Some aspects of the life history of the subarctic copepodNeocalanus cristatus (Calanoida) in Sagami Bay,central Japan

A typical subarctic copepod,Neocalanus cristatus, occurred in the mesopelagic layer (500 to 1000 m) in Sagami Bay, central Japan, throughout the year. Specimens were collected from 1982 to 1986. A small number of adult females were distributed from 800 to 900 m only, but no adult males were collected. This species appeared to be abundant in April and August, when intermediate Oyashio water flowed strongly into Sagami Bay. Mean prosome lengths of copepodite stages IV and V and adults were 4.33, 6.87 and 6.87 mm, respectively. The condition factor [wet wt/(prosome length)³ × 100] of copepodite stage V did not vary remarkably, and mean values ranged from 4.7 to 5.0. Prosome length, body weight and condition factor ofN. cristatus collected from Sagami Bay were smaller than those of copepods in the northern North Pacific.N. cristatus transported from the north cannot molt to adult stages (except for those originating in mesopelagic waters) due to the adverse environmental conditions in Sagami Bay; instead, they die in the mesopelagic layer and sink to the bathypelagic layer (1 000 to 1 400 m), close to the bottom. Since nauplii and early copepodite stages did not occur in any season,N. cristatus probably do not reproduce in Sagami Bay.     

Feeding and predation impact of two chaetognath species, Eukrohnia hamata and Sagitta gazellae, in the vicinity of Marion Island (southern ocean)

The predation impact of the two chaetognaths Eukrohnia hamata and Sagitta gazellae on mesozooplankton standing stock were investigated in three depth layers during two 24 h stations occupied in the vicinity of Marion Island in late austral summer (April/May) 1986. The zooplankton community at both stations was dominated by small copepods (Oithona spp., Microcalanus spp.), which accounted for >95% of total zooplankton abundance. Chaetognaths comprised <2% of total zooplankton abundance. E. hamata constituted >95% of the total chaetognath stock. The general trend in both species was decreasing abundance with increasing depth, which appeared to be correlated to the distribution of copepods (r ² = 0.45; P <0.05). Gut-content analysis showed that copepods (mainly Oithona spp., Calanus spp. and Rhincalanus gigas) and ostracods were the main prey of both species, accounting for 87 and 61% of the total number of prey in E. hamata and S.␣gazellae stomachs, respectively. In the guts of S.␣gazellae, pteropods (Limacina spp.) and chaetognaths were also well represented. The mean number of prey items (NPC) for E. hamata ranged from 0.02 to 0.06 prey individual⁻¹ which corresponds to an individual feeding rate (Fr) of between 0.05 and 0.12 prey d⁻¹. For S.␣gazellae, the NPC values were higher, varying between 0.04␣and 0.20 prey individual⁻¹, or between 0.15 and 0.76 prey d⁻¹. The daily predation impact of the two chaetognaths was estimated at between 0.3 and 1.2% of the copepod standing stock or between 7 and 16% of the daily copepod production. Predation by S. gazellae on chaetognaths accounted for up to 1.6% of the chaetognath standing stock per day. Received: 26 November 1996 / Accepted: 31 October 1997     

Prey, feeding rates, and asexual reproduction rates of the introduced oligohaline hydrozoan Moerisia lyonsi

