Spermatozoan ultrastructure and spermatogenesis in aplousobranch ascidians,with some phylogenetic considerations

Spermatozoa and some stages of spermatogenesis were studied for four species of aplousobranch ascidians. Spermatozoa of Clavelina lepadiformis (Müller) (family Clavelinidae) are plesiomorphous in that they have apical acrosomal vesicles and a moderately elongated, cylindrical nucleus. The elongated mitochondrion is twisted ea. 11/2 times around the nucleus. In their ultrastructural morphology, C. lepadiformis sperm conform in some respects to the least-derived ascidian sperm, those of the phlebobranch ascidian Ciona intestinalis L.; however, the sperm of Clavelina lepadiformis have two apomorphies not shared with those of Ciona sp.: (1) the mitochondrion of Clavelina lepadiformis is long and spiralled along the entire nucleus rather than being comparatively compact and not at all helical; (2) the mitochondrial cristae are elongated parallel to the long axis of the nucleus, whereas in Ciona sp. sperm the cristae are unmodified. In Distaplia sp., Aplidium sp. and Synoicum pulmonaria (Ellis and Solander) the spermatozoa are more derived and consist of a proximal cylindrical and a distal corkscrew-like part. The mitochondrion in Distaplia sp. and Aplidium sp. contains electron-dense material and extends in a long thread around the nucleus. In S. pulmonaria the mitochondrion surrounds the anterior part of the nucleus in mature spermatozoa, and an elongated, dense structure displaying fine striation is enclosed in the mitochondrion in late spermatids. The sperm ultrastructural morphology observed in this study is consistent with the majority view that clavelinids are closest to the ancestral ascidian but is also consistent with other conclusions, particularly that the Cionidae are closest to the stem ascidian.     

Seasonal variations in condition,reproductive activity and biochemical composition of the flat oyster,Ostrea edulis,from San Cibran (Galicia,Spain)

The influence of environmental parameters (temperature, salinity and available food) on the condition, reproductive activity and biochemical composition of a native population of Ostrea edulis L. in San Cibran (Galicia, Spain) was studied between September 1988 and December 1989. Histological preparations of gonads showed that gametogenesis started when temperature was at its lowest in winter. The water temperature in San Cibran never fell low enough to interrupt it. Gametogenesis proceeded slowly and spawning took place in May–June, although the predicted time of ripening was early March. Salinity in San Cibran was relatively stable throughout the year; it did not seem to have any influence on gametogenesis. Available food appeared to be a very important factor in controlling gonad growth, once gametogenesis was initiated. The major concentration of suspended organic particulate matter was present in the spring at the time of rapid gonadal maturation. Only one spawning period was observed. Larvae were released when the food in the water was high. Seasonal changes in the main biochemical components of this oyster were determined for a standard individual. Lipids and carbohydrates presented a similar time-course whereas proteins were constant. When food was abundant, energy reserves were built up. Spawning produced a decrease in biochemical constituent levels, and recovery coincided with the phytoplankton bloom. The stored reserves, mainly lipids, were used to overcome a state of energy imbalance in late autumn associated with low food availability. Results show this oyster to be an opportunist organism which concentrates its reproductive effort during a short period of favourable conditions and which is directly dependent on nutritive availability in the environment.     

Heavy metals and organic carbon in sediments from the Tuy River basin,Venezuela

The Tuy River basin, located in north-central Venezuela with an annual average temperature of 27°C and precipitation of 140 cm, was selected to conduct a geochemical study of bottom sediments, with the object of establishing the natural and human influences in the abundance and distribution of Fe, Mn, Cr, Co, Cu, Ni, Pb, Zn and organic carbon. The basin is lithologically divided into two sub-basins, north and south. The north sub-basin drains a iow-grade metasedimentary terrain with a population density of 800 persons km^–2 and approximateiy 600 industrial sites, while the south sub-basin in underlain by metavolcanic and ultramafic rocks, with a population density of less than 10 persons km^–2.Stream bottom sediment samples (150) were collected during the years of 1979–1986 in 16 unpolluted sites and 13 polluted sites. The sediments were air dried at room temperature and sieved through a 120 stainless steel mesh (125 m). Samples of grain size smaller than 125 m were analysed, the heavy metals being determined by atomic absorption spectrometry and the organic carbon (Corg) by dry combustion.The higher concentrations of heavy metals and organic carbon found in the pristine areas were in the south sub-basin, especially in those areas with higher annual precipitation and tropical forest. This indicated that the metavolcanic and ultramafic rocks yield higher concentrations of heavy metals than the metasedimentary rocks. It was also noted that the higher concentrations of Cr and Ni are associated with the ultramafic rocks. The results obtained from the sediment samples collected in the polluted sites showed that the elements Pb, Zn and Corg are enriched up to 4 times as a result of ail the human activities taking place in the basin. Organic carbon is an excellent indicator of domestic wastewater, Pb and Zn are good indicators of the automotive traffic and industrial effluents. The concentrations of each heavy metal did not show any significant correlation with grain size fractions; however, the concentration of organic carbon did show a negative correlation with grain size. The lithological, climatic and vegetation influence in the abundance of heavy metals and organic carbon in stream sediments clearly indicates the necessity of establishing background levels for the area under study when carrying out studies in environmental geochemistry.     

