Dynamics of residency and egress in selected estuarine fishes: evidence from acoustic telemetry

In an attempt to determine the extent and periodicity of several large juvenile and adult fishes (smooth dogfish Mustelus canis, black drum Pogonias cromis, hickory shad Alosa mediocris) in a temperate estuary, we tracked acoustically tagged individuals with passive and active telemetry techniques during the summer, fall and winter. This approach confirmed summer residency for all of these species and species-specific and individual patterns of egress ranging from summer through fall. Other species (striped bass Morone saxatilis, summer flounder Paralichthys dentatus, bluefish Pomatomus saltatrix) that were previously tracked in the same estuary with the same techniques were reanalyzed for the same characteristics. There were species-specific and individual differences in residency and egress of these species as well, but all species left the estuary by the end of December. Together, these observations confirm the importance of temperate estuaries in the summer and fall, but not during the winter for the juveniles and adults of these migratory species. However, the duration of residency and timing of egress may vary if warming of estuarine waters continues.     

Top-down impact by copepods on ciliate numbers and persistence depends on copepod and ciliate species composition

Experiments with the copepods Acartia clausi, Centropages hamatusand Pseudocalanus sp. were performed to assess the species-specificeffect of these copepods on the development of monospecificalgae (Nephroselmis pyriformis) and ciliate communities (Strombidiumvestitum, Strombidium conicum, Strombidium sp. and Lohmanniellaoviformis). It was hypothesized that potentially switching copepodslike A. clausi will stabilize the algal community by switchingbetween ciliate and algal food, in contrast to copepods withstereotypic filter feeding behaviour (Pseudocalanus sp.). Intreatments with Pseudocalanus sp. and C. hamatus, all ciliatespecies were wiped out in 2 days, resulting in blooms of N.pyriformis. In treatments with A. clausi, two of the ciliatespecies were able to persist, but the combined ciliate and copepodcommunity was not able to control the algal bloom. Ciliatesbecame abundant in control treatments without copepods, butonly S. vestitum and S. conicum seemed able to establish grazingcontrol. Hence, when evaluating the role of ciliates in foodwebs, their actual numbers and species composition should betaken into account. Likewise, the species composition of copepodsmay be crucial; these experiments demonstrate that small filterfeeding copepods may have tremendous impact on ciliate numbers.     

Weak 'top-down' control of dinoflagellate growth in the coastal Skagerrak

Effects of planktivorous fish (Sprattus sprartus), copepods(mainly Oithona sp.) and benthic filter feeders (Mytilus edulis)on the biomass and species composition of a natural marine planktoncommunity, dominated by dinoflagellates, was studied experimentallyin September 1988. Plastic 300 I cylinders were filled withSkagerrak surface water, filtered through a 90 µm nylonnet in order to remove mesozooplankton. Copepods were addedin three different concentrations (0, 1x and 6x the ambientdensity) with and without nutrient additions (nitrate, silicateand phosphate). In two nutrient-enriched cylinders with addedcopepods (1x and 6x), 5 cm sprats were also added (10 per cylinder).In two nutrient-enriched cylinders without added copepods. bluemussels (20 per cylinder. 3 cm length) were introduced. Chlorophylla. inorganic nutrients and densities of phytoplankton, ciliatesand copepods were measured throughout the experimental periodof 10 days. Gyrodinium aureolum increased in cell numbers inall but the mussel cylinders, while the other dominating dinoflagellate,Ceratium furca, decreased. This decrease was not due to grazing,but probably a container artefact. Diatoms, Rhizosolenia fragilissinia,Leptocylindrus danicus and Chaetoceros spp., increased slowlyin treatments without fish. In the fish treatments, diatomshad appreciably higher growth rates and cell numbers, probablydue to predation by fish on the copepods. Gyrodiniun aureolunalso had a somewhat higher growth rate in the presence of fish.although this was probably not due to reduction of copepod grazing.The results show that planktivorous fish can affect the biomassand species composition of natural phytoplankton communitiesin marine waters, but that the effect on the late summer-autumndinoflagellate dominated community is weak. Blue mussels areefficient grazers of the whole phytoplankton community and arenot particularly selective with respect to either size or species.     

