Examples of post-mortality alteration in Recent brachiopod shells and (paleo)ecological consequences

Post-mortality alteration of brachiopod shells under normal environmental conditions leads to high taphonomic loss, and to a poor contribution to the biodetrital soft sediment. The successive stages of alteration which shells undergo are: (bio)degradation of the organic matrix shell softening structural disaggregation or/and mechanical fragmentation; these processes depend on the shell structure (number of layers) and composition (organic and inorganic components), but very little on environmental conditions, except for the dissolution of inorganic shell constituents. Among the Brachiopoda, three types of alteration occur to different types of shells — Type I: two-layered chitino-phosphatic shell (species ofLingula andGlottidia) displays a rapid degradation of the organic matrix and mechanical abrasion, leading to total disappearance of the shell in 2 to 3 wk; Type II: two-layered carbonate shell, e.g.Terebratulina spp. [but notNotosaria nigricans (Sowerby) which may constitute a fourth shell type], exhibits degradation of the organic matrix of the secondary layer, shell softening, and structural disaggregation leading to shell disintegration in 6 to 7 mo with a concomitant contribution of calcitic microfibres to the sediment; Type III: three-layered carbonate shell, e.g.Gryphus vitreus (Born), undergoes organic degradation of the secondary layer, fragmentation of the anterior two-thirds of the shell, and slow degradation (because of the thick tertiary layer) of the posterior portion of the shell, with dissolution of the inorganic components (mainly in the tertiary layer) which make a relatively minor contribution to the sediment.     

Observations on shell deformities,ultrastructure, and increment formation in the bay scallop Argopecten irradians

Shell morphology and ultrastructure were examined in the bay scallop Argopecten irradians, cultured in recirculating seawater systems under various conditions of feeding, lighting, and handling. On a unialgal diet of Thalassiosira pseudonana, scallop growth ranged from 120 to 183 m d^-1 at 20°C in the laboratory, about two-thirds of the growth rate found in the field. However, shell deposited in the laboratory differed from that in the field in several ways. In the field, scallops formed costae as an unpigmented, corrugated marginal shell layer; shell deposited in the laboratory lacked this layer and was therefore darker. Also, microstructure of the exterior shell surface of field scallops was coarsely granular, while that of cultured scallops was relatively smooth. Excessive handling of scallops in the laboratory resulted in marginal thickening of valves, a deformity which was completely arrested by a change from daily to weekly handling. Scallops cultured in the same tank with oysters developed shell-thickening on the interior of the valves. It is postulated that shell abnormalities in bivalves result from disruption of complex behavioral processes associated with shell deposition and may be elicited by a variety of natural and experimental irritants. Under natural lighting regimes and optimal conditions for growth, scallops deposited exactly one shell increment per day, but under continuous lighting, deposition of growth increments often became aphasic. In one 28-d experiment, there was a strong correlation between number of growth increments formed and increase in shell height, suggesting that shell ridge formation occurred intermittently, rather than daily, when shell growth rates fell below approximately 150 m d^-1.     

Modification of shell formation in the giant clam Tridacna gigas at elevated nutrient levels in sea water

The effect on shell formation of Tridacna gigas by sea water supplemented for 3 mo with ammonium (5, 10 M, N) and phosphate (2, 5, 10 M, P), separately or in combination, was examined. Exposure to N and N+P significantly enhanced shell-extension rates, but significantly reduced shell weights at equivalent size. Scanning-electron microscopy further revealed structural alterations in the outer shell layer, such as misshapen aragonite crystals, irregular crossed-lamellar orientation, and relatively porous shell microstructure. These observations are consistent with results of X-ray diffractometry on the shells which show distinct shifts in the positions of reflections from the (012) and (200) crystal planes relative to the control, indicating changes in crystal lattice parameters following addition of nutrients.     

