nm microfilaments which react with heavy meromyosin producing unidirectional arrowheads. Numerous actin filaments are present in the vicinity of the lateral cell membranes and basal processes. It is possible that the actin filaments regulate the extracellular spaces for prourine passage. It is postulated that the actin-rich kidney epithelium may generate hydrostatic pressure for ultrafiltration. Na⁺-K⁺ ATPase is located on the luminal side of the kidney epithelium, which may regulate intracellular fluid level of the kidney epithelial cells, and thereby regulate their cell volume. Thus Na⁺-K⁺ ATPase may be involved in the regulation of extracellular spaces in kidney epithelial cells. The enzyme may participate in the production of hyposmotic urine.     

Podocytes in bivalve molluscs: Morphological evidence for ultrafiltration

Summary In representatives from a survey of three major taxa of bivalves the pericardial glands were found in two distinct positions. In protobranches (Acila castrensis) and filibranch bivalves (Glycymeris subobsoleta, Chlamys hastata, Pecten caurinus, Placopecten magellanicus, Mytilus edulis andMytilus californianus) the pericardial glands are located on the auricular surface. In heterodonts (Mercenaria mercenaria, Clinocardium nuttallii andMya arenaria) the pericardial glands are found in an anterodorsal position to the pericardial cavity.The sites of ultrafiltration are described. They consist of podocytes with basally extending pedicels forming an interdigitating network apposed to a basal lamina. Other characteristics of this ultrafiltration barrier described are anionic sites on the basal lamina and presence of substructural components within the ultrafiltration slits between pedicels.The pathway for the ultrafiltrate in protobranchs and filibranchs is from the hemocoel through the basal lamina, through the ultrafiltration slits of the pedicel network, into the urinary spaces between the podocyte cell bodies and into the pericardial cavity. The pathway for the ultrafiltrate in heterodonts is from the hemocoel through the basal lamina, through the ultrafiltration slits of the pedicel network, into urinary spaces between the podocyte cell bodies, into the lumen of the pericardial gland tubules and into the pericardial cavity.All podocyte cells have electron dense granules, Golgi apparatus and vacuoles associated with their cytoplasm. Heterodont species have microvilli on the cell surfaces of the podocytes apposed to urinary spaces.In all cases the morphological sites of ultrafiltration were associated with the pericardial glands of the heart-pericardial complex.     

Fine structure,mechanism of heart function and haemodynamics in the prosobranch gastropod molluscLittorina littorea (L.)

Summary The heart, main blood vessels, and associated structures ofLittorina littorea were examined by scanning and transmission electron microscopy. The auricle is subdivided into two compartments, one receiving blood from the gill and opening to the nephridial gland vein, the other connecting with the latter anteriorly and the ventricle posteriorly.Video recordings were made of the beating heart in vivo and revealed that the auricle expelled blood not only to the ventricle, but also the nephridial gland vein at systole and provided further evidence of tidal flow of blood in the vein. There is clear indication that the constant volume mechanism of auricular re-filling is not strictly true inLittorina.Blood pressure in the heart and major vessels was measured using a servo-nulling micropressure system. The rate of formation of urine (derived by filtration of blood through the auricular wall) was measured using [⁵¹Cr] EDTA as a blood marker.Basal blood pressure was slightly above ambient (0.7 cm H₂O). Peak systolic pressure in the ventricle (3.8 cm H₂O) was synchronised with a subambient trough in pericardial pressure (–1.0 cm H₂O); these pressure pulses were out of phase with that of the auricle (2.3 cm H₂O) at systole. The observations are consistent in broad terms with a constant volume mechanism, but this does not take into account urine formation or filling of the nephridial gland vein.A filtration pressure of 1.5 cm H₂O has been demonstrated across the auricular wall throughout the cardiac cycle. Colloidal back pressure appears to be negligible. The mean rate of urine formation is 0.26 l g^–1 min^–1.     