Comparison of routine oxygen consumption rates of three species of pleuronectids at three temperatures

The oxygen consumption rates of three species of pleuronectids, the yellowtail flounder, Pleuronectes ferrugineus (Storer), the winter flounder, Pleuronectes americanus (Walbaum), and the American plaice, Hippoglossoides platessoides (Fabricius), were examined under simulated, land-based, aquaculture conditions. Routine oxygen consumption (ROC) rates for groups of each species were measured simultaneously using single-pass, flow-through respirometry. This study was conducted over three seasons at temperatures from 2°C to 14°C. An analysis of variance identified a significant interaction between temperature and species on the oxygen consumption rates of these flounder. The analysis indicated that at each temperature, ROC rates were significantly different among the three species (P < 0.05). A subsequent test of each species'ROC rate across the three temperatures indicated that both the yellowtail flounder and the winter flounder had significantly different ROC rates at each temperature experiment (P < 0.05). The ROC of yellowtail and winter flounder responded similarly to changes in experimental temperatures.     

Phase transitions of phospholipids as a criterion for assessing the capacity for thermal adaptation in fish

This study examines the crystal-liquid crystal phase transitions of the major phospholipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE), from muscle tissue of marine fish living at temperatures of 0–4.1°C (the Pacific cod Gadus macrocephalus, banded Irish lord Hemilepidotus gilberti, Pacific halibut Hippoglossus hippoglossus, black edged sculpin Gymnocanthus herzensteini, dark colored flounder Pleuronectes obscurus, and plain sculpin Myoxocephalus jaok), as well as of fish living at 14 and 18°C (Pacific redfin Tribolodon brandti). The PC and PE phase-transition thermograms of all the investigated species displayed specific profiles. The largest share of the thermogram area at temperatures higher than those of the habitat was found for the PC (28–40%) and PE (47–82%) of the black-edged sculpin, dark-colored flounder, and the plain sculpin, which have reduced physiological activity at low temperatures. In the Pacific cod, banded Irish lord, and the Pacific redfin, this parameter was much lower, 0–18% (PC) and 0–39% (PE). The thermotropic behavior PC and PE was symbate in all fish, except for the cod and the plain sculpin. The transition enthalpy of PC in all the investigated species was 2.8 times higher than that of PE. To interpret the varied PC and PE thermogram profiles of fish with similar fatty-acid compositions, the data on the composition of the molecular species of these phospholipids appeared to be the most informative. This study suggests that each fish species has its own strategy for thermal adaptation, which is realized through a certain set of phospholipid molecular species.     

Novel microsatellites from the European plaice (Pleuronectes platessa)––identification by data mining and cross-species amplification in other flatfishes

The European plaice (Pleuronectes platessa; Pleuronectidae) is considered the most important flatfish for fisheries in Europe. Overall catches have been reduced drastically throughout the past decades, indicating a severe decline in the natural populations. However, a limited set of genetic tools are available to improve stock assessment and management for the species. Sequences of European plaice and its congener, the winter flounder (Pleuronectes americanus) deposited in the public database were searched for short tandem repeats. Out of 22 loci derived from database sequences, eight were amplified successfully and genotyping of 23 wild-caught individuals showed expected heterozygosities ranging from 0.18 to 0.84. The average allele number was 4.75 per locus (range 2–11). Successful cross-species amplification of the markers was demonstrated from two related flatfishes. All eight markers were successfully amplified from the European flounder (Platichthys flesus), a phylogenetically closely related species, whereas three of them worked in Achiroides melanorhynchus, an endemic species of Southeast Asia for which no microsatellites have been described to date.     