Population genetics of the swimming crab<Emphasis Type="Italic"> Callinectes bellicosus</Emphasis> (Brachyura: Portunidae) from the eastern Pacific Ocean

The population genetics and historical demography of the swimming crab Callinectes bellicosus from the eastern Pacific were assessed using mitochondrial DNA (mtDNA) sequences from portions of the cytochrome c oxidase subunit I (COI) and cytochrome b (Cytb) genes. Analysis of molecular variance of sequence data from crabs collected from nine localities, ranging from the upper to lower Gulf of California and the outer coast of the Baja California peninsula, revealed an absence of population structure, suggesting a high level of gene flow over a wide geographic area. Maximum-likelihood estimates of long-term effective population size obtained with the program FLUCTUATE, in addition to highly significant values obtained from neutrality tests (Tajimas D, Fu and Lis D, and Fus F_S) and mismatch analysis, are consistent with a population expansion dating to the Pleistocene epoch. Phylogenetic analysis of C. bellicosus sequences using both neighbor-joining and Bayesian methods revealed a widely distributed subclade (clade II) cryptically embedded at low frequency in the main (clade I) population. Although sequence divergence between the two clades was low (1.1% COI; 0.6% Cytb), statistical support for the split was high. The Kimura-2-parameter genetic distance between C. bellicosus and the sympatric and morphologically similar C. arcuatus was high (d=0.17) and similar to the genetic distance between C. bellicosus and the allopatric C. sapidus from the western Atlantic (d=0.18), suggesting an ancient (Miocene) divergence of C. bellicosus and C. arcuatus.Communicated by P.W. Sammarco, Chauvin     

Differences in sterol composition of clams (<Emphasis Type="Italic">Ruditapes decussatus</Emphasis>) from three rías in Galicia (NW Spain)

The sterol composition of three different populations of Ruditapes decussatus from three localities close to each other, but where different environment conditions prevail, was investigated over a period of 14 months. Sterols of adult clams were isolated by thin layer chromatography and identified by gas chromatography/mass spectrometry. In all samples, the major sterol component was cholesterol (>40% of total sterols); other sterols identified were 24-norcholesta-5,22-dienol, 22-cis-dehydrocholesterol, 22-trans-dehydrocholesterol, brassicasterol, 24-methylene-cholesterol, campesterol, stigmasterol, -sitosterol and isofucosterol. At each locality, changes in sterol levels followed specific and different patterns, which remained constant for the period studied and allowed the origin of the clams to be distinguished. Stepwise discriminant analysis, based on the percentage amounts of the sterols at each locality, indicated brassicasterol, 22-cis-dehydrocholesterol, cholesterol, 24-norcholestadienol and -sitosterol as discriminant variables that distinguish clam populations. Correct identification of the clam origin was achieved in 100% of the samples, demonstrating that sterols can justifiably be used as molecular biomarkers for determining the origin of this bivalve species.Communicated by O. Kinne, Oldendorf/Luhe     