Microsensor studies of photosynthesis and respiration in larger symbiotic foraminifera. I The physico-chemical microenvironment of Marginopora vertebralis, Amphistegina lobifera and Amphisorus hemprichii

 The physico-chemical microenvironment of larger benthic foraminifera was studied with microsensors for O₂, CO₂, pH, Ca²⁺ and scalar irradiance. Under saturating light conditions, the photosynthetic activity of the endosymbiotic algae increased the O₂ up to 183% air saturation and a pH of up to 8.6 was measured at the foraminiferal shell surface. The photosynthetic CO₂ fixation decreased the CO₂ at the shell down to 4.7 μM. In the dark, the respiration of host and symbionts decreased the O₂ level to 91% air saturation and the CO₂ concentration reached up to 12 μM. pH was lowered relative to the ambient seawater pH of 8.2. The endosymbionts responded immediately to changing light conditions, resulting in dynamic changes of O₂, CO₂ and pH at the foraminiferal shell surface during experimentally imposed light–dark cycles. The dynamic concentration changes demonstrated for the first time a fast exchange of metabolic gases through the perforate, hyaline shell of Amphistegina lobifera. A diffusive boundary layer (DBL) limited the solute exchange between the foraminifera and the surrounding water. The DBL reached a thickness of 400–700 μm in stagnant water and was reduced to 100–300 μm under flow conditions. Gross photosynthesis rates were significantly higher under flow conditions (4.7 nmol O₂ cm⁻³ s⁻¹) than in stagnant water (1.6 nmol O₂ cm ⁻³ s⁻¹), whereas net photosynthesis rates were unaffected by flow conditions. The Ca²⁺ microprofiles demonstrated a spatial variation in sites of calcium uptake over the foraminiferal shells. Ca²⁺ gradients at the shell surface showed total Ca²⁺ uptake rates of 0.6 to 4.2 nmol cm⁻² h⁻¹ in A. lobifera and 1.7 to 3.6 nmol cm⁻² h⁻¹ in Marginopora vertebralis. The scattering and reflection of the foraminiferal calcite shell increased the scalar irradiance at the surface up to 205% of the incident irradiance. Transmittance measurements across the calcite shell suggest that the symbionts are shielded from higher light levels, receiving approximately 30% of the incident light for photosynthesis. Received: 6 July 1999 / Accepted: 28 April 2000     

Microsensor studies of photosynthesis and respiration in the symbiotic foraminifer Orbulina universa

Oxygen and pH microelectrodes were used to investigate the microenvironment of the planktonic foraminifer Orbulina universa and its dinoflagellate endosymbionts. A diffusive boundary layer surrounds the foraminiferal shell and limits the O₂ and proton transport from the shell to the ambient seawater and vice versa. Due to symbiont photosynthesis, high O₂ concentrations of up to 206% air saturation and a pH of up to 8.8, i.e. 0.5 pH units above ambient seawater, were measured at the shell surface of the foraminifer at saturating irradiances. The respiration of the host–symbiont system in darkness decreased the O₂ concentration at the shell surface to <70% of the oxygen content in the surrounding air-saturated water. The pH at the shell surface dropped to 7.9 in darkness. We measured a mean gross photosynthetic rate of 8.5 ± 4.0 nmol O₂ h⁻¹ foraminifer⁻¹. The net photosynthesis averaged 5.3 ± 2.7 nmol O₂ h⁻¹. In the light, the calculated respiration rates reached 3.9 ± 1.9 nmol O₂h⁻¹, whereas the dark respiration rates were significantly lower (1.7 ± 0.7 nmol O₂ h⁻¹). Experimental light–dark cycles demonstrated a very dynamic response of the symbionts to changing light conditions. Gross photosynthesis versus scalar irradiance curves (P vs E _o curves) showed light saturation irradiances (E _k) of 75 and 137 μmol photons m⁻² s⁻¹ in two O. universa specimens, respectively. No inhibition of photosynthesis was observed at irradiance levels up to 700 μmol photons m⁻² s⁻¹. The light compensation point of the symbiotic association was 50 μmol photons m⁻² s⁻¹. Radial profile measurements of scalar irradiance (E _o) inside the foraminifera showed a slight increase at the shell surface up to 105% of the incident irradiance (E _d). Received: 26 January 1998 / Accepted: 11 April 1998     

