“Seed bank” of coastal planktonic diatoms in bottom sediments of Hiroshima Bay, Seto Inland Sea, Japan

Abundance and temporal distribution of viable (able to germinate) resting stage cells of planktonic diatoms in bottom sediments have been investigated almost monthly during 1989 to 1992 in Hiroshima Bay, western part of Seto Inland Sea, Japan. The abundance of viable resting stages in bottom sediments was enumerated with the extinction dilution method (most probable number method, MPN). In bottom sediments of Hiroshima Bay, dominantly distributed species and/ or genera of the diatom resting stages were Skeletonema costatum, Chaetoceros spp. and Thalassiosira spp. Viable resting stages of these diatoms were densely distributed on the orders of 10³ to 10⁶ (MPN g⁻¹ wet sediments), and persisted in bottom sediments throughout the investigation period. Conversely, vegetative cells of these diatoms fluctuated remarkably in the water column and disappeared sporadically. Survival of the resting stages in a collected sediment sample was also determined with the MPN method, at different storage temperatures (5, 10, 15, 20, 25 °C). The survival test demonstrated that the diatom resting stages could survive in the dark for several months or years in sediments. Resting stages survived longer at the lower storage temperature, and the order of longevity was consistent within three diatoms (Chaetoceros spp. > Thalassiosira spp. > S. costatum) at each storage temperature. The present study suggests that these diatom resting stages in the coastal bottom sediments could serve as a “seed bank”, analogous to those of terrestrial plants. The seed bank would ensure the survival of diatoms within highly fluctuating coastal environments, while it would also be the source of sporadic and autochthonous diatom blooms in coastal waters. Received: 29 November 1996 / Accepted: 16 December 1996     

Survival and recovery of resting spores and resting cells of the marine planktonic diatom Chaetoceros pseudocurvisetus under fluctuating nitrate conditions

Of the two resting life-forms of the planktonic diatom Chaetoceros pseudocurvisetus Mangin formed during periods of nitrate depletion, resting spores survived at least 1 month after spore formation at 24 °C, while resting cells survived only for about 10 d at the same temperature. Under nitrogen limitation, resting cells exhibited higher specific death rates than resting spores at temperatures ranging from 5 to 30 °C. After nitrogen replenishment, resting spores required a certain lag period of about 1 d to initiate vegetative growth at levels of nitrate supply from 0.5 to 20 M, while resting cells initiated vegetative growth almost immediately. Resting spores exhibited an intracellular accumulation of the supplied nitrate during germination and initial vegetative growth. The resting cells, however, exhibited more active vegetative growth, closely coupled with the uptake of the supplied nitrate. The resting spores and resting cells appear to play different roles in the maintenance of populations under nutrient fluctuations depending on the interval length between nutrient fluxes in natural waters. Received: 27 April 1998 / Accepted: 1 March 1999     

Nuclear ITS region of the alga Heterosigma akashiwo (Chromophyta: Raphidophyceae) is identical in isolates from Atlantic and Pacific basins

The internal transcribed spacer (ITS) region from 19 isolates of the algal genus Heterosigma (Chromophyta: Raphidophyceae) was amplified by polymerase chain-reaction (PCR) and sequenced. Isolates were obtained from both the Atlantic and Pacific basins, including Europe, eastern North America, western North America, Japan and New Zealand. This study presents evidence that all Heterosigma isolates in this study are representatives of one species (H. akashiwo). All 19 isolates, except one (LB 2005) had identical ITS sequence (98.31% similar by pairwise comparison); Isolate LB 2005 may represent a separate subspecies. Such high degree of ITS sequence identity implies that the organism has spread between oceanic regions in geologically recent times, possibly by human means. In addition to those from Heterosigma spp., the ITS regions from other marine Raphidophyceae (Chattonella antiqua, C. marina, C. subsalsa, Fibrocapsa japonica, and Olisthodiscus luteus) were amplified and sequenced using PCR. Total ITS lengths differed among the Raphidophyceae (C. antiqua, 577 base pairs (bp); C. marina, 577 bp;. C. subsalsa, 579 bp; F. japonica, 830 bp; H. akashiwo, 561 and 563 bp; O. luteus, 829 bp), but 5.8S rDNA sequences were similar in size (13 to 142 bp). The high ITS sequence identity between C. antiqua and C. marina (>99.9% by pairwise comparison) suggests the need for a taxonomic review of these species encompassing all morphological, genetic, physiological and biochemical information. Additionally, a number of cultures of Raphidophyceae were positively identified. In general, ITS comparisons among the Raphidophyceae may be most useful at the level of species determination rather than at the population level. Received: 12 July 1999 / Accepted: 16 March 2000     