Cladoceran filtration rate-body length relations: model improvements developed for a Microcystis-dominated hypertrophic reservoir

Filtering rates were measured for zooplankton species in Situon single-celled Chlorella and on four Microcystis colony sizefractions (5–20, 20–40, 40–60 and 60–100µm) in a hypertrophic reservoir. Natural-log-transformedfiltration rates of five cladoceran species, one copepod andone rotifer were included in an all-food-particle, all-speciesmultiple regression model which explained 43% of the variancein filtration rate as a function of animal body length. An additional14% and 7.6% of the variance was attributable to food type andzooplankton species respectively, with temperature accountingfor <4% of the variance. Restricting the filtration ratemodel to cladocerans alone explained 51% of the variance asa function of animal length, 16% as a function of food type,7.5% as a function of species and only 0.2% as a function oftemperature. In linear filtration rate models for each foodtype, cladoceran body length explained 70% of the variance whenfeeding on Chlorella and between 57 and 67% of the varianceon the four Microcystis colony fractions. Models describingcladoceran filtration rates on Chlorella and the 5–20µm Microcystis colony fraction were significantly differentfrom the three models on larger colonies due to cladoceran responsesto increasing food particle size. Accordingly, a combined modelfor Microcystis colonies >20 µm was developed. Inclusionof food quality factors such as cyanophyte colony size seemsjustified in models aimed at estimating clearance rates, resourceutilization and phytoplankton grazing losses in plankton orecosystem studies when applied to eutrophic or hypertrophiclakes where large cyanophyte particles are abundant.     

THE FINE STRUCTURE AND FUNCTION OF THE ANAL GLAND OF THE MURICID NUCELLA LAPILLUS (NEOGASTROPODA) (AND A COMPARISON WITH THAT OF THE TROCHID GIBBULA CINERARIA)

The fine structure of the anal gland of Nucella lapillus isdescribed, and compared with that of Gibbula cineraria. It isconcluded that the glands are not analogous in function andare unlikely to be homologous. The anal gland of Gibbula isa small rectal diverticu-lum near the anal papilla, lined bymucus-secreting cells. The evidence suggests that the largeanal gland of Nucella extracts macromolecules and cations fromthe blood, metabolizes or sequesters them in lyso-somes andultimately expels the resultant residual bodies by apocrinesecretion. It has large reserves of lipid and glycogen, andthe abundance of melanin indicates that it is a major site oftyrosine degradation. The invariable presence of bacteria inthe lumen, and of pits in the epithelium to house them, impliesa symbiotic relationship in which the bacteria metabolize anddegrade the cell debris, some of which is resorbed. The lossof the anal gland in bucci-nids and nassariids is accompaniedby great reduction in size and importance of the rectal sinus,and increase in complexity of the kidney. (Received 31 December 1991; accepted 23 January 1992)     

THE ANATOMY OF PACHYDERMIA LAEVIS (ARCHAEOGASTROPODA: 'PELTOSPIRIDAE')

The anatomy of Pachydermia laevis Warén & Bouchet,1989 is investigated. It is a deep-sea gastropod and restrictedto hydrothermal vents of the East Pacific Rise. Its anatomyresembles that of Melanodrymia aurantiaca Hickman, 1984 (Melanodrymiidae)in most respects, except that its gonopericardial duct opensinto the renopericardial duct, not into the peri-cardial chamber,and that it lacks a copulatory organ. Examination of M. aurantiacamain confirms earlier work. The two species have a cerebralbuccal connective fused with the cerebral ganglion (not freefrom it), a synapomorphy that has not been described for anyother archaeogastropod. The investigation suggests that Melanodrymiaand Pachydermia are closely related and together with Cyathermiidaeand Neomphalidae form a monophyletic group within the Neomphalina.No evidence was found to support earlier speculations aboutclose relations of Neomphalina to Viviparoidea. (Received 10 January 1996; accepted 3 April 1997)     

MORPHOLOGICAL BASIS OF EXCRETORY FUNCTION IN NAUSITORA FUSTICULA (JEFFREYS, 1860) (BIVALVIA: TEREDINIDAE)

The function of the excretory system of the teredinid bivalveNausitora fusticula is discussed on the basis of new informationon histology and ultrastructure. The wall of the auricles islined with podocytes that allow haemolymph ultrafiltration tothe pericardial cavity. These podocytes also show apical microvilliwith absorptive activity. The primary urine is drained fromthe pericardial cavity to the afferent ducts by a citiated bulb-likestructure. Theafferent and efferent ducts together form thekidney body. The afferent duct shows structures related to absorption,excretion and conduction of the urine. The efferent ducts, however,have structures concerned only with urine absorption and conduction. (Received 13 January 1997; accepted 15 July 1997)     