Shell structure and behaviour related to cementation in oysters

Shell microstructure and mantle behaviour relating to shell cementation was studied on adult oysters,Crassostrea gigas, Saccostrea mordax, andS. kegaki (collected from Shirahama, Wakayama Prefecture, Japan, in 1989 and 1990). At the place of cementation, the prismatic structure of the outermost shell layer is modified to a significant structure. This structure, named the ridge-and-furrow structure, consists of calcified ridges a few microns wide separated by furrows, both arranged parallel to the shell growth direction. The furrows are ultimately filled by shell material. The prismatic and ridge-and-furrow structures gradually merge in a transitional area where an intermediate type of the two structures occurs. The small size of the crystal units of the ridges and furrows is due to the close distribution of crystal seeds, especially close to pre-existing ridges. This is the basis of the difference between the ridge-and-furrow and the prismatic structures and also makes the former structure functional for cementation, in contrast to the latter. At the site of ongoing shell cementation, the mantle margin adpresses the shell margin onto the substrate. Experiments show that this pressing action is essential for cementation and probably also for the formation of the ridge-and-furrow structure. Even the right valve, which oysters nerve use to cement in natural conditions, forms the ridge-and-furrow structure and cements to the substrate if the pressing action of its mantle margin is induced under artificial conditions. Behavioural changes probably led oysters to switch from byssal attachment to cementation within a short time span when they acquired their cementing habit.     

A pelagic bryozoan from Antarctica

Spherical or sub-spherical bryozoan colonies were collected from the surface waters of a coastal polynya in the southeastern Weddell Sea near Halley Station in February 1992. These are the first truly pelagic marine bryozoan colones yet recorded. The collection site is the edge of the Brunt ice shelf, which is between 150 and 250 m thick in this area, and the depth of water to the seabed is -400 m. The colonies were hollow, composed of a single layer of autozooids, and appeared complete and undamaged. They were between 5.0 and 23.0 mm in diameter, were brownish in colour in life, and pale yellowish brown after preservation. Light and scanning electron microscope investigations of the colonies indicated that they belonged to the genus Alcyonidium, and they are here compared with A. flabelliforme Kirkpatrick, a known antarctic benthic species. Both a pelagic existence and hollow spherical colony form are new attributes for the phylum Bryozoa. However, because of the plasticity of form of species belonging to the genus Alcyonidium, these is not enough evidence for the introduction of a new species at this time. The colonies found may represent a previously undescribed juvenile stage of a known Alcyonidium species. It is postulated that these colonies may obtain nutrition from the often abundant populations of ice algae present in the lower layers of permanent sea-ice.     

Vertical distribution of mesoplankton in the open area of the Black Sea

In April–May 1984 mesoplankton vertical distribution in the Black Sea was studied by sampling with a 150-l waterbottle, vertical plankton nets with mesh-sizes of 180 and 500 mkm and by direct counting of the jelly-fish Aurelia aurita, the ctenophore Pleurobrachia pileus, Calanus helgolandicus and the chaetognath Sagitta setosa from the manned submersible Argus. During daytime throughout the whole deep-water body of the sea near the lower oxycline boundary, plankton forms a layer of high concentration (from 2.5 to 38 g m^-3); its thickness varies from 5 to 10–20 m and it has an unchangeable vertical structure; its upper portion is formed by the ctenophore P. pileus, its middle portion by V–VI copepodites of C. helgolandicus, and its lower portion by the population of S. setosa. The lower boundary of this layer coincides with 0.4 to 0.5 ml O₂ l^-1 isooxygen surface, and the depth of its location varies in different areas of the sea from 150 to 50 m, depending on the depth location of 0.5 ml O₂ l^-1 isooxygen surface. By night the animals, which form this layer, migrate towards the surface.     

Spermatozoon structure of some North American prosobranchs from the families Lottiidae (Patellogastropoda) and Fissurellidae (Archaeogastropoda)

The spermatozoa of four species of the patellogastropod family Lottiidae (Lottia pelta, L. digitalis, L. strigatella, Tectura scutum) and one species of the archaeogastropod family Fissurellidae (Diodora aspera) were examined in 1990 using transmission electron microscopy. All have primitive or ect-aquasperm, typical of invertebrates using external fertilization. Sperm of the lottiid limpets are characterized by a 5 to 9 m-long head composed of a conical acrosome which constitutes >50% of the head length, and a cylindrical nucleus. The acrosome of all species of lottiids is differentiated internally, and has a posterior invagination 0.9 to 1 m in depth, into which an elongate acrosomal lobe protrudes. Between the posterior acrosomal lobe and the nucleus, the subacrosomal material is aggregated as a fibrous column. The midpiece of the sperm has a ring of 4 to 5 spherical mitochondria of 0.6 m diam, posterior to which is a collar of cytoplasm 1 m long, which sheaths the anterior portion of the axoneme. The size and morphology of the acrosome and large cytoplasmic collar clearly distinguish the spermatozoa of the Lottiidae from other families of Patellogastropoda. The sperm of D. aspera (Fissurellidae) is typical of the family of archaeogastropod; the head has a length to breadth ratio of 4:1, and the cylindrical nucleus is capped by a small acrosome, <25% of the total head length, which is deeply invaginated.     