Tidally driven distribution of phytoplankton blooms in a shallow, macrotidal estuary

The influence of the ebb tide on the abundance and distributionof bloom-forming species, as well as the mechanisms for theselection of those species which remain, were investigated inthe shallow, tidal-flushed Urdaibai estuary, north Spain. Phytoplanktonwas collected monthly from May to September 1998 during differenttidal conditions (neap-spring) at five stations along the salinitygradient of the estuary. During the neap tides of May, Leptocylindrusdanicus dominated in the lower estuary, together with Thalassiosiraguillardii and Peridinium foliaceum in the upper segment; T.guillardiiexperienced the broadest displacement along the estuary. Duringthe June and July cruises, coinciding with mid-tidal amplitudesand high temperatures, Peridinium quinquecorne densely aggregatedin the upper estuary at the slack high tide, whereas Chaetocerossalsugineum bloomed at the intermediate stations. Cyclotellaatomus and Protoperidinium achromaticum reached high concentrationsin the upper zone only during the late stages of the ebb. Duringthe spring tides of September, Prorocentrum minimum, Heterocapsapygmaea and Heterocapsa rotundata appeared in the lower marineestuary, being washed out at low tide. By contrast, the diatomsSkeletonema costatum, Thalassiosira guillardii and Cyclotellaatomus were the most abundant species in the upper reaches,peaking during the ebb. Physical trapping and high water residencetimes served to retain blooming species in the upper estuary.The intense growth of the estuarine diatoms may compensate forthe advective seaward losses of cells during the ebb, thus allowingthe development of stable populations in the estuary. Only Peridiniumquinquecorne seems to combine an endogenous tidal rhythm witha photic response to remain in the tidal area of the estuary.     

A 91-year record of seasonal and interannual variability of diatoms from laminated sediments in Saanich Inlet, British Columbia

Diatom seasonal succession and interannual variability werestudied using laminated sediments from Saanich Inlet, BritishColumbia, for the years 1900–1991. Frozen sediment coresallowed fine-scale sampling of laminae for each year. Thus,three ‘seasons’ for each year were identified basedon species composition. Thalassiosira species were indicatorsof spring deposition. Skeletonema costatum was abundant in samplesfollowing Thalassiosira, probably deposited in late spring andsummer. Rhizosolenia sp. was most abundant in fall/winter samples.Diatom stratigraphies were related to sea surface temperature,salinity, sea level and the Pacific North American Index (PNA)using canonical correspondence analysis (CCA). CCA showed thatspecies of a particular season generally had optima for temperatureand salinity characteristic of that time. Interannual changesin diatom species composition and abundance were most prevalentin the decades 1920–1940, with the exception of S.costatumwhich showed cyclic changes in abundance. Skeletonema was moreabundant during periods of cool temperatures, while littoraldiatoms were more abundant during times of heavy winter rains.Sea level was an important variable in CCA and while its relationshipto diatoms is not clear, it may be related to variations innutrient supply to diatoms in surface waters.     

Behavioral Adaptations of Terrestrial Isopods

In their response to environmental stimuli, terrestrial isopodsshow various trends that are correlated with their ecology andphysiology. With the transition from sea to the littoral zone (Ligia), orientationto light changes from positive to negative. Yet, since theseisopods are positively hygrokinetic even at very high humidities,their hygroreaction is of greater significance than their photoreaction. In isopods from mesic habitats (Oniscus, Porcellio, Armadillidium)photoreaction becomes of less significance than in littoralspecies, until in some species it reverts to positive (Armadillidium).Sometimes the positive photoreaction occurs only at high temperatures(Porcellio), a pattern of behavior correlated with thermoregulationby evaporative cooling. Similarly, in mesic species the responseto humidity becomes less significant than in littoral species,resuming importance mainly when the isopods become dehydrated(Armadillidium). Finally, in isopods from xeric habitats in semi-arid and desertregions (Armadillo, Venezillo), photoreaction is strongly negativeexcept in Hemilepistus at lower temperatures. All of these isopodsare positively hygrokinetic only at low humidities and are stronglynegatively thermoactive, indicating a drop in activity at hightemperatures. The photonegative response in these isopods isstronger than the hygroreaction.     