Moerisia lyonsi Boulenger (Hydrozoa) medusae and benthic polyps were found at 0 to 5‰ salinity in the Choptank River subestuary of Chesapeake Bay, USA. This species was introduced to the bay at least 30 years before 1996. Medusae and polyps of M. lyonsi are very small and inconspicuous, and may occur widely, but unnoticed, in oligohaline waters of the Chesapeake Bay system and in other estuaries. Medusae consumed copepod nauplii and adults, but not barnacle nauplii, polychaete and ctenophore larvae or tintinnids, in laboratory experiments. Predation rates on copepods by medusae increased with increasing medusa diameter and prey densities. Feeding rates on copepod nauplii were higher than on adults and showed no saturation over the range of prey densities tested (1 to 64 prey l⁻¹). By contrast, predation on copepod adults was maximum (1 copepod medusa⁻¹ h⁻¹) at 32 and 64 copepods l⁻¹. Unexpectedly, M. lyonsi colonized mesocosms at the Horn Point Laboratory during the spring and summer in 4 years (1994 to 1997), and reached extremely high densities (up to 13.6 medusae l⁻¹). Densities of copepod adults and nauplii were low when medusa densities were high, and estimated predation effects suggested that M. lyonsi predation limited copepod populations in the mesocosms. Polyps of M. lyonsi asexually produced both polyp buds and medusae. Rates of asexual reproduction increased with increasing prey availability, from an average total during a 38 d experiment of 9.5 buds polyp⁻¹ when each polyp was fed 1 copepod d⁻¹, to an average total of 146.7 buds polyp⁻¹ when fed 8 copepods d⁻¹. The maximum daily production measured was 8 polyp buds and 22 medusae polyp⁻¹. The colonizing potential of this hydrozoan is great, given the high rates of asexual reproduction, fairly wide salinity tolerance, and existence of a cyst stage. Received: 29 October 1998 / Accepted: 3 March 1999     

Predation on Euphausia pacifica by demersal fishes: predation impact and influence of physical variability

The importance of Euphausia pacifica Hansen to the demersal fish community off Sendai Bay, northern Japan, was studied based on a total of 256 trawl samples collected during May, October and November, from 1989 to 1992. The samples were classified into 16 assemblages according to year, month and depth. The importance of E. pacifica to the total diet of each of the assemblages was measured using an index considering both fish diet and species composition. A total of 24 fish species out of 87 were found to ingest E. pacifica. Of these, Gadus macrocephaus and Theragra chalcogramma had the highest predation impact due to their dominance in the fish assemblages and the high proportion of E. pacifica in their diets. The contribution of E. pacifica to the total diet of the fish assemblage was highest in the shallow (≤300 m) regions during May, accounting for an average of 38.5% for the 4 years. However, during May 1990, when the warm Kuroshio Extension prevailed, the contribution was higher in the deep (>300 m; 22.6%) region, reflecting active and/or passive movements of E. pacifica. The average predation impact was maximal in the May/shallow assemblage (mean ± SE; 4.6 ± 1.4 kg wet wt km² d⁻¹) and was minimal in the November/shallow assemblage (0.4 ± 0.3). Annual estimates of consumption by demersal fish fishes ranged from 43 to 128 metric tons, representing 15 to 64% of the annual commercial catch of E. pacifica by local fisheries. It is therefore suggested that the fisheries on E. pacifica have the potential to considerably impact the demersal fish assemblages. Received: 3 December 1997 / Accepted: 10 June 1998     

Variability over space and time in the genetic structure of the winged kelp Alaria marginata

The genetic structure of Alaria marginata Postels & Ruprecht was investigated spatially and seasonally using amplified-fragment length polymorphisms (AFLP). Using one primer, 206 scoreable bands were produced. Individual plants that were separated by as little as a few decimeters to >100 km could be distinguished, and followed an isolation-by-distance model. Genetic similarity (average April data) ranged from 76% for patches (a few decimeters in diameter), to 71% for individual kelp stands (15 m across) and 67% for a group of stands separated by 185 km. Greater genetic similarity of patches occurred at a wave-sheltered site than at a wave-exposed site. The lower wave action may limit meiospore dispersal, resulting in patches of related sporophytes. Genetic similarities between A. marginata stands indicated a pattern of gene flow that is consistent with local currents. In one stand, genetic similarities were markedly different between seasons. This seasonal pattern may be the result of different age groups dominating the sampled stands over time. The genetic structure of A. marginata comprises local scale (patch and within-stand) heterogeneity and larger scale (between-stands) homogeneity. Received: 30 September 1998 / Accepted: 26 October 1999     

Zooplankton responses to hypoxia: behavioral patterns and survival of three species of calanoid copepods