Experimental studies into the feeding biology of rotifers in brackish water

Mass developments of rotifers of the genus Brachionus, and especiallyof B.quadridentatus, occur regularly in the largely hypertrophicchain of shallow waters (‘boddens’) south of theDarss-Zingst peninsula (Southern Baltic). Interest in the autecologyof the species is, therefore, considerable. Various food sourceswere used in laboratory experiments to ascertain the food requirementsof B.quadridentatus, determine its filtration and ingestionrates, and assess its food particle size-selection ability.Growth experiments showed that the chlorophyceans Nannochlorissp. and Chlorella vulgaris possess considerable nutritionalvalue for the species, whereas abundances declined when Microcystisfirma, detritus from Enteromorpha sp. and only bacteria (Pseudomonas),respectively, were provided as food sources. Filtration ratesvaried between 0.02 and 1.73 µl ind.^–1 h^–1,and ingestion rates between 121 and 5560 cells ind.^–1h^–1, depending on the filtration rate and algal concentration.Investigations into food particle size selection using fluorescentlatex particles revealed that particle size influences foodparticle intake. When particles of different sizes were mixed,the animals showed a preference for the larger particles andingested the smaller ones with a diameter of 1–2 µmless efficiently. The brackish water species Brachionus plicatiliswas studied besides B.quadridentatus in all experiments. Theformer species proved to be superior both in its range of utilizableparticle sizes and its growth rate. The experiments with laboratorycultures were backed up by studies performed with various rotiferspecies taken from natural populations.     

Zur Struktur und Funktion des Exkretionssystems von Ascaris lumbricoides L.

Zusammenfassung Das Exkretionssystem von Ascaris lumbricoides besteht aus einer einzigen großen etwa H-förmigen Zelle, deren lange Sehenkel in den lateralen Epidermisleisten der Tiere liegen. Die Schenkel sind jeweils von einem intrazellulären Kanal durchzogen, dessen Lumen nach den elektronenmikroskopischen Untersuchungen nach hinten zu schnell abnimmt. Die Kanalwandung ist aus geschichteten Cytoplasmaplatten aufgebaut, die anscheinend aus einer dicht gepackten Vesikelschicht hervorgehen. Auch die äußere Zellmembran besitzt Oberflächendifferenzierungen in Form tiefer Einfaltungen. Die Zellschenkel sind von schwammartigen Hüllzellen zur Leibeshöhle hin umgeben. Eine Basalmembran bildet die Grenze zur Leibeshöhle.Die physiologischen Untersuchungen zeigen, daß das Exkretionssystem für die Osmoregulation ohne Bedeutung ist; dagegen spielt das System für die Ionen-Regulation eine Rolle: Nach Aufenthalt in stark Na⁺-haltigen Medien steigt die Konzentration an Na⁺ im Endharn stark an, die Konzentration an K⁺ sinkt; nach Aufenthalt in stark K⁺-haltigen Medien war es genau umgekehrt. Die Konzentration an K⁺ und Na⁺ in der Leibeshöhlenflüssigkeit blieb erhalten.Die morphologischen Befunde lassen es unwahrscheinlich erscheinen, daß sich der Harn durch Filtration bildet. Gegen eine Filtration spricht auch der Befund, daß in die Leibeshöhle eingebrachtes Inulin im Endharn nicht nachweisbar war. Die Harnbildung erfolgt daher wahrscheinlich durch Sekretion.

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Structure and function of the excretory system of Ascaris lumbricoides L.