TWO NEW SPECIES OF THE GENUS ISLAMIA RADOMAN, 1973 FROM ITALIAN ISLANDS (PROSOBRANCHIA, HYDROBIIDAE)

Two new Italian species of the genus Islamia (Prosobranchia:Hydrobiidae), one living in eastern Sicily (I. cianensis), andone on Elba Island (Tuscan Archipelago, Italy) (I. gaiteri)are described. The two species are distinguished on the basisof shell and anatomical characters, mainly those of the malegenitalia. I. cianesis n. sp. is characterized by a valvatoidshell and a penial lobe with internal band of glandular tissuenot distinct in its lower portion from the penis body but bulgingapically as a small knob. I. gaiteri n. sp. is characterizedby a planispiral shell and a small lateral penial lobe withoutinternal glandular tissue. (Received 20 December 1993; accepted 27 June 1994)     

Estuarine colonization, population structure and nursery functioning for 0-group sea bass (Dicentrarchus labrax), flounder (Platichthys flesus) and sole (Solea solea) in a mesotidal temperate estuary

The function of the Mondego estuary as a fish nursery habitat was investigated from June 2003 to June 2004 by comparing the timing of estuarine colonization with juveniles of sea bass Dicentrarchus labrax, flounder Platichthys flesus, and sole Solea solea, while also analysing their population structure, growth and diet composition. Differences in the onset of estuarine colonization were observed, since sole juveniles were the first to enter the estuary (in January), followed by flounder in April and sea bass in June. The estuarine population of these species consisted of several age‐groups, although the majority of individuals belonged to age‐groups 0 and 1. The growth rates determined for 0‐group fish were within the range of those reported for other European estuarine systems. Some differences were also recognized regarding the timing of estuarine colonization and the length of the growing season. Diet of 0‐group sea bass consisted mainly of Crustacea, Polychaeta and Mollusca. Flounder juveniles fed chiefly on Amphipoda (especially Corophium spp.), with Polychaeta, Isopoda and Decapoda also being common prey. The diet of 0‐group sole was dominated by Polychaeta, with Amphipoda, Mollusca and Decapoda ranking highest, with other important benthic organisms also being present. Dietary overlap among these species was relatively low.     

Composition, temporal changes and ecological guild classification of the ichthyofaunas of large European estuaries – a comparison between the Tagus (Portugal) and the Elbe (Germany)

Fish species densities from 1025 hauls obtained with beam trawl and stow net catches from the southern Tagus and northern Elbe estuaries were analysed for two different periods of time between 1978 and 1997. The data were related with environmental factors to compare species composition and densities, contributions of different ecological guilds, structuring environmental key factors, intra‐annual cyclicity and interperiodical stability of ichthyofaunas in large European estuaries at different latitudes. Although the total number of species in both the Tagus and Elbe estuaries (Tagus: 57 species, Elbe: 58 species) was very similar, significant differences were estimated in species compositions, species densities and number of species, and individuals of the different ecological guilds between both estuaries. A total of 15 species occurred in both estuaries but only two of these species, the catadromous Anguilla anguilla and the marine estuarine opportunist Pleuronectes flesus, contributed substantially to the total abundance. Marine species together contributed more than 5% of the total number of species in the Tagus and Elbe estuaries, whereas their contribution in number of individuals amounted to 22% in the Tagus and only 9% in the Elbe. In terms of number of individuals, the ichthyofauna of the Tagus estuary was dominated by estuarine species (especially Pomatoschistus spp.) contributing 68% of the total abundance. In contrast, the fish fauna of the Elbe estuary was dominated by high numbers of anadromous species (88%) individuals, especially Osmerus eperlanus. Canonical corresponding analyses (CCA) indicated that, besides intra‐annual variables, salinity and mouth distance were two of the most important environmental factors structuring the fish communities in both estuaries. Moreover, water depth was found to be an important environmental factor in the Tagus estuary, whereas the water temperature substantially affected the ichthyofaunal composition in the Elbe estuary. In both estuaries, contributions by number of species and number of individuals of the different ecological guilds were not statistically different between the two periods considered, 1978–80 and 1995–97 for the Tagus, and 1981–86 and 1989–95 for the Elbe. Nevertheless, from 1981–86 until 1989–95 a remarkable increase of about 4.5 times in the densities of O. eperlanus was observed in the Elbe estuary, probably related to improved water quality, especially of oxygen conditions in the nurseries since 1990. This was also supported by the results of CCA, suggesting important influences of the years 1989 and 1990 on the Elbe estuary fish assemblage. Climate change as a result of global warming may be evident for the observed changes of the densities of P. flesus which decreased in the southern Tagus estuary and, in contrast, increased in the northern Elbe estuary during comparable periods of time. It is recommended to use the ecological guild classification scheme which was applied for the Tagus and Elbe estuaries for further studies of European estuarine fish assemblages.     