Seasonal vertical distribution and population dynamics of the chaetognath Parasagitta elegans in the water column and hyperbenthic zone of Conception Bay, Newfoundland

The vertical distribution and population dynamics of the chaetognath Parasagitta elegans Verrill were determined in the water column and hyperbenthic zone of Conception Bay, Newfoundland from April 1997 to June 1998. The water column depth at the study site (47°32.2′N; 53°07.9′W) was 235 m. The temperature below the thermocline was <0 °C the year round. Chaetognath samples from the water column were collected with a Tucker Trawl. Those from the hyperbenthic zone, were collected with an epibenthic sledge. Depending upon whether the hyperbenthic zone was assumed to extend either 1 m or 10 m above bottom, the grand mean, areal abundance of chaetognaths in the hyperbenthic zone ranged from 6% to 40% of the total abundance in the water column (including the hyperbenthic zone), and the grand mean, areal biomass ranged from 25% to 77%. Large, mature individuals were collected only in the hyperbenthic zone, whereas small, immature individuals were collected primarily in the water column. According to body length and ovary maturity data, three cohorts were identified in the hyperbenthic zone during the study period. Within each cohort, the length frequency of reproductively mature individuals was bimodal, with groups of mean length 33 mm and 41 mm reproducing from May to October. The recruitment period of juvenile chaetognaths extended from July to February, coinciding with the recruitment period of copepods. The estimated individual growth rate of P. elegans was 1.0 mg C year⁻¹. The approximate generation time of the two groups of individuals with mean length at maturity of 33 mm and 41 mm was 450 and 780 days, respectively. This study demonstrates that a failure to sample the large, mature P. elegans living in the hyperbenthic zone leads to serious underestimates of the total abundance and biomass of chaetognaths and an inaccurate picture of seasonal population dynamics. Received: 8 September 1999 / Accepted: 15 September 2000     

Feeding and bacteriolytic responses of the deposit-feeder Abarenicola pacifica (Polychaeta: Arenicolidae) to changes in temperature and sediment food concentration

Deposit-feeders can respond to seasonal fluctuations in food concentration both functionally (e.g. by adjusting feeding rates) and physiologically (e.g. by changing the concentration of bacteriolytic agents in gut fluids). Laboratory feeding experiments were carried out (11 to 21 July 1997) with the arenicolid polychaete worm Abarenicola pacifica (Healy and Wells). Objectives were to test for separate and interactive effects of sediment food concentration and temperature (6, 11, and 16 °C) on deposit-feeder functional (feeding rates) and physiological (bacteriolytic activity of gut fluids) responses. Food concentration was varied experimentally using sieved (1 mm) natural sediments (Md φ=2.00; 0.6% organic) mixed with combusted (500 °C, 8 h) sediments for final concentrations of 25, 50, and 100% natural sediment. Sediment food quality was measured as: (1) bioavailable amino acids (EHAA), (2) chlorophyll a (chl a), and (3) bacterial abundance. Feeding rates were inferred from egestion rates (ER, g h⁻¹) and analyzed with respect to worm size. Bacteriolytic activity of midgut fluids was assayed turbidimetrically against two bacterial isolates, after worms had fed on experimental sediments for 15 d. Temperature and food concentration both significantly affected feeding rates, with maxima occurring at 50 and 100% natural sediment mixtures, and at high (16 °C) temperature. ER was positively, but not significantly correlated with EHAA and chl a; a positive, significant correlation was detected between ER and sediment bacterial abundance. Overall, functional responses agreed with earlier compensatory intake models for deposit-feeders. However, the size and direction of these responses was temperature-sensitive, suggesting that these models need to be adjusted for changes in absorption rates. No effects of ambient temperature or food concentration on bacteriolytic rates were observed, possibly due to compensatory mechanisms or the presence of multiple bacteriolytic agents in gut fluids. Received: 28 June 1999 / Accepted: 14 March 2000     