Histology and ultrastructure of the skin of Lepadichthys lineatus (Gobiesocidae: Teleostei)

The skin of the Red Sea clingfish Lepadichthys lineatus Briggs, 1966, which lives on shallow-water crinoids, consists of a continuous layer of giant mucus-producing cells. Upon stimulation, these cells are able, in seconds, to envelope the fish entirely with a thick layer of their secretion. The cells are 75 to 290 high, and are enveloped by septa of compressed epithelium. They rest on a strong collagenous layer, and extend to the skin surface, where they open. The nuclei of the giant cells are large, and irregularly shaped; their cytoplasm has a very extensive endoplasmic reticulum and undergoes deep structural changes during maturation. Their ability to produce large amounts of mucus seems to be an adaptive device to protect the fish from contact with the rough surface of the crinoid host.     

Lead uptake from sea water and food,and lead loss in the common mussel Mytilus edulis

Common mussels, Mytilus edulis (shell length 19 to 21 mm, average dry weight 30 mg) were maintained for 6 weeks in sea water containing different concentrations of lead (0.005 to 5 mg · l^-1). The lead concentration in the mussels' whole soft parts was analysed at different times during the experiment. A constant rate of lead uptake, linearly dependent on the lead concentration of the medium, was observed. Thus, the temporal change of the concentration factor is also linear (regression coefficient 149.9 daily). Rate of lead loss, measured after transferring the mussels into natural sea water, is linearly dependent on the original lead concentration in the soft parts. Rates of uptake and loss in large mussels (shell length 45 to 55 mm, average dry weight 750 mg) are less than those in small mussels (shell length 19 to 21 mm, average dry weight 30 mg). During a much more extended experimental period, adjustment to a steady state is expected to occur; rates of lead uptake and loss are then non-linear. Lead uptake by individual organs (kidney, gills, adductor muscle, digestive gland, foot, mantle with gonads) of large M. edulis (shell length 45 to 55 mm, average dry weight 750 mg) was analysed in 2 test series. In the test series medium, the mussels were kept in a seawater medium containing 0.01 mg. Pb.l^-1. In the test series food, the mussels were kept in natural sea water but fed with the green algae Dunaliella marina containing lead (approximately 600 g.g^-1 dry weight). The lead quantity given per mussel per day was about 2 g in both test series. Within 35 days, the mussels of test series medium took up 29% of the total amount of lead given, those of test series food took up 23.5%. In all organs, lead concentration increased, but rates of uptake differed; the kidney displayed by far the highest rate of uptake. With these physiological properties M. edulis is an ideal indicator organism for lead pollution in the marine environment. A biologic calibration curve, the relationship between lead concentration in the mussels' whole soft parts at equilibrium and lead concentration in sea water, is presented.This paper forms part of a doctoral thesis in biology at Hamburg University     

Growth and survival of Mytilus edulis larvae exposed to low levels of dibutyltin and tributyltin

Two studies were conducted to observe effects of dibutyltin (DBT) and tributyltin (TBT) on larvae of Mytilus edulis for an exposure period of 25 d. Endpoints for evaluation were shell growth and mortality measured at 33 d. Larvae were cultured in a new laboratory assay chamber in a recirculating static test. The control, 2, 20, and 200 g/l DBT-treated populations had mean shell lengths of 527, 523, 417, and 180 m, respectively. Survival was 1% for the 200 g/l DBT-treated population, but ranged from 73 to 83% for controls, 2, and 20 g/l treatments. The no-observed-effect concentration (NOEC) was 2 g/l for DBT, while the lowest-observed-effect concentration (LOEC) was 20 g/l. The chronic toxicity value was 6.3 g/l. In the TBT bioassay, mean shell lengths for the control, 0.006, 0.050, and 0.130 g/l-treated populations were 565, 437, 385, and 292 m, respectively. Control survival was 74%, whereas TBT-treated populations survival ranged from 52 to 58%. The NOEC for TBT was 0.006 g/l TBT and the LOEC was 0.050. A chronic toxicity value of 0.017 g/l was calculated. The results of this study indicated that the toxicity of DBT was less than that of TBT. It was concluded that shell length was inversely related to exposure level in both DBT and TBT bioassays. In this study, we have observed TBT effects at lower exposure levels in the laboratory than previously reported, and also report the first data for DBT effects on mussel larvae.     