Maternal Influences on Variation in Egg Sizes in Temperate Marine Fishes

SYNOPSIS. We examine the variation in egg sizes of marine teleostsand evaluate the maternal contribution to this variability.At the species level, egg sizes in 309 North Atlantic fishesrange from 0.3 to 18.0 mm diameter (median = 1.1), size at hatchingvaries directly with egg size, and large adult size is associatedwith large eggs but the relationship is weak. Within populations,egg sizes are distributed normally with a median coefficientof variation of 4% (n = 56 species). Egg size varied among femalesin all cases for which female-level data were found. Estimatesof the variance components of egg size due to females were foundfor three species and, as a percentage of total variance, are71 % for capelin, Mallotus villosus, 46% for winter flounder,Pleuronectes americanus, and 35% for Atlantic cod, Gadus morhua.For cod, which spawn multiple egg batches per year, an additionalbatches-within-females component was estimated to be 26%. Sizeat hatching also differs among sibgroups and is generally directlyrelated to egg size at the individual level. We modelled fishgrowth by allowing individuals to grow at exponential ratesfrom a normal distribution of initial sizes. Comparing sizevariation in model fish to empirical evidence suggests thatvariation in initial sizes, propagated by growth, could accountfor a large fraction of the size variation observed months afterhatching in natural populations. We view size variation in youngmarine fishes to be largely of maternal origin and environmentallymodulated, which if true has special consequences for fisheriesand aquaculture.     

Feeding ecology of juvenile turbot Scophthalmus maximus and flounder Pleuronectes flesus at Gotland, Central Baltic Sea

Food and feeding of juvenile turbot Scophthalmus maximus and flounder Pleuronectes flesus were studied in five nursery areas at Gotland, Central Baltic Sea, ICES SD 27 and SD 28. Ontogeny involved partitioning of available food resources. The food choice of turbot <30 mm standard length (L_S) included both planktonic‐hyperbenthic prey (calanoid copepods and mysids) and epibenthic–endobenthic prey (chironomids and amphipods), whereas turbot ≥30 mm L_S fed mainly on hyperbenthic species (mysids and fishes). Conversely, for flounder, epibenthic–endobenthic prey were the most abundant prey items throughout development (harpactocoid copepods, oligochaetes and chironomids for fish <40 mm L_S and oligochaetes, chironomids and amphipods for flounder ≥40 mm L_S). Thus, the highest degree of dietary overlap occurred between turbot <30 mm and flounder ≥40 mm. Food composition for both turbot and flounder varied, however, according to exposure and predominant wind direction in the nursery area. For example, expressed as the ratio between the biomass of mysids and fishes consumed, the relative importance of mysids v. fishes as food source for turbot, varied from <1 in the most sheltered area to 16 and 27 in the more open areas. Considerable differences in feeding incidence were recorded; mean ±s .d . 58 ± 20% for turbot <30 mm L_S and 83 ± 8% for turbot ≥30 mm L_S, as opposed to ≥85–90% for flounder irrespective of size. The lower feeding success of turbot <30 mm L_S was related to mysid abundance, shown to vary spatially and temporally, and to density of flounder, indicating that food availability, and potentially interspecific competition, influence feeding of early juvenile turbot with implications for survival following settlement. Regarding variability in abundance, hyperbenthic prey, as mysids, are considered more variable than epi‐ and endobenthic organisms. Hence, in addition to the ‘nursery size hypothesis’, i.e. the positive relationship between abundance of recruits and extension of nursery areas, variability in food availability may explain the average lower recruitment of turbot as compared to other flatfishes, e.g. flounder.     