Irradiance and temperature regulation of oxygenic photosynthesis and O2 consumption in a hypersaline cyanobacterial mat (Solar Lake, Egypt)

 Short-term effects of temperature and irradiance on oxygenic photosynthesis and O₂ consumption in a hypersaline cyanobacterial mat were investigated with O₂ microsensors in a laboratory. The effect of temperature on O₂ fluxes across the mat–water interface was studied in the dark and at a saturating high surface irradiance (2162 μmol photons m⁻² s⁻¹) in the temperature range from 15 to 45 °C. Areal rates of dark O₂ consumption increased almost linearly with temperature. The apparent activation energy of 18 kJ mol⁻¹ and the corresponding Q ₁₀ value (25 to 35 °C) of 1.3 indicated a relative low temperature dependence of dark O₂ consumption due to mass transfer limitations imposed by the diffusive boundary layer at all temperatures. Areal rates of net photosynthesis increased with temperature up to 40 °C and exhibited a Q ₁₀ value (20 to 30 °C) of 2.8. Both O₂ dynamics and rates of gross photosynthesis at the mat surface increased with temperature up to 40 °C, with the most pronounced increase of gross photosynthesis at the mat surface between 25 and 35 °C (Q ₁₀ of 3.1). In another mat sample, measurements at increasing surface irradiances (0 to 2319 μmol photons m⁻² s⁻¹) were performed at 25, 33 (the in situ temperature) and 40 °C. At all temperatures, areal rates of gross photosynthesis saturated with no significant reduction due to photoinhibition at high irradiances. The initial slope and the onset of saturation (E _k = 148 to 185 μmol photons m⁻² s⁻¹) estimated from P versus E _d curves showed no clear trend with temperature, while maximal photosynthesis increased with temperature. Gross photosynthesis was stimulated by temperature at each irradiance except at the lowest irradiance of 54 μmol photons m⁻² s⁻¹, where oxygenic gross photosynthesis and also the thickness of the photic zone was significantly reduced at 40 °C. The compensation irradiance increased with temperature, from 32 μmol photons m⁻² s⁻¹ at 25 °C to 77 μmol photons m⁻² s⁻¹ at 40 °C, due to increased rates of O₂ consumption relative to gross photosynthesis. Areal rates of O₂ consumption in the illuminated mat were higher than dark O₂ consumption at corresponding temperatures, due to an increasing O₂ consumption in the photic zone with increasing irradiance. Both light and temperature enhanced the internal O₂ cycling within hypersaline cyanobacterial mats. Received: 30 November 1999 / Accepted: 11 April 2000     

Fluctuations of the red tide flagellates Chattonella spp. (Raphidophyceae) and the algicidal bacterium Cytophaga sp. in the Seto Inland Sea, Japan

A marine algicidal gliding bacterium Cytophaga sp. strain J18/M01 was isolated in 1990 from a station in northern Harima-Nada, the Seto Inland Sea, Japan, using the harmful red tide alga Chattonella antiqua (Hada) Ono as a susceptible organism. The bacterium can prey upon various species of microalgae. Temporal fluctuations of this bacterium and Chattonella spp. [C. antiqua and C. marina (Subrahmanyan) Hara et Chihara] were investigated weekly at the above station in the summer of 1997 and 1998, using immunofluorescence assay employing highly specific polyclonal antibodies for the bacterium. In the summer of 1997, the cell density of Chattonella spp. showed a maximum value (70 cells ml⁻¹) on 8 July, and decreased thereafter. The bacterium Cytophaga sp. J18/M01 was commonly detected around a few hundreds of cells per milliliter or less. The number of Cytophaga sp. J18/M01 increased after the peak of Chattonella spp., and the maximum cell number of the bacterium was 1350 ml⁻¹. This algicidal bacterium also followed the changes of total amounts of microalgal biomass (chlorophyll a+pheophytin) when Chattonella spp. were absent. In the summer of 1998, Chattonella spp. were relatively less abundant (maximum 21 cells ml⁻¹), and the algicidal bacterium Cytophaga sp. J18/M01 showed a close relationship with the change of total microalgal biomass. The present study suggests that the algicidal bacterium Cytophaga sp. J18/M01 preyed upon, not only harmful red tide microalgae, but also other common microalgae such as diatoms, and the bacterium presumably plays an important role in regulating microalgal biomass in natural marine environments. Received: 20 April 2000 / Accepted: 1 December 2000     