Prodissoconch morphology is environmentally modified in the brooding bivalve Lasaea subviridis

Embryonic shell sculpture of intertidal and laboratory brooded individuals of the direct developer Lasaea subviridis was studied using scanning electron microscopy. Intertidally brooded individuals develop a distinct prodissoconch I (PI) and prodissoconch II (PII) of unusual morphologies. The PI is relatively small (50–100 m in length), oval in outline, lacks a radial sculpture and is restricted to the initial pitted zone of the prejuvenile shell. A larger PII is formed (510–680 m in length) and it is dominated by pronouced commarginal striae but also includes a belt of radial ridges extending from the PI/PII boundary. Laboratory brooded individuals kept constantly submerged do not develop the pronounced commarginal striae characteristic of PII. This implies that PII formation in L. subviridis is not a direct result of the brooding habit, and can be modified by environmental factors, which possibly include low-tide exposure. There is no single feature of external prodissoconch morphology that unambiguously indicates a direct development mode combined with mantle cavity brood protection. General guidelines are presented to help recognize this developmental mode, based on prodissoconch morphology. In comparing prodissoconch morphologies of brooding bivalves, the habitats as well as the phylogenies of species should be considered, especially in comparisons of intertidal and subtidal species.     

Some new data on skin-digestion and absorption in urchins and sea stars (Asterias and Henricia)

An adult Psammechinus miliaris, and another specimen 8 mm in diameter were both fed apically by deposition of a protein-powder extracted from a heavily ¹⁴C-tagged urchin. The organs of the former were counted and the latter was totally sectioned and autoradiographed. The gut was more labelled than expected after an apical meal. This could be explained by a pumping of some nutritive powder into the gut, the bright marking of the superficial tissues being obviously due to direct uptake. The localized proteolysis produced by a podial disc of P. miliaris on a black gelatin-film may be due to the secretion of special cell-masses just underlying the calcareous plates of the disc. Extracts from mucus, small spines and dorsal podia from the anterior furrow of Echinocardium cordatum produce a conspicuous digestion of Remazol-brilliant blue hidepowder in vitro. Several in vivo tests with artificial substrates suggest the existence of a trypsin-like enzyme and not chymiotrypsin in the mucus. Fresh frozen sections in podia of Asterias rubens treated with A.S. naphthol-propionate showed that special cells, scattered in the connective tissue, gave a dark-red stain. Strains of red granules issued by them found their way to the surface of the disc between the columnar epithelial cells. These chimiotrypsin-positive cells and their products seem identical to those seen by Chaet and Philpot (1964) in A. forbesi. These authors did not suspect their digestive function. The nutrition of Henricia sanguinolenta through the body wall was tested either with radioactive diatoms or ¹⁴C-labelled amino acids in sea water selectively offered to one or two arms. Both countings and autoradiographs demonstrated the importance of skin-digestion and absorption, especially in the ambulacra. The absorbed nutrients reach the muscle layer, not only the epiderm as mentioned in Ferguson's (1967) esperiments.     

Photosynthetic characteristics of nanoplankton and picoplankton from the surface mixed layer