Zooplankton and ichthyoplankton communities in a temperate estuary: spatial and temporal patterns

Zooplankton and ichthyoplankton assemblages were studied fromJanuary 2003 to June 2004 in a temperate shallow estuary (Mondegoestuary, Portugal). Monthly sampling was performed at five stationsat high and low tides, with subsurface tows with 335 and 500µm mesh Bongo nets. Analysis of variance (ANOVA) showeda significant effect (P < 0.05) of the mesh size of the neton the abundance of main zooplankton groups. On average, theabundance of the 500 µm taxocenosis was 67 and 102 timeslower than the 335 µm taxocenosis at high and low tidesrespectively, especially in the south arm. More than 80 specieswere identified in the zooplankton samples. The upper reachesof northern arm are dominated by freshwater crustacean mesozooplanktonlike Acanthocyclops robustus and Diaptomus spp. and the cladoceransDaphnia, Ceriodaphnia and Bosmina, often being codominant. Inthe southern arm, the resident estuarine copepod Acartia tonsawas dominant, eventually being the most abundant taxon. Marinereaches of estuary are usually dominated by the marine calanoidcopepods Acartia clausi and Temora longicornis and the siphonophoresMuggiaea atlantica. Concerning the ichthyoplankton, this wasdominated by the larvae of estuarine resident species, mainlyPomatoschistus sp., and eggs of Engraulis encrasicolus. Theabundance of Pomatoschistus sp. larvae was positively correlatedwith water temperature. Statistical analysis (canonical correspondenceanalysis) used to determine the spatiotemporal structure ofthe zooplankton assemblages and its correlation with environmentalvariables showed that salinity and temperature were the mainfactors influencing the distribution of zooplankton. The resultsobtained also showed that abundance was strongly influencedby the hydrological circulation pattern and direct or indirecthuman impacts that occur in each arm of the estuary. This article was presented at Plankton Symposium III, held atFiguera da Foz, portugal, between 17 and 20 March 2005, underthe auspices of the University of Coimbra and the Universityof Aveiro, and coordinated by Mário Jorge Pereira andUlisses M. Azeiteiro.     

Prey selection by flounder, Platichthys flesus, in the Douro estuary, Portugal

Prey selection by the flounder, Platichthys flesus (Linnaeus, 1758), in an estuarine nursery was investigated and the major factors influencing food choice by this species were assessed. Diet breadth was narrow, reflecting the low prey diversity observed in the benthos. A gradual ontogenetic shift from small prey such as amphipods to larger prey like polychaetes and bivalves was observed. Amphipods had positive electivity values in the upper estuary and negative values in the lower estuary.Polychaetes showed the inverse pattern. Bivalve electivity values were always positive. Differential selectivity throughout the estuary was mainly related to spatial segregation of flounders according to size, with the smaller individuals concentrating in the upper estuary and larger individuals concentrating in the lower estuary. Amphipods such as Corophium spp. play a crucial role in the flounder diet because of their small size, low mobility and diel activity pattern. As prey, the polychaete value increases throughout flounder ontogeny since the flounder size range is compatible with the larger mouth gapes and detection ability of larger fish. Bivalve electivity values for flounder are mainly related to high calorific values. The absence of Crangon crangon (Linnaeus, 1758) in the diet may be due to low water temperature since the cost–benefit involved in the capture of highly mobile prey is too high at low temperatures. It was concluded that flounder must use several sensory features to detect and capture prey in turbid estuarine waters and that field studies provide important background information on the actual predator preferences under natural conditions.     