Seasonal reproductive biology of the egg-carrying calanoid copepod Pseudodiaptomus marinus in a eutrophic inlet of the Inland Sea of Japan

In situ egg production of the egg-carrying calanoid copepod Pseudodiaptomus marinus was investigated in Fukuyama Harbor, a eutrophic inlet of the Inland Sea of Japan, at 3- to 5-d intervals for a year. This species reproduced throughout the year, and the adults showed a large abundance peak in June/July and a small peak in September/October. Females usually outnumbered males, comprising 61.4% of the annual mean. The composition of ovigerous females varied from 7.9 to 100%, with an annual mean of 55.7%. Adult prosome length was consistently large throughout winter and spring, and decreased with increasing temperature in summer and fall. Egg diameter varied from 98 to 121 μm, and was negatively correlated to temperature. The seasonal variation in clutch size (range: 15.1 to 38.2 eggs) was bicyclical, with peaks in May and December. The egg production rate of breeding females was low in January to March (mean: 2.3 eggs female⁻¹ d⁻¹), while it was constantly high from mid-May to early October (mean: 12.1 eggs female⁻¹ d⁻¹). The specific egg production rate for the breeding females was highly correlated to temperature; it increased linearly from 0.03 d⁻¹ at 9 °C to 0.27 d⁻¹ at 26 °C. Compared to other co-occurring copepods, the reproductive rate of P. marinus was lowest, which is one of the reasons why this species never dominates in this inlet. Received: 11 November 1996 / Accepted: 7 December 1996     

Differential increment-deposition rate in embryonic statoliths of the loliginid squid Loligo vulgaris

 Apart from one study that reported growth of less than one increment per day in statoliths of the squid Alloteuthis subulata, most studies so far have presumed that one increment was laid down per day in the statoliths of the squid species they examined. The present study provides evidence of differential daily growth rates in embryonic statoliths of the squid Loligo vulgaris Lamarck, 1798, thus confirming a previous report for A. subulata. Incremental growth rates of L. vulgaris statoliths differ as a function of temperature. Squid embryos were incubated in the laboratory at three temperatures (12.0, 15.5 and 21.1 °C), and tetracycline staining was used to follow statolith growth. This growth slowed in squid exposed to the lowest temperature, but recovered when the squid were returned to warm conditions, indicating statolith adaptation. Statolith growth rate after incubation at 12 °C was 1.3% d⁻¹ and reached 6.1% d⁻¹ for squids exposed to 21.1 °C. Statoliths from embryos incubated at 15.5 °C yielded a rate of 1 increment d⁻¹ and a mean daily growth of 2.2 μm in the dorsal dome area of the statolith. In contrast, the slow growth of statoliths incubated at 12 °C yielded a mean daily growth of only 0.9 μm in the dorsal dome and the readings resulted in a less-than-daily increment-deposition rate. Received: 9 October 1999 / Accepted: 30 March 2000     

Resource acquisition in two intertidal fucoid seaweeds, Fucus serratus and Himanthalia elongata : seasonal variation and effects of reproductive development