Seasonally recurrent and persistent hypoxic events in semi-enclosed coastal waters are characterized by bottom-water dissolved oxygen (d.o.) concentrations of < 2.0 ml l⁻¹. Shifts in the distribution patterns of zooplankters in association with these events have been documented, but the mechanisms responsible for these shifts have not been investigated. This study assessed interspecific differences in responses to hypoxia by several species of calanoid copepods common off Turkey Point, Florida, USA: Labidocera aestiva (Wheeler) (a summer/fall species), Acartia tonsa (Dana) (a ubiquitous year-round species), and Centropages hamatus (Lilljeborg) (a winter/spring species). Under conditions of moderate to severe hypoxia 24-h survival experiments were conducted for adults and nauplii of these species from August 1994 to October 1995. Experiments on adults used a flow-through system to maintain constant d.o. concentrations. Adults of A. tonsa showed no decline in survival with d.o. as low as 1.0 ml l⁻¹, sharp declines in survival at d.o. = 0.9 to 0.6 ml l⁻¹, and 100% mortality with d.o. = 0.5 ml l⁻¹. Adults of L. aestiva and C. hamatus were more sensitive to oxygen depletion: both species experienced significant decreases in survival for d.o. = 1.0 ml l⁻¹. Nauplii of L. aestiva and A. tonsa showed no significant mortality with d.o. = 1.1 to 1.5 ml␣l⁻¹ and d.o. = 0.24 to 0.5 ml l⁻¹, respectively. In addition, experiments investigating behavioral avoidance of moderate to severe hypoxia were carried out for adults of all three species. None of the three species effectively avoided either severely hypoxic (d.o. < 0.5 ml l⁻¹) or moderately hypoxic (d.o. ≈ 1.0 ml l⁻¹) bottom layers in stratified columns. These results suggest that in␣nearshore areas where development of zones of d.o. < 1.0 ml l⁻¹ may be sudden, widespread, or unpredictable, patterns of reduced copepod abundance in bottom waters may be due primarily to mortality rather than avoidance. Received: 31 August 1996 / Accepted: 24 September 1996     

Variation in cadmium uptake by estuarine phytoplankton and transfer to the copepod Eurytemora affinis

The pathways of cadmium (Cd) uptake and transfer within an estuarine planktonic community from the Patuxent River, Maryland, USA, were investigated using an assemblage of natural phytoplankton and the copepod Eurytemora affinis Poppe. The experiment was carried out in October 1992 in replicated 500-liter, flow-through, fiberglass tanks. Growth rate, species composition, and Cd loading affected the accumulation of Cd by the phytoplankton. Uptake of Cd by phytoplankton was proportional to the amount of Cd available in the water column. Partition coefficients (K _d) for phytoplankton uptake averaged 4.4 × 10⁴. As metal loading rates and phytoplankton species composition changed during the 12-d experiment, Cd partitioning declined. Transfer of Cd to E. affinis occurred from Cd-laden phytoplankton, with levels in the copepods being approximately the same as, or somewhat less, than in the phytoplankton. Some Cd uptake occurred in copepods exposed to dissolved Cd only; however, the uptake was considerably less than that seen from food. Thus, Cd content of higher trophic levels, such as copepods, can be affected by the degree of Cd incorporation in their food source, and by ecological factors regulating phytoplankton ingestion. Received: 13 September 1995 / Accepted: 29 October 1998     

Population dynamics and production of the planktonic copepods in a eutrophic inlet of the Inland Sea of Japan. IV. Pseudodiaptomus marinus, the egg-carrying calanoid

Population dynamics and production of the egg-carrying calanoid copepod Pseudodiaptomus marinus were studied for a year in Fukuyama Harbor, a eutrophic inlet of the Inland Sea of Japan. This species was perennial, with a large numerical peak in June and small peaks in September/October and November/December. During the study period, at least 11 generations could be detected. For each generation, the stage-specific survival from egg to Copepodite Stage (C) V was determined; it was very high during early life stages (egg to NIII), and gradually decreased beyond. On average, 94% of eggs recruited into NIII, which is strongly contrasted with very high (>ca. 90%) mortality during the corresponding stages for free-spawning copepods, i.e. Acartia omorii, Centropages abdominalis and Paracalans sp. This demonstrates that the egg-carrying strategy has a great advantage to reduce mortality in egg stage. The biomass of this species showed marked seasonal variations largely in parallel with numerical abundance. The instantaneous somatic growth rate increased linearly with temperature. The population production rate was estimated as the sum of somatic growth of larval stages and egg production of adult females; the annual integration was 51.0 mg C m⁻³ yr⁻¹ or 0.38 g C m⁻² yr⁻¹. Received: 11 November 1996 / Accepted: 7 December 1996     