Summary The excretory system of Ascaris lumbriocoides L. is formed by one single cell. This cell is H-shaped, with the two long tubules running in each lateral chord on both sides of the animal. The diameter of tubular lumen declines rapidly along tubular length. The cellular wall is formed by layers of cytoplasma plates which seem to originate in a densely packed vesicle layer (Fig. 2). The outer cellmembrane has deep basal infoldings. Between cell and body cavity spongelike cells are located. The body cavity is lined with a basal membrane.The constancy of final urine osmolality during immersion in hypo- or hypertonic solutions demonstrated no important role of the excretory system in osmoregulation, whereas the Na⁺-and K⁺-excretion varied remarkably with the electrolyte composition of the bathing solution (Fig. 9), indicating the importance of the excretory system in ion regulation of body fluid. Inulin-C¹⁴ injected into body cavity was not excreted in the final urine, a result consistent with the finding that no filtration elements were detected by morphological examination. Therefore in this animal urine is formed by secretion.

     

THE SYSTEMATIC POSITION OF LAROCHEA FINLAY, 1927, AND INTRODUCTION OF A NEW GENUS AND TWO NEW SPECIES (GASTROPODA: SCISSURELLIDAE)

Species of the gastropod genus Larochea Finlay, 1927 are shownto be scissurellids without an anal shell slit or foramen. TheNew Zealand species, L. miranda Finlay, 1927 and L. secundaPowell, 1937, brood their young in the right subpallial cavityagainst a modified inner lip that is set well behind the aperturalplane. Larochea scitula n.sp. is based on shells from WanganellaBank, southern Norfolk Ridge. Larocheopsis n. gen. is introducedfor a minute species from off northern New Zealand that lacksa shell brood chamber. Larochea miranda and Larocheopsis amplexan.sp. are either gonochoristic with smaller males or consecutivehermaphrodites, while Larochea secunda and L. scitula are evidentlygonochoristic, having mature males and females of similar size. (Received 23 July 1992; accepted 10 December 1992)     

FUNCTIONAL MORPHOLOGY AND FEEDING OF DONAX SERRA ROODING AND DONAX SORDIDUS HANLEY (BIVALVIA: DONACIDAE)

The functional morphology of Donax serra and D. sordidus fromSouth African beaches was examined, and comparative measurementsmade of the rate of water filtration. The two species differin the structure and ciliation of the ctenidia, the number andspacing of the ridges on the labial palps, the direction ofciliary currents on the surface of the mantle and visceral mass,and the length and coiling of the mid-gut. The maximum ratesof water filtration recorded showed similar relationships tosize for the two species, but in D. sordidus the rate declinedrapidly with time during the observations. These differencesindicate differences in feeding strategy related to differencesin distribution and behaviour of the two species on beaches.     

Salt Glands in Pappophorum (Poaceae)

Salt glands were observed in two species of Pappophorum, belongingto the tribe Pappophoreae (Chloridoideae, Poaceae). Glands resemblethose described in other genera of the Gramineae; they comprisetwo cells, a large basal one and smaller upper one. Gland densityper unit surface was much higher in P. philippianum, a facultativehalophyte, than in P. pappiferum, a glycophyte. The relevanceof the recretion process for the elimination of accumulatedNa in these two species is considered. The evolutionary significanceof the presence of glands in Pappophoreae and other membersof the Chloridoideae is discussed. Salt glands, Pappophorum philippianum, P. pappiferum, Poaceae     

LIFE HISTORY TRAITS OF THREE MORPHS OF MELANOIDES TUBERCULATA (GASTROPODA: THIARIDAE), AN INVADING SNAIL IN THE FRENCH WEST INDIES

The life history traits of three morphs of the parth-enogeneticsnail Melanoides tuberculata were studied under laboratory conditions.Differences in growth size, mortality and reproduction rateswere observed between morphs, indicating adaptation to differentdemographic strategies. These results are discussed in relationto the invasion of the species which occurred in Guadeloupeand Martinique. (Received 24 February 1992; accepted 30 April 1992)     

HAMINAEA EXIGUA (GASTROPODA, OPISTHOBRANCHIA), A NEW CEPHALASPID SPECIES FROM THE MEDITERRANEAN SEA

A new species of the genus Haminaca has been found in samplesfrom the northern Adriatic Sea. The new species is very similarto H. orteai, but may be clearly distinguished by body size,the non-pigmented area around the eyes, number of leaves ofHancock's organ, number of ridges on the gizzard plates andby the morphology of male genital tract. A comparison and adiscussion of the values of the most significant characterswithin the genus Haminaca is provided herein. (Received 19 September 1991; accepted 28 February 1992)     