Seasonal variations and the effect of reproductive development on resource acquisition by two intertidal fucoid species, the iteroparous Fucus serratus L. and the semelparous Himanthalia elongata (L.) S. F. Gray were examined. The oxygen-exchange characteristics of vegetative apical tissue of both non-fertile and fertile plants and receptacle tissue were compared at monthly intervals throughout reproductive development. Respiratory rates in non-fertile F. serratus varied seasonally between 1.5 and 8.0 μmol g⁻¹ fresh wt h⁻¹; in fertile plants the receptacle had a significantly lower respiratory rate than the vegetative tissue. The respiratory rate of the vegetative button of fertile H. elongata displayed less seasonal variation and was lower than that of the receptacle, which varied from a maximum of 9.5 μmol g⁻¹ fresh wt h⁻¹ at receptacle initiation in October to a minimum of 2.0 μmol g⁻¹ fresh wt h⁻¹ in February. The maximum photosynthetic rate (P _max) of non-fertile plants of both species did not vary in a distinct seasonal manner (∼60 μmol g⁻¹ fresh wt h⁻¹ for F. serratus and ∼12 μmol g⁻¹ fresh wt h⁻¹ for H. elongata). In fertile plants, the P _max of the receptacle tissue was (∼50% lower in F. serratus, and at its peak three times higher in H. elongata, than that of vegetative tissue. The stable carbon-isotope ratio (δ¹³C) did not differ between different tissue types in F. serratus, but values did vary seasonally, being less negative in the summer than in the winter (−13.5‰ compared to −18‰). The receptacle tissue of H. elongata also displayed a distinct seasonal variation in δ¹³C values (−12‰ in summer, −16‰ in winter), whilst the δ¹³C of the vegetative button did not vary seasonally. The rate of uptake of inorganic nitrogen by the vegetative thallus was lower in H. elongata than in F. serratus. The receptacle tissue of F. serratus had lower uptake rates than the vegetative tissue, whilst the uptake rate by H. elongata receptacle tissue was higher than that of the vegetative button. Received: 14 March 1997 / Accepted: 22 April 1997     

Metabolic rate and lipofuscin accumulation in juvenile European lobster (Homarus gammarus ) in relation to simulated seasonal changes in temperature

 The accumulation of lipofuscin, which is an indictor of physiological age, in the brain of juvenile European lobster (Homarus gammarus L.) was monitored for 22 mo in three experimental temperature regimes that simulated seasonal variation in temperature in the geographic range of this species. Metabolic rate responses to changes in temperature were estimated by measuring the activity of the electron transport system (ETS) in muscle tissue and in vivo rates of oxygen consumption. Lipofuscin accumulation oscillated with simulated seasonal changes in temperature and was described by seasonalised von Bertalanffy growth functions. The incremental accumulation in lipofuscin between sampling dates was linearly related to the number of degree days that accumulated between dates, irrespective of the amplitude of temperature fluctuation that had occurred. ETS activity increased with acclimation temperature and was modelled using a polynomial function. This indicated a lower temperature sensitivity in the temperature mid-range (12 to 16 °C), although the Q₁₀ for this mid-range was 2.1. ETS activity in lobsters acclimated to 8 and 18 °C and assayed at 13 °C was similar, indicating no compensation for changes in environmental temperature. Oxygen consumption rate was significantly higher at 14 °C than at 10.5 °C and had a Q₁₀ of 3.6, again suggesting no compensation to temperature change. The absence of metabolic compensation in response to temperature change in H. gammarus is consistent with the predictability of changes in temperature and food availability in the sub-littoral environment of this species. As lipofuscin accumulates according to metabolic rate, and metabolic rate in H. gammarus is directly correlated with temperature, geographic differences and long-term temporal trends in temperature will need to be considered when converting physiological age indices, obtained from lipofuscin estimates, to a chronological scale. Received: 27 April 2000 / Accepted: 21 July 2000     

A comparison of transport-related gill enzyme activities and tissue-specific free amino acid concentrations of Baltic Sea (brackish water) and freshwater threespine sticklebacks, Gasterosteus aculeatus, after salinity and temperature acclimation

The osmoregulatory abilities of one freshwater and two brackish water (Baltic Sea) populations of the euryhaline teleost fish Gasterosteus aculeatus were studied with respect to evolutionary physiology. Plasma osmolality, activities of Na⁺K⁺-ATPase, citrate synthase, creatine kinase in the gill and free amino acids in liver, axial muscle and pectoral fin muscle were measured. After transfer from 10 to 35 ppt at 15 °C, time-course changes of plasma osmolality and gill Na⁺K⁺-ATPase showed no significant fundamental differences between the freshwater and one of the Baltic Sea populations. In a multi-factorial experiment, each population was exposed to four different abiotic regimes. Both brackish water populations had high mortality in freshwater at 4 °C, which is discussed as a failure of osmotic regulation (reduced taurine concentrations). Freshwater specimens had higher levels of glycine in the axial and pectoral fin muscles compared to the brackish water populations. This is interpreted as a genetically based effect. In brackish (20 ppt) water of 15 °C, the freshwater population had high activities of Na⁺K⁺-ATPase, but low activities of creatine kinase, whereas both brackish water populations behaved in the opposite way. A fundamental difference between the freshwater and brackish water populations on the level of the osmoregulatory machinery was not observed. Received: 10 December 1998 / Accepted: 22 September 1999     