Observations of copepod feeding and vertical distribution under natural turbulent conditions in the North Sea

We present results of simultaneous measurements of turbulent-dissipation rate, zooplankton vertical distribution and copepod gut pigments in the northern North Sea. Analysis shows that some, but not all, copepods (by species, sex and stage) exhibit significant dependence on turbulence in respect to vertical distribution and feeding rate. Oithona similis (female and copepodite stages) exhibits an avoidance of the surface layer when turbulence is strong there. For the range of turbulence (10⁻⁷ to 10⁻³m² s⁻³) and ambient chlorophyll concentration (0.5–0.8 μg l⁻¹) encountered, Calanus spp. and Metridia lucens exhibited a significant negative response in feeding-rate index with increasing turbulence. Centropages typicus and Pseudocalanus spp. also exhibited a negative response but of less significance. Received: 12 October 2000 / Accepted: 11 December 2000     

Weight and chemical composition of some important oceanic zooplankton in the North Pacific Ocean

Wet and dry weight, total carbon, nitrogen, hydrogen, and ash contents were determined on 33 species of zooplankton distributed predominantly in the open sea region of the North Pacific. Sampling covered the waters from 44°N to the equator. Average percentage of dry weight to wet weight was about 19% of all samples from the whole area. Percentage dry weight of carbon in copepods was on an average 51.5%. The highest value, 66.6%, was obtained in eggs of the copepod Pareuchaeta sarsi. Mixed zooplankton was assumed to contain carbon comprising about 35 to 45% of the dry weight. Carbon contained in the zooplankton biomass existing in the upper 200 m in the western parts of the northern North Pacific and Bering Sea during spring and summer was estimated to range from 20 to 85 mg C/m³. Nitrogen content varied considerably with localities. Average ratio of carbon to nitrogen was 8.5 in subarctic copepods, and 4.1 in subtropic-tropic copepods. This ratio also varied with season. In the copepod Calanus cristatus the ratio was highest (10.0) in May, immediately after the spring bloom of phytoplankton, when the animals contained much fat. The ratio fell to 5.1 in December. There seemed to be a large seasonal variation in boreal zooplankton due to great fluctuations of environmental conditions, especially the amount of food available; in tropical species the range was small because of environmental uniformity. Average hydrogen content was about 6 to 10%. The percentage of ash to dry weight amounted to 39.3% in pteropods and 3.4% in copepods.     

Relationships between diet and body size, mouth morphology, habitat and movements of six sillaginid species in coastal waters: implications for resource partitioning

The dietary compositions and breadths of sequential 50 mm size classes of the six whiting species found in nearshore (<1.5 m), shallow inner-shelf (5 to 15 m) and/or deep inner-shelf (20 to 35 m) waters of the lower west coast of Australia were determined. Comparisons between the results of principal components analysis of head and mouth dimensions and the dietary compositions of Sillago bassensis, S. vittata, S. burrus, S. schomburgkii, S. robusta and Sillaginodespunctata suggests that any differences in the dietary composition of similar-sized representatives of different species, when they occur in the same habitat, are more likely to be due to differences in foraging behaviour than mouth morphology. Classification, ordination and Schoener's overlap indices showed that, in nearshore waters, the juveniles of Sillago bassensis, which colonise relatively exposed areas, have a different diet to those of the smallest representatives of the other whiting species that occupy more sheltered habitats. S. bassensis consumes mainly amphipods, whereas the smaller representatives of S. vittata, S. burrus, S. schomburgkii and Sillaginodes punctata ingest large volumes of copepods, which are typically abundant in protected nearshore waters. Although the mouth dimensions of S. punctata tend to be smaller than those of Sillago schomburgkii, the larger individuals of the former species ingest greater quantities of larger prey, such as crabs and carid shrimps. As S. bassensis, S. vittata and S. burrus increase in size and migrate out into shallow inner-shelf waters, the latter two species tend to concentrate more on benthic prey, while the former species ingests fauna that is more epibenthic. The largest S. bassensis subsequently migrate out into deep inner-shelf waters, where they co-occur with S. robusta, which is restricted to those waters. In these waters, S. bassensis feeds to a far greater extent on large benthic prey, whereas S. robusta consumes a greater quantity of small epibenthic crustaceans, differences that reflect the far larger lengths of the former species in that region. The above data emphasise that the distribution and ontogenetic movements of the six abundant species of whiting play a major role in facilitating a partitioning of food resources amongst these species found in coastal waters of the lower west coast of Australia. Received: 7 October 1996 / Accepted: 31 January 1997     