Lobster burrows in Swedish waters

Summary 1. The typical habitat of the European lobster(Homarus vulgaris) is rocky or soft bottom with crevices, boulders and stones.2. At the Swedish west coast it favours a bottom with stones and boulders on sand or shellsand, where it can dig burrows in the soft bottom material under the stones or boulders.3. Such burrows have one, or more than one, opening; in the latter case usually two. A relationship between the size of a lobster and its burrow is often, but not always, noted. The lengths of tunnel-formed burrows, for instance, vary with the size of the overlying stones.4. External factors more or less important in the choice of the site of a burrow are, for example, the composition of the bottom material, the occurrence of slightly overhanging edges of stones or rocks, an outwards-downwards sloping bottom, highest possible values of salinity and temperature, and a sparse algal vegetation or no vegetation at all. Lobsters tend to be absent from polluted areas where the sedimentation rate of organic particles and the turbidity of the water are high.5. Besides being shelters, the burrows also serve as look-outs.6. Burrows may be dug in soft-bottom material in rocky crevices or in flat mud bottoms.7. The burrowing behaviour is characterized by three main patterns: (a) loosening of the bottom material with the walking legs and sometimes with the big claws, (b) shuffling off of piles of soft bottom material in a basket formed by the 3rd pair of maxillipeds and one or two pairs of walking legs, and (c) fanning backwards with the pleopods.8. Small lobsters behave like adults.9. The burrowing behaviour is very similar to that of the American lobster and resembles that ofNephrops norvegicus.

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Hummerhöhlen in schwedischen Gewässern

Kurzfassung Der europäische Hummer(Homarus vulgaris) wird vorzugsweise auf felsigen Böden mit Löchern und Spalten und felsigen oder sandigen Böden mit Steinblöcken und kleineren Steinen angetroffen. Vereinzelte Individuen sieht man auch auf ebenen Weichböden. Besonders tagsüber weilen die Tiere gern in Löchern bzw. Höhlen, welche die Hummer unter Steinen, Blöcken oder an Felswänden selbst ausgegraben haben. Viele Faktoren wirken bei der Wahl des Standortes zusammen, an dem eine solche Höhle ausgegraben werden soll. Unter Steinen und Felsen — vorzugsweise mit überhängenden Kanten — muß geeignetes Bodenmaterial vorhanden sein. Günstige hydrographische Verhältnisse sind eine weitere Voraussetzung. Fernerhin muß der Boden außerhalb der Höhle etwas schräggelagert sein; die Algenvegetation darf nicht zu dicht sein, damit die Sicht nicht eingeschränkt wird. Eine von einem anderen Hummer oder von einem Taschenkrebs verlassene Höhle nimmt der Hummer gerne in Besitz und verbessert sie, falls es notwendig ist. Verschmutzte Gewässer mit erheblicher Sedimentation und starker Trübung werden nicht besiedelt. Die meisten der gegrabenen Hummerhöhlen haben eine Öffnung. Einige sind in der Art eines Tunnels gebaut, mit zwei — selten drei — Öffnungen. Eine gewisse Relation zwischen der Größe einer Höhle und der des einwohnenden Hummers ist vorhanden. Oft aber bestimmt das Aussehen der unmittelbaren Umgebung die Größe der Höhle: so ist die Dimension eines oben liegenden Steines entscheidend für die Länge eines Tunnels. Auch in Lehmböden findet man ausnahmsweise gegrabene Höhlen. Die Art und Weise, in der der Hummer seine Höhle gräbt, wird kurz beschrieben. Die Arbeitsweise ist im Grunde stereotyp, sie wird aber zum Teil den speziellen Verhältnissen der Umgebung angepaßt. Kleine Hummer agieren in der gleichen Weise wie die adulten. Die Wahl des Aufenthaltsplatzes und das Ausgraben der Löcher ist dem Verhalten sehr ähnlich, das vonHomarus americanus beschrieben worden ist. Die Art und Weise des Grabens erinnert auch an die vonNephrops norvegicus.