Potential dietary effects on the fatty acid composition of the common jellyfish Aurelia aurita

Fatty acid composition of the natural and aquarium-reared common jellyfish Aurelia aurita was investigated. Fatty acid composition of the aquarium-reared A. aurita clearly reflected that of the diet, brine shrimp (Artemia). In the same way, fatty acid composition of the natural A. aurita was assumed to reflect those of natural diets. Samples of natural A. aurita were collected from April 1995 to September 1995 in the Seto Inland Sea, Japan, and their fatty acids were analyzed by gas chromatography and mass spectrometry. Variation of fatty acid compositions was seasonal rather than dependent on body size. Two major seasonal groups were divided by the cluster analysis of the A. aurita fatty acid composition: the April–June and the August–September clusters. The April–June cluster was characterized by high contents of the (n − 3)-fatty acids of diatom origin, accumulated via the grazing food chain. By contrast, the August–September cluster was characterized by an increase in (n − 6)-fatty acids of macroalgal origin, probably transferred via the detritus food chain. These results suggest that the diet of natural A. aurita may shift between the diatom-based food chain and the detritus-based food chain. Received: 12 April 2000 / Accepted: 1 December 2000     

Oxygen transport by the hemocyanin of the protobranch mollusc Solemya reidi

Oxygen transport by the hemocyanin of the protobranch mollusc Solemya reidi Bernard was studied in native hemolymph samples. Clams were collected from two different reducing environments, beneath log booms in Alberni Inlet, British Columbia, Canada, and from the sewage effulent in Santa Monica Bay, California, USA. The hemocyanin concentration in a pooled hemolymph sample (n = 10 individuals) was 33.5 mg ml⁻¹. The mean hemolymph pH of five Alberni clams maintained for 3 wk in mud was 7.96 ± 0.06. No significant variation in hemocyanin oxygen-affinity or cooperativity was found for hemocyanin in whole hemolymph samples from these five individually studied clams. There was a significant difference only at 15 °C in the oxygen affinity of hemocyanin in pooled whole-hemolymph samples from S. reidi collected from Alberni Inlet compared with clams collected from Santa Monica Bay. Little effect of temperature on hemocyanin oxygen-affinity was found for temperatures below 20 °C; above 20 °C, the oxygen affinity was reduced for clams from both sites. Temperature and pH had no apparent affect on hemocyanin cooperativity. Moderate Bohr shifts were found at all temperatures examined. The presence of physiologically relevant concentrations of thiosulphate in hemocyanin samples resulted in a decrease in hemocyanin oxygen-affinity, opposite to the effect on hemocyanin found for the hydrothermal vent crab Byth-ograea thermydron, but thiosulphate had no effect on hemocyanin cooperativity. Received: 2 September 1997 / Accepted: 6 February 1998     

Facultative lecithotrophy during larval development of the burrowing shrimp Callianassa tyrrhena (Decapoda: Callianassidae)

Survival, developmental and consumption rate (Artemia nauplii ingested per day) as well as predation efficiency (ingested per available Artemia nauplii) were studied during the larval development of the shallow-water burrowing thalassinid Callianassa tyrrhena (Petagna, 1792), which exhibits an abbreviated type of development with only two zoeal stages and a megalopa. The larvae, hatched from berried females from S. Euboikos Bay (Aegean Sea, Greece), were reared at 10 temperature–food density combinations (19 and 24 °C; 0, 2, 4, 8 and 16 Artemia nauplii d⁻¹). Enhanced starvation resistance was evident: 92 and 58% of starved zoeas I molted to zoea II, while metamorphosis to megalopa was achieved by 76 and 42% of the hatched zoeas at 19 and 24 °C, respectively. The duration of both zoeal stages was affected by temperature, food density and their interaction. Nevertheless, starvation showed different effects at the two temperatures: compared to the fed shrimp, the starved zoeae exhibited accelerated development at 19 °C (8.4 d) but delayed metamorphosis at 24 °C (5.9 d). On the other hand, both zoeal stages were able to consume food at an increased rate as food density and temperature increased. Predation efficiency also increased with temperature, but never exceeded 0.6. Facultative lecithotrophy, more pronounced during the first zoeal stage of C.tyrrhena, can be regarded as an adaptation of a species whose larvae can respond physiologically to the different temperature–food density combinations encountered in the wide geographical range of their natural habitat. Received: 28 February 1998 / Accepted: 21 October 1998     