Nitrate storage by phytoplankton in a coastal upwelling environment

The storage of nitrate by phytoplankton cells during the early phases of upwelling was studied in coastal stations off northern Spain (southern Bay of Biscay) between 1990 and 1994. In this region, a persistent upwelling during summer is characterised by intermittent pulses of variable intensity, and increased nutrient concentrations in the surface layer. The main effect of an upwelling pulse on phytoplankton distribution is the shifting of the chlorophyll a and primary production maxima to near the surface. When the upwelling relaxes, thermal stratification of the water column occurs, and a distinct subsurface chlorophyll maximum develops below the production maximum. An accumulation of intracellular nitrate characterized the early phases of upwelling (mean = 2.73 μmol N m⁻³), maximum concentrations being attained at depths where biomass and production values were moderate. In contrast, phytoplankton cells from non-upwelling situations contained significantly lower concentrations of intracellular nitrate (mean = 0.17 μmol N m⁻³). The variations in the intracellular pool of nitrate may result from the differential allocation of resources within the cell as a result of variations in the energy available, since the uptake and assimilation of nitrate is a relatively expensive process involving several enzymatic systems. We hypothesize that nitrate storage by phytoplankton cells is characteristic of early phases of upwelling and is linked to patterns of carbon fixation. Average nitrogen budgets for upwelling and non-upwelling situations indicate that intracellular nitrate reserves are not responsible for maintaining high phytoplankton growth rates, since they only account for <2% of daily primary production during upwelling events. Received: 28 August 1996 / Accepted 3 December 1996     

Reproductive effort of colonial and solitary breeding tree sparrows Passer montanus L.

A total of 250 nestboxes were arranged in five plots in a suburban area of Budapest, Hungary (19°04E, 47°41N). In each plot, 25 were placed at 50 m intervals to simulate solitary breeding and 25 3–5 m apart to simulate colonial breeding. Length of nest building period, feeding frequency, nestling mortality, nestlings' diet, productivity and parental condition were compared for colonial and solitary breeding tree sparrows Passer montanus. Parents with long nest-building periods, including the majority of first-year females, produced fewer young than parents which built over short periods. Parents fed nestlings morefrequently and nestlings had lower mortality in second than first broods; whether or not a third brood was reared was determined by the costs invested in first and second broods. Females that laid a third clutch had reared fewer young in first and second broods and were heavier than females that reared many young in two broods. Colonial birds had higher feeding frequencies, more similar diets and suffered lower nestling mortality than solitary parents for first broods, but they fed less frequently, diets were less similar, and nestling mortality was higher in second and third broods. It is suggested that colonial breeders benefited from the social stimulation of simultaneous feeding in first broods, but the advantage of synchronicity in feeding declined in second broods and the sparser breeding spacing of solitary parents was more advantageous for feeding in second and third broods. Birds that changed nest spacing between broods had fed nestlings less frequently and had higher nestling mortality before changing than birds which retained their spacing. Parents which changed from colonies to solitary nests fed more frequently with lower nestling mortality in the next brood than parents which retained colonial nests for their second (and third) brood. Solitary parents did not show such a relationship. The rearing of three broods caused higher weight loss in colonial than solitary parents.Correspondence to: L. Sasvári     

Accumulation of mesozooplankton in a wake area as a causative mechanism of the “island-mass effect”

Mesozooplankton biomass (as displacement volume and protein content) and electron-transport system (ETS) activity have been studied in the coastal waters around the Canary Islands. Increases in biomass and ETS activity, on a unit volume basis, were observed in the wakes of the islands. The biomass values obtained in the leeward area for Gran Canaria Island were up to ten times higher than those observed for typical oceanic waters around this archipelago. Sampling was performed in May 1986 at the end of the characteristic vertical mixing period in these waters. Relatively strong winds were recorded prior to sampling. Specific ETS activity was higher at the windward stations on the island shelf. This correlates with the observation in a recent study of increased primary production on the shelf area, where turbulence produced by the northerly trade winds has a marked effect. The persistence of this turbulence during a prolonged wind-pulse results in an accumulation of organisms in the wake of the islands due to current dynamics. This process is proposed as being an important causative mechanism of the island-mass effect. Different biomass values were recorded between circular and oblong islands. The latter are oriented towards the current and the northerly trade winds, and display significantly lower biomass values than the former.     

Critical thermal maxima and upper lethal temperatures for the calanoid copepods Acartia tonsa and A. clausi

Determination of the critical thermal maxima and upper lethal temperatures for Acartia tonsa Dana and A. clausi Giesbrecht revealed the relative temperature tolerance of the two species. A. tonsa exhibited a wider range of acclimation temperatures compared to A. clausi. The response of A. tonsa is related to its ubiquity in Atlantic Ocean estuaries (temperate and tropical), and its capacity to live in temperate waters throughout most of the year. Differences between upper lethal temperatures and critical thermal maxima were very narrow for A. tonsa from tropical and subtropical areas. This suggests that, in warm climates, this copepod lives close to its upper lethal temperature limits.Based in part on a dissertation submitted in partial fulfillment for the Ph.D. degree at the University of Rhode Island.