Grazing of phytoplankton by copepods in eastern Antarctic coastal waters

Chlorophyll a, primary productivity and grazing by copepods on phytoplankton were measured in the upper water column during the summer of 1994/1995 at a coastal site near Davis Station, East Antarctica. Chlorophyll a was at a maximum in mid-December, then dropped markedly as the coastal fast ice melted and broke‐out. Phytoplankton biomass increased again from mid‐ to late‐February. Copepods accounted for at least 65% of zooplankton biomass in the water column before sea ice break‐out, whereas larval polychaetes and ctenophores dominated after ice break‐out. Oncaeacurvata was the numerically dominant species throughout the study. The highest grazing rate (8.7 mg C␣m⁻³␣d⁻¹) was recorded on 21 December when O.␣curvata accounted for 64% of the total. Grazing had decreased markedly by 28 December (0.9 mg C m⁻³ d⁻¹); again O. curvata accounted for over 50% of the total ingested. Copepod grazing increased after ice break-out until the last experiment on 20 February (⋍5 mg C␣m⁻³␣d⁻¹). The main species responsible for grazing during this period were O. curvata, Oithonasimilis, Calanoidesacutus and unidentified copepod nauplii. It was estimated that copepods removed between 1 and 5% of primary productivity. Received: 11 October 1996 / Accepted: 22 October 1996     

Trophic upgrading of food quality by protozoans enhancing copepod growth: role of essential lipids

 Protozoa are known for their intermediary trophic role in transferring organic matter from small size planktonic particles to mesozooplankton. This study concentrates on the possible addition of biochemical value during this transfer, by new production of compounds that are essential in copepod food. In laboratory experiments, copepods could not be raised on a diet of the chlorophycean Dunaliella sp., though they readily consumed this alga. Dunaliella sp. contained all essential amino acids, but was deficient in highly unsaturated fatty acids and in sterols. In contrast to copepods, the heterotrophic dinoflagellate Oxyrrhis marina grew well on Dunaliella sp., producing significant amounts of the long-chain fatty acids docosahexaenoic acid and eicosapentaenoic acid, in addition to cholesterol and brassicasterol. Using this O. marina grown on Dunaliella sp. to feed Temora longicornis and Pseudocalanus elongatus, both copepod species rapidly developed from young nauplius larvae to maturity on the dinoflagellate diet. Hence, in this experimental food-chain the inadequate chlorophycean food was biochemically upgraded by the protozoan to high-quality copepod food. The results indicate that highly unsaturated fatty acids and/or sterols are essential compounds, which can be produced by protozoans. Due to their intermediate size, the mechanism of trophic upgrading by protozoans may bridge the gap of essential nutrients between the microbial loop and higher trophic levels. Received: 11 January 1999 / Accepted: 3 June 1999     

Production and fate of copepod fecal pellets across the Southern Indian Ocean

The vertical distribution of copepods, fecal pellets and the fecal pellet production of copepods were measured at seven stations across the Southern Indian Ocean from productive areas off South Africa to oligotrophic waters off Northern Australia during October/November 2006. We quantified export of copepod fecal pellet from surface waters and how much was retained. Furthermore, the potential impact of Oncaea spp. and harpacticoid copepods on fecal pellets degradation was evaluated and found to be regional substantial. The highest copepod abundance and fecal pellet production was found in the western nutrient-rich stations close to South Africa and the lowest at the central oligotrophic stations. The in situ copepod fecal pellet production varied between 1 and 1,000 μg C m⁻³ day⁻¹. At all stations, the retention of fecal pellets in the upper 400 m of the water column was more than 99% and the vertical export of fecal pellets was low (<0.02 mg m⁻² day⁻¹).     