Exfoliation of epithelial cells by the scallop Placopecten magellanicus : seasonal variation and the effects of elevated water temperatures

Previous studies have shown that large numbers of ciliated and nonciliated epithelial cells (diam.: 6 to 15 μm) are released by adult sea scallops, Placopecten magellanicus (Gmelin), during summer months in Newfoundland when water temperatures are at a maximum and gonads are well developed. Such exfoliation of cells could be a response to stresses associated with elevated water temperatures and/or spawning activity. In the present study an electronic particle counter/sizer was used to further investigate the factors that influence exfoliation of epithelial cells by juvenile and adult scallops throughout the year. We observed release of epithelial cells from juveniles, and from adults collected in months when gametogenic activity was minimal, indicating that exfoliation does not occur as a result of reproductive activity alone. SEM analysis revealed little difference in surface characteristics of the gills, mantle and gonad between scallops that had released cells and those that had not, suggesting that exfoliation of small numbers of cells may be a consequence of cellular turnover and normal physiological function. Adult scallops were monitored in a second experiment to determine the effects of raising water temperatures from 8.5 to 14.7 and to 21.0 °C on the frequency (proportion) and rate of cellular exfoliation. Only at the highest experimental temperature (21.0 °C) were exfoliation rates significantly higher than rates recorded at 8.5 or 14.7 °C. SEM analyses revealed some damage to gill, mantle and gonad tissues when scallops were exposed to 14.7 and then to 21.0 °C for a total of 8 d. Received: 21 August 1996 / Accepted: 13 September 1996     

Patterns of time and space utilisation in coelacanths (Latimeria chalumnae), determined by ultrasonic telemetry

Acoustic telemetry was used to examine patterns of activity and space utilisation of coelacanths, nocturnal predators which spend the day in submarine caves. Nine coelacanths (Latimeria chalumnae) were tracked, each for a period of 1 to 16 nights at Grande Comore, West Indian Ocean. Activities lasted on average 9 h, usually starting shortly after sunset and ending before sunrise. Vertically, coelacanths moved up and down at and below cave level by following the bottom contour, mainly between 180 and 400 m depth. The deepest record was 698 m, the shallowest 133 m. Most time was spent between 200 and 300 m depth. Large individuals performed deep excursions to depths below 400 m, usually once per night. The fish spent most time in water temperatures of 15 to 19 °C; they rarely ventured into waters warmer than 22 °C measured at depths shallower than 160 m depth. Horizontally, coelacanths stayed in narrow areas ranging from <1 to 10 km of coastline. Coelacanths are extremely slow drift-hunters with an estimated average swimming speed of 3.2 m min⁻¹, often travelling not more than 3 km per night. They probably take advantage of local upwelling and downwelling and slow currents occurring parallel to the steep slopes. This study shows that coelacanths are inhabitants of the subphotic zone, where they are active mainly below the depth of their daytime refuges. Received: 7 July 1999 / Accepted: 11 February 2000     

Intraspecific physiological variability of the gastropod Littorina saxatilis related to the vertical shore gradient in the White and North Seas

 Physiological responses to desiccation and temperature stress as well as behavioural responses to fast and abrupt environmental changes were investigated in high- and low-shore Littorina saxatilis (Olivi) from several populations from the White and North Seas. Variations in evaporation rates, resistance to air exposure and to acute and chronic temperature stress between animals from different shore levels were similar in White and North Sea periwinkles, consistent with the adaptive nature of these variations. High-shore snails were found to be able to conserve body water reserves better, to resist higher temperatures and to survive longer under conditions of combined temperature and desiccation stress than their low-shore counterparts. In a temperature range of 25 to 35 °C, the rate of evaporative water loss was positively correlated with temperature in low-shore snails while being largely temperature-independent in high-shore snails. Median lethal time during air exposure in L. saxatilis was negatively but not linearly related to the temperature of exposure. In a temperature range of 30 to 38 °C, the resistance to heat exposure in air was only slightly dependent on the temperature, with Q ₁₀ = 1.4 for the median lethal time; the heat resistance dropped drastically at temperatures above 38 °C, with Q ₁₀ = 593.8. This suggests different mechanisms of temperature resistance in different parts of the studied temperature range. In contrast, behavioural response to extreme salinity fluctuations was not uniform in the high- and low-shore periwinkles from the White and North Seas, which may reflect specific environmental conditions at different shore levels in the two areas studied. Observed physiological and behavioural variations are discussed from the viewpoint of different adaptive strategies employed by eulittoral and eulittoral-fringe animals within populations of a single species. Received: 13 December 1999 / Accepted: 11 April 2000     