Alimentary tract helminths of four pleuronectid flatfish in relation to host phylogeny and ecology

Eighteen species of helminths were analysed in relation to host–parasite specificity and the effect of host ecological preferences on the establishment of the parasite fauna in the alimentary tract of four pleuronectid flatfish, flounder Pleuronectes flesus , witch flounder Glyptocephalus cynoglossus , American plaice Hippoglossoides platessoides , and Atlantic halibut Hippoglossus hippoglossus in northern Norway. Thirteen species were generalists and the diversity of the parasite faunas decreased with increasing depths inhabited by the host. Similarities were greatest between the parasite faunas of flounder and American plaice and least between flounder and witch flounder. Host ecology, rather than phylogenetic relationships of these hosts, mainly influences the composition and diversity of the parasite communities of flatfishes in northern Norway.     

Smelling Danger – Alarm Cue Responses in the Polychaete Nereis (Hediste) diversicolor (Müller, 1776) to Potential Fish Predation

The harbour ragworm, Nereis (Hediste) diversicolor is a common intertidal marine polychaete that lives in burrows from which it has to partially emerge in order to forage. In doing so, it is exposed to a variety of predators. One way in which predation risk can be minimised is through chemical detection from within the relative safety of the burrows. Using CCTV and motion capture software, we show that H. diversicolor is able to detect chemical cues associated with the presence of juvenile flounder (Platichthys flesus). Number of emergences, emergence duration and distance from burrow entrance are all significantly reduced during exposure to flounder conditioned seawater and flounder mucous spiked seawater above a threshold with no evidence of behavioural habituation. Mucous from bottom-dwelling juvenile plaice (Pleuronectes platessa) and pelagic adult herring (Clupea harengus) elicit similar responses, suggesting that the behavioural reactions are species independent. The data implies that H. diversicolor must have well developed chemosensory mechanisms for predator detection and is consequently able to effectively minimize risk.     

Descriptions of five new species of Haplostylus (Mysidacea-Crustacea) from South West Australia

Five new species, Haplostylus flagelliforma sp. nov., H.similissp. nov., H.multispinosus sp. nov., H.uthupus sp. nov. and H.disparsp. nov., are described from the south-west coast of Australia.These new species are distinguishable from each other and fromthe other allied species by the structure of the pleopods ofthe male, and by the armature of the telson and the uropods.     

Autotrophic and heterotrophic nanoplankton in the diet of the estuarine copepods Eurytemora affinis and Acartia bifilosa

The ingestion of autotrophic and heterotrophic nanoplanktonby two estuarine copepods, Eurytenora affinis and Acarith bifilosa,was measured in various environmental conditions using the incubationmethod and epifluorescence microscopy. Egg production of thespecies was also deter mined in order to estimate their carbonrequirements. Assuming a gross efficiency of egg productionof 0.3, nanoplanktonic carbon ingested always met the carbonrequirements suggesting that, most of the time, other carbonsources could be unnecessaly. Nanoplankton ingestion by A.bifilosa(from 128 to 1693 cells ind.^–1 h^–1) was dominatedby autotrophic forms (60–97%) and was seriously affectedby high (>100 mg l^–1 suspended particulate matter (SPM)concentrations. Nanoplankton ingestion by E.affinis (from 300to 1049 cells ind^–1 h^–1) was relatively stable incomparison, but this latter species seemed to switch its grazingpressure from autotrophic to heterotrophic forms when SPM concentrationsincreased. Thus, two copepod species, living in the same estuary,presented two different feeding behaviours, probably to maximizeenergy input per unit of energy expenditure. Such differencescould contribute to the spatial and seasonal segregation ofthese species which is usually observed.