Nanoplankton and picoplankton primary production has been studied at two oceanic stations in the Porcupine Sea-bight and at one shelf station in the Celtic Sea. At both sites, low wind conditions in June and July 1985 resulted in greatly reduced vertical turbulent mixing and a secondary, temporary thermocline developed in what is usually a well-mixed surface layer; as a result, there was physical separation of the phytoplankton within two zones of the surface mixed layer. The photosynthetic characteristics of three size fractions (>5 m, <5 to >1 m and <1 to >0.2 m) of phytoplankton populations from the two zones have been measured. Phytoplankton was more abundant at the oceanic stations and chlorophyll a values were between 1.3 and 2.2 mg chlorophyll a m^-3, compared with 0.3 to 0.6 mg chlorophyll a m^-3 at the shelf station; at both stations, numbers of cyanobacteria were slightly higher in the lower zone of the surface mixed layer. There was no effect of the temporary thermocline on the vertical profiles of primary production and most phtosynthesis occurred in the surface 10 m. Photosynthetic parameters of the three size fractions of phytoplankton have been determined; there was considerable day-to-day variation in the measured photosynthetic parameters. Assimilation number (P _m ^B ) of all >5 m phytoplankton was lower for the deeper than for the surface populations, but there was little change in initial slope (a ^B ). The small oceanic nanoplankton (<5 to >1 m) showed changes similar to the >5 m phytoplankton, but the same size fraction from the shelf station showed changes that were more like those shown by the picoplankton (<1 m) viz, little change in P _m ^B but an increase in a ^B with depth. Values of a ^B were generally greater for the picoplankton fraction than for the larger phytoplankton, but values of adaptation parameter (I _k )(=P _m ^B /) were not always less. There was little evidence to support the hypothesis that these populations of picoplankton were significantly more adapted to low light conditions than the larger phytoplankton cells. When photosynthetic parameters of the picoplankton were normalised to cell number (P _m ^C /a ^C ) rather than chlorophyll a, P _m ^C was comparable to other published data for picoplankton, but a ^C was much lower. The maximum doubling time of the picoplankton at saturating irradiance is calculated to be ca. 8.5 h for the oceanic population and ca. 6.2 h for the shelf population.     

Diurnal migration and vertical distribution of phyllosoma larvae of the western rock lobster Panulirus cygnus

Studies off the west coast of Australia showed that the phyllosoma larvae of Panulirus cygnus George undergo a diurnal vertical migration, with light as an important factor influencing the depth distribution of all 9 phyllosoma stages. The early stages (I to III) occurred at the surface at night regardless of moonlight intensity, whereas late stages (VI to IX) concentrated at the surface only on nights with less than 5% of full moonlight. Midday peak densities of early-stage larvae occurred in the 30 to 60 m depth range while those of mid (IV to VI) and late stages were in the 50 to 120 m range. Depths of peak densities of larvae increased with distance offshore. The limits of vertical distribution of the phyllosoma remained within ranges of illuminance which were estimated to be in the order of 50 to 250 E m^-2 sec^-1 for early stages, 20 to 200 E m^-2 sec^-1 for mid stages and 5 to 50 E m^-2 sec^-1 for late stages. Minimal rates of net vertical movement were estimated for the larvae. Early stages exhibited mean net rates of ascent and descent of 13.7 and 13.0 m h^-1, respectively, while the rates for mid stages were 16.0 and 16.6 m h^-1 and for late stages 19.4 and 20.1 m h^-1. Diurnal migrations and vertical distribution are shown to have a vital role in the relationships between circulation in the south-eastern Indian Ocean and the transport and dispersal of the phyllosoma larvae. The diurnal migrations of early stages place them at the surface at night, when offshore vectors of wind-driven ocean-surface transport dominate, and below the depth of wind-induced transport during the day, when offshore vectors are small or negative, thus accounting for their offshore displacement. Mid and late stages, because of their deeper daytime distribution and absence from the surface on moonlight nights, are predominantly subject to circulation features underlying the immediate surface layer. This is hypothesized to account for the return of the phyllosoma to areas near the continental shelf edge by subjecting them to a coastward mass transport of water which underlies the immediate surface layer.The western rock lobster is referred to as P. longipes or P. longipes cygnus in some of the literature quoted; these are synonymous with P. cygnus.     

The gastropod statolith: a tool for determining the age of<Emphasis Type="Italic"> Nassarius reticulatus</Emphasis>

The microstructure, shape and appearance of the growth rings in statoliths of Nassarius reticulatus (L.) were investigated. This species possesses two statocysts, each containing a single spherical statolith of calcium carbonate of up to 0.22 mm in diameter in the largest animals. The relationship between statolith diameter (SD) and total shell height (TSH) is exponential [ln(TSH)=26.3SD–0.842], although the function is site specific. Statoliths of the largest whelks (>29 mm) contained three or four clearly defined rings, corresponding to TSH values of ~1.1, 4.6–5.3, 12.0–13.5 and 18.5 mm, respectively. The first ring likely represents the metamorphic ring that was deposited at the time of larval metamorphosis when the post-larval whelk adopted a benthic lifestyle. The estimated size of the whelks at formation of the second, third and fourth statolith rings closely matched the TSH inferred from the shell rings. It is concluded that the patterns of growth rings present in statoliths can provide information about the age and growth of N. reticulatus.Communicated by O. Kinne, Oldendorf/Luhe     