     

The Pericardial Sacs of Ocypode in Relation to the Conservation of Water, Molting, and Behavior

The surface area of pericardial sacs of the semiterrestrialcrab, Ocypode cordimana, is greater than that of pericardialsacs of the intertidal crabs, Ocypode macrocera and Ocypodeplatytarsis. Responses of the pericardial sacs with relationto conservation of water and molting vary with the species.In the semiterrestrial crab, Ocypode cordimana, the pericardialsacs store water during proecdysis which is used to stretchthe new cuticle after ecdysis. In contrast, the pericardialsacs of the intertidal crabs, Ocypode macrocera and Ocypodeplatytarsis, do not store water during proecdysis. But the sacsof these crabs swell for a brief period during ecdysis, apparentlyto accommodate excess fluid entering the body while the legsare being withdrawn from the old exoskeleton. Although the significanceremains unknown, deposits of uric acid are found in the pericardialsacs of Ocypode cordimana but not in the pericardial sacs ofOcypode macrocera and Ocypode platytarsis. Exposure of specimensof Ocypode cordimana to dry sand reduces the swelling of pericardialsacs, while proecdysial growth of the limbs remains unaffected.The pericardial sacs of Ocypode cordimana swell at a rapid rateif the crabs are maintained in darkness rather than in light.Implications of the present findings are discussed with relationto the habitat of the crabs.     

Natural filtration rates of zooplankton in a closed system: the derivation of a community grazing index

Measurements of individual filtration rates by natural suspensionsof zooplankton are presented. The products filtration rate xanimal concentration, calculated for each of several species-sizecategories, are summated to estimate the total volume of waterfiltered by the zooplankton per unit time, here defined as the"Community Grazing Index" (CGI). Seasonally, CGI varies overnearly three orders of magnitude. The Daphnia hyalina individualspresent frequently contributed >90% of CGI. Individual filtrationrates were highest when the concentrations of suitable foodswere limiting (equivalent to 0.1–0.2 µg C ml^–1),but were depressed after long (3-week) periods of low food availabilityand when large Microcystis colonies dominated the phytoplankton.Approximately one order of magnitude separated these extremes.Fluctuations in CGI owe relatively more to change in the concentration,size and species distribution of the animal population thanto changes in individual filtration rate. ¹Present address: Anglian Water Authority, Oundle, PeterboroughPE8 4AS, UK     

Gill Structure in Zebra Mussels: Bacterial-Sized Particle Filtration

SYNOPSIS. The filtration mechanics of the gill of the zebramussel, Dreissena polymorpha, allow this organism to captureparticles less than 1 µm. The organization of gill cirriand the architecture of the cirri appear to be important inproviding the organism with the ability to filter small particles.Bacteria may provide a useful nutrient source for these animalsas bacterial proteins can be digested and assimilated into musselproteins. Laboratory experiments indicate that D. polymorphais capable of filtering and assimilating a wide range of bacteriaranging in size from 1–4 µm. Unionid species appearto be at least an order of magnitude less efficient at filteringbacteria than D. polymorpha. Because of its relatively smallergill size, C. fluminea also filters bacteria less efficientlythan D. polymorpha. We suggest that bacterial utilization byfreshwater mussel species has important population and evolutionaryimplications.     

DISTRIBUTION, HABITAT AND MORPHOLOGY OF THE CARIBBEAN CORAL- AND ROCK-BORING BIVALVE,LITHOPHAGA BISULCATA (d'ORBIGNY) (MYTILIDAE: LITHOPHAGINAE)

Lithophaga bisulcata is the most common Caribbean and Atlanticlithophagine and is the only species of the genus known to occurfrequently in both living and dead coral. The abundance in livingcorals is non-random and variable. Most common hosts are Siderastereasiderea and Stephanocoenia michelini. The bivalves are moreabundant in their preferred hosts than in dead coral. Individualsfrom the two habitats are indistinguishable in shell shape,musculature and size of boring and posterior pallia! glands,indicating a single population. Boreholes differ in the twohabitats with respect to size and lining. Linings are formedat the "inactive" end of the burrow; therefore living coralinhabitants line the anterior end of the burrow and dead coralborers line the posterior end. Recruitment rates are unknownin dead coral but were very low in living coral (Received 9 June 1987;