Impact of temperature on juvenile growth and age at first egg-laying of the Pacific reef squid Sepioteuthis lessoniana reared in captivity

Cephalopod mollusks exhibit highly plastic life cycle traits influenced primarily by the interactive effects of food availability, light cycle and temperature, with the latter perhaps the most influential. Hatchlings of the tropical reef squid Sepioteuthis lessoniana were hatched from field-collected eggs in the laboratory and cultured at different temperatures to evaluate the effect of temperature on growth rates. All groups showed rapid, sustained growth rates from hatching to a size of 10–25 g. Beyond this size range, growth was slower and not clearly exponential in form. Growth rate was closely linked to temperature. Squids grown at approximately 27 °C attained a size of 10 g in as little as 45 days at sustained growth rates of 12.2% body weight day⁻¹ (%bw day⁻¹), while squids cultured at 20 °C required almost 100 days to attain the same size at rates of 5.7%bw day⁻¹. At an age of 55 days and approximately 1 g body weight, juvenile squids cultured at 20 °C were able to accelerate growth rates from 5.7%bw day⁻¹ to over 12%bw day⁻¹ when temperature was raised to 27 °C. They maintained this growth rate to a size of about 10 g and an age of at least 75 days post-hatching, indicating that body size and not age is the limiting factor for this rapid post-hatching growth. By comparison, conspecifics cultured near 27 °C from hatching had shifted out of the rapid post-hatching growth phase by day 50 at sizes between 10 and 50 g. The hatchlings from temperate to subtropical Japan had consistently higher growth rates at comparable temperatures than hatchlings from tropical Okinawa. When plotted as growth rate versus temperature, the Japanese group had a clearly higher slope to the relationship than the tropical populations, equivalent to a 2%bw day⁻¹ difference in growth rate at 25 °C. Age at first egg-laying was decreased at higher culture temperatures; however, overall life span was not. Received: 21 February 2000 / Accepted: 6 September 2000     

Beat frequency of cilia in the branchial basket of the ascidian Ciona intestinalis in relation to temperature and algal cell concentration

To elucidate the effects of temperature and algal cell concentration on pumping of water in the ascidian Ciona intestinalis a number of different experiments were performed. Beat frequency of the lateral cilia in the openings of the branchial sac was measured in intact specimens using a microprojection objective and a monochrome CCD video camera. At constant low algal cell concentration, beat frequencies increased linearly with temperature from 4.0 Hz (±0.5) at 7.4 °C to 13.6 Hz (±1.6) at 20.1 °C. At a constant temperature of 15 °C, beat frequency decreased with increasing algal cell concentration from approximately 3000 to >10 000 Rhodomonas sp. cells ml⁻¹. The decrease was observed both in experiments where the ascidians had been acclimated to a fixed algal cell concentration and in experiments with changing concentrations. Effect of algal cell concentration on squirting/siphon closure and flow velocity in the exhalent siphon was measured using a thermistor. At low algal cell concentrations, flow velocity in the exhalent siphon was stable, apart from a few short squirts. At very high algal cell concentrations, the flow velocity was reduced and much less stable, with prolonged squirting. The effect of gut content on filtration was studied in experiments with specimens acclimated to high algal cell concentrations. Results showed a close relation between gut clearance and filtration rate. From the experimental results and a qualitative analysis of the Ciona-pump it was concluded that the ciliary beat frequency is proportional to the water flow through the sea squirt and that changes in pumping caused by temperature or algal cell concentration are under nervous control or governed by enzyme kinetics, rather than being a result of physico-mechanical properties, i.e. pump efficiency versus flow resistance, of the ascidian pump. Received: 6 October 1997 / Accepted: 8 October 1998