Mesoscale, finescale and microscale distribution of micro- and nanoplankton in the Irish Sea, with emphasis on ciliates and their prey

We examined the degree of mesoscale (km), finescale (m), and microscale (cm) patchiness of ciliates and their prey in waters of varying hydrographic conditions. Samples were taken throughout the water column, along a transect across the Irish Sea (54°N), at scales ranging from 0.15 to 10⁵ m. We examined physical, chemical, and biological characteristics. The eastern and western Irish Sea were stratified, with a pycnocline at ∼20 to 30 m. The central waters were mixed and had adjacent frontal regions. Euphotic depth was ∼20 to 35 m. Generally, the upper waters were nitrogen-limited, with elevated levels associated with frontal regions and deeper waters. Microphytoplankton exhibited fine-mesoscale patchiness: diatom numbers were low in stratified waters, with higher levels in mixed and frontal regions; dinoflagellates were abundant in subsurface waters near the fronts. Nanoflagellate numbers and biomass decreased with depth below the pycnocline, and exhibited microscale distribution in upper waters; these micropatches may provide increased food levels for ciliates. Microscale distribution of ciliates was rare and only occurred at mixed/frontal sites; finescale ciliate patches were a more prominent feature of the water column. These finescale patches can be composed of a variety of taxa but can also be virtually monospecific. Finescale patches may produce localised regions of high productivity that is available to fishes and copepods, but may also be a sink, as patches can be short-lived and thus unavailable to predators. Received: 28 September 1998 / Accepted: 26 January 1999     

Zooplankton: pre-southwest and northeast monsoons of 1993 to 1994, from the North Arabian Sea

Zooplankton samples from the North Arabian Sea Environment and Ecosystem Research (NASEER) cruises were analyzed to determine the basic taxonomic composition, biomass (standing stock) and the total and copepod numeric abundance; these characteristics are discussed with reference to the different monsoon periods. Cruises carried out during March 1993 and May 1994, categorized as pre-southwest monsoon periods, and a cruise in December 1994, categorized as a northeast monsoon period, are discussed in detail. The biomass of January 1992 versus August 1992 and August 1992 versus March 1993 differed significantly (F = 6.44, P≤ 0.05). Ranges of highest and lowest biomass from each cruise are also given. Distinct “high” and “low” production areas of statistically significant difference (F = 12.67, P≤ 0.05) were observed. The “high” and “low” production areas were mobile and followed the surface wind circulation patterns (wind reversal pattern) during the northeast and southwest monsoon periods. Overall zooplankton showed a patchy distribution. The overall zooplankton abundance and total copepod counts differed significantly between the Cruises 3 versus 4 and 4 versus 5 (F = 15.67, P≤ 0.05 and F = 34.39, P≤ 0.05, respectively). There was no significant difference (P≥ 0.05) in biomass, between eutrophic and oligotrophic stations, suggesting no difference between near shore and offshore waters. Thirty-eight taxonomic groups were identified from the samples, with copepods being the most dominant group, followed by chaetognaths and siphonophores. Copepods constitute an average of 52.50 to 74.93% of the total zooplankton count and reach maxima of 92.14% of the total zooplankton count at the outset of the southwest monsoon (March 1993) and 91.39% at the outset of the active northeast monsoon (December 1994). Received: 27 February 1998 / Accepted: 8 October 1999     

Vertical distribution, population structure and life cycle of Neocalanus cristatus (Crustacea: Copepoda) in the Oyashio region, with notes on its regional variations