Abundance of picophytoplankton in the subsurface chlorophyll maximum layer in subtropical and tropical waters

A distinctive chlorophyll maximum was detected around 60-m depth in the western North Pacific Ocean and the South China Sea, and almost 55% of the total chlorophyll in the entire water column was found within 50 m around the subsurface chlorophyll maximum (SCM) layer. More than 70% of the chlorophyll was contained in picoplankton which passed through a 3-m Nuclepore but retained on 0.22-m Millipore filters at the SCM as well as the surface layers. By transmission electron microscopic observations, the picoplankton were identified as aChlorella-like coccoid green alga having a section size of 1.2 to 1.5 m and cyanobacteria of 0.5 to 2 m. No obvious difference in these two dominant groups was observed in the SCM and the surface samples except in numerous and heavily stacked thylakoids in the former samples.     

Floristic and physiological differences between the shallow and the deep nanophytoplankton community in the euphotic zone of the open tropical Atlantic revealed by HPLC analysis of pigments

Suspended matter sampled in 1982 in the North Equatorial Current, in the open Atlantic to the west of West Africa, was analyzed by high performance liquid chromatography. The pigment fingerprint of samples taken in the surface mixed layer was dominated by zeaxanthin and chlorophyll a, in agreement with observed dominance of coccoid cyanobacteria. Near the bottom of the euphotic zone the fingerprint was more complicated, with a sharp transition at the depth of the deep chlorophyll maximum layer to dominance of chlorophyll b, 19-hexanoyloxyfucoxanthin and an unknown fucoxanthin derivative in the lower part of this layer; this fingerprint suggests dominance of eukaryotes (green algae, Prymnesiophyceae and Chrysophyceae) at depth. Up to 90% of the chl a was contained in particles smaller than 8 m, and in the surface mixed layer even more than 50% in particles smaller than 1 m. The high concentration of zeaxanthin relative to chl a near the surface suggests adaptation of the cyanobacteria to exposure to high irradiance. Evidence of this adaptation was the very high specific phytoplankton growth rate between sunrise and sunset (=0.16 h^-1), measured by recording ¹⁴C incorporation into organic carbon and into chl a carbon after isolation of the latter by HPLC. The high concentration of chl b relative to chl a at depth was possibly caused by shade-adapted green algae containing more chl b than chl a. The specific growth rate of the deep shade community was low (<0.04 h^-1), yet net primary production, calculated on the basis of chl a increase during incubation, was greatest at depth.     

Organogenesis and histogenesis in the planktotrophic veliger of Doridella steinbergae (Opisthobranchia: Nudibranchia)

Development of the planktotrophic veliger of the dorid nudibranch Doridella steinbergae (Lance) was studied by histological examination of 4, arbitrarily defined larval stages. Following an embryonic period of 7 1/2 to 8 days (12° to 15°C), the newly hatched veligers possess a functional digestive tract, a pair of nephrocysts, a secondary kidney, a pair of cerebral ganglia, a larval shell consisting of a two-thirds whorl, and the metapodial component of the foot. Development during Stage I mainly involves growth of the larval shell and the visceral organs. Stage II is marked by the retraction of the mantle fold from the shell aperture and the appearance of the eyespots, gonadal rudiment, larval heart, and the optic, pedal, and pleural ganglia. At Stage III the radular sac rudiment evaginates from the esophageal wall, the buccal ganglia differentiate, and the propodial rudiment begins to develop on the ventral surface of the metapodium. Stage IV veligers, which are competent to metamorphose, possess 6 pairs of radular teeth, lipid deposits in the left digestive gland, rudiments of the adult kidney and the oral lip glands, an hypertrophied mantle fold, a propodium, and densely packed cilia over the entire ventral surface of the foot. The length of the obligatory larval period, from hatching of the veliger until the attainment of metamorphic competence, is 25 to 26 days under laboratory culture conditions and the larval shell grows from 142 to 168 m in length. The sequence of morphogenetic events and the structure of the competent veliger of D. steinbergae is compared to that of other opisthobranch veligers. It is suggested that the relatively small maximal shell size attained by D. steinbergae results from precocious retraction of the mantle fold. It is further suggested that interspecific differences in the kinds of structures that develop during the veliger phase of opisthobranchs may relate to variations in the requirements of the juvenile phase. The functional adaptations of the gut of planktotrophic veligers are discussed and compared to those of lecithotrophic veligers.