Vertical distribution and population structure of Neocalanus cristatus were investigated at Site H in the Oyashio region from September 1996 through October 1997 to evaluate their life cycle mode. Additional temporary samplings were also made at several stations covering the entire subarctic Pacific, Okhotsk Sea and Japan Sea, as a basis for regional comparison of life cycles of this species. At Site H, N. cristatus spawned throughout the year below 500 m depth, with a peak from October to December. The resulting eggs and nauplii floated/migrated upward, and formed an abundance peak of Copepodite Stage 1 (C1) in the surface layer in February. In the surface layer, the C1 developed and reached C5 by early June through a phytoplankton bloom which occurred in mid-March to end of June. The C5 migrated to deeper layers in July and August, where they molted to adults. Apparently, the developmental time from C5 to adults was highly variable (>1 month), and some might overwinter. The life cycle of N. cristatus appeared to be annual for the major portion of the population. Taking into account sampling season, temporal changes in vertical distribution and population structure data collected from regions other than Site H, there was a close correlation in the timing of the life cycle over the entire subarctic Pacific, but the reproduction season (April to June) was observed to be different in the Okhotsk and Japan Sea populations. Regional comparison of prosome length of C5 individuals, including those in the Bering Sea, indicated significantly larger sizes of specimens from the Japan Sea and Okhotsk Sea, as compared with those from the entire subarctic Pacific. Possible causes for regional variability in life cycle patterns and body sizes are discussed. Received: 18 December 1998 / Accepted: 19 April 1999     

Zooplankton growth rates: the influence of female size and resources on egg production of tropical marine copepods

Egg production was measured in 17 species of copepods from the genera Acartia, Calanopia, Centropages, Clausocalanus, Corycaeus, Eucheata, Euterpina, Oithona, Oncaea, Paracalanus, Parvocalanus, Temora and Undinula in Jamaican waters. At the high local temperatures (∼28 °C), mean egg production ranged from 3.2 to 88 eggs female^–1 d^–1, and instantaneous female growth (g, as egg production) ranged from 0.04 to 0.87 d^–1. Female growth was positively related to ambient chlorophyll concentration (r ² = 0.44) and negatively to female body size (r ² = 0.29). Together these two variables explained 60% of the variation in growth. When quadratic terms for chlorophyll and a term for interaction of body size and chlorophyll were introduced, 82% of the variance in growth rate was explained. Egg production rates represent an extension of the resource and size-dependent relationship established for copepodites. In smaller species (<3.5 μg), egg production was comparable to prior copepodite somatic growth; in larger species (>3.5 μg), egg production is compromised at lower resource concentrations than copepodite somatic growth. Thus, it appears that egg production in tropical copepods may be frequently limited by resources in a size-dependent manner. Under conditions where growth is resource limited, we caution against the application of egg production rates for the calculation of total copepod production. Received: 30 May 1997 / Accepted: 13 May 1998     

Effects of contrasting modes of larval development on the genetic structures of populations of three species of prosobranch gastropods

 In south-eastern Australia, the prosobranch gastropods Morula marginalba (Blainville), Cominella lineolata (Lamarck) and Bedeva hanleyi (Angas) have similar fine-scale distributions, but appear to possess very different dispersal capabilities due to contrasting modes of larval development. M.marginalba produce planktonic larvae, whereas C. lineolata and B. hanleyi undergo direct development in benthic egg capsules and emerge as crawling juveniles. To test for possible effects of contrasting life histories on levels of genetic variation within and among populations, a survey was conducted of allozyme variation at six polymorphic loci in 8 to 9 local populations of each species. Collections of snails were made between June 1992 and November 1993. Sampling ranges spanned between 162 and 180 km of coast. Regardless of larval type, proportions of single-locus genotypes in each collection were consistent with the recruitment of offspring which had been generated through random mating. However, genotypic diversity was lower in those species that undergo direct development. Loci surveyed in C. lineolata and B. hanleyi were polymorphic (i.e. frequency of most common allele <95%) in fewer populations than those examined for M.␣marginalba (P <0.001) and, where polymorphisms occurred, also possessed significantly fewer alleles (P <0.001). Consequently, average levels of expected heterozygosity were greater in populations of M. marginalba than in those of either of the other species (P <0.001). Genetic variation among populations, expressed as the standardised variance in allele frequencies (F _ST ), was inversely related to expected larval dispersal capability. The nine collections of M. marginalba showed little overall differentiation (F _ST  = 0.017; P <0.001), reflecting the ability of planktonic larvae to interconnect local populations, and so limit divergence due to drift and natural selection. In contrast, there were high levels of allelic heterogeneity among the nine collections of C. lineolata (F _ST  = 0.523; P <0.001) and eight collections of B. hanleyi (F _ST  = 0.140; P <0.001). These data imply that for species which undergo direct development, local populations are effectively closed and evolve largely independent of one another. Received: 3 May 1996 / Accepted: 12 July 1996