Evolutionary genetics of marine Mysis spp. (Crustacea: Mysidacea)

Inter-and intraspecific allozyme differentiation in the mysid crustacean genus Mysis in the North Atlantic region was studied in order to evaluate earlier concepts of evolutionary and systematic relationships and to assess patterns of subdivision within widespread taxa. The results support a relatively ancient divergence of the marine and non-marine species of the genus, and are generally in line with the current subgeneric tridivision into Mysis s.str., Michteimysis and Auricomysis. However, the North American littoral species M. gaspensis should be returned to subgenus Mysis s.str. from its present position in Michteimysis with M. mixta. The closest observed affinities within Mysis s.str. were between M. gaspensis and the freshwater M. relicta group, and between M. oculata and M. litoralis. Intraspecific differentiation among North European coastal populations of M. oculata and M. litoralis was moderately strong (F _ST0.1), suggesting population bottlenecks and limited dispersal in the post-glacial time. On the other hand, ransoceanic differences were not essentially greater, indicating the systematic homogeneity and long-term dispersal capacity in the marine species. This contrasts with the strong genetic and systematic fragmentation earlier found within the circumboreal M. relicta species group.     

Food utilisation of pelagic mysids, Mysis mixta and M. relicta, during their growing season in the northern Baltic Sea

The dietary habits of the pelagic mysid Mysismixta were studied during its growing season at an open sea location in the Gulf of Finland, the northern Baltic Sea. Stomach samples were taken twice a month from June to September 1997. The most abundant phytoplankton taxa in the stomachs were diatoms and dinoflagellates, and copepods and cladocerans were the most abundant zooplankton identified. A clear change was found in the diets during the study period. Small mysids (3 to 6 mm) fed on sedimented phytoplankton in the early summer (90% benthic particles in June) but shifted gradually to a more pelagic and carnivorous diet (>40% pelagic particles, consisting of ca. 60% zooplankton in September). Seasonal changes in mysid capture ability as well as food availability were suggested to affect the diet composition of mysids during their growth. The ratio of pelagic and benthic food particles could – irrespective of the season – be explained by mysid size, whereas the zooplankton:phytoplankton ratio was better explained by season. The stomach analysis suggests that the mysids needed to attain a threshold size of 8 to 11 mm to initiate feeding on the more evasive copepods. Mysids also started to grow faster at the same time as the proportion of copepods increased in the diet, which suggests that copepods are an important energy source for M. mixta in late summer. Finally, a comparison was made between the M. mixta diet and that of the less abundant M. relicta. The diets of the two pelagic mysid species overlapped by 75% (Schoener's index). The main difference was due to M. mixta eating more zooplankton and pelagic material than M. relicta. Received: 15 September 1999 / Accepted: 18 January 2000     

Larval development in the Tasmanian mysids Anisomysis mixta australis, Tenagomysis tasmaniae and Paramesopodopsis rufa (Crustacea: Mysidacea)

We describe the brood duration and marsupial development of three temperate coastal mysid species, Anisomysis mixta australis (Zimmer), Tenagomysis tasmaniae Fenton and Paramesopodopsis rufa Fenton, found commonly in Tasmanian waters. Larvae cultured in vitro had brood durations at 13 °C (17 °C) of 22 (15), 23 (15), and 28 (20) d, respectively. Development through seven larval stages, and brood durations for these three species are similar to those reported for coastal mysids from other temperate areas throughout the world. Received: 10 June 1997 / Accepted: 1 July 1997     

Diel vertical migration of Sagitta setosa as inferred acoustically in the Black Sea

Swimming trajectories of chaetognaths Sagitta setosa Müller in the Black Sea were studied using an echosounder operating at 120 and 200 kHz and an acoustic Doppler current profiler (ADCP) operating at 150 kHz. S. setosa were acoustically discriminated with respect to vertical migration and swimming speed, according to dissolved oxygen (DO) concentration and the timing of migrations. S. setosa formed a concentration layer thicker than Calanus euxinus did (1–3 m). The migration was completed in about 2.5–4 h, upward migration starting before C. euxinus and downward migration after C. euxinus. Adult Sagitta swam fast only in the well-oxygenated layer (subsurface maximum DO). The DO was found to be a significant (p<0.05) variable by partial correlation between the speed and hydrographical parameters. This feature constituted an oxygen-dependent influence on S. setosa’s vertical swimming and distinguished S. setosa from C. euxinus. Chaetognaths migrated daily between the nearsurface and the oxycline or the suboxic zone (OMZ, see Fig. 3b for the layers characterized by DO). Whether the deepest depth limit of migration was the oxycline or the OMZ depended on the relative abundance of adult and immature (young) individuals in the concentration layer. In July and September, individuals belonging to a new generation did not migrate but stayed in subsurface water day and night.     

Diel vertical migration and feeding of copepods at an oceanic site near South Georgia

Twelve Longhurst Hardy Plankton Recorder (LHPR) profiles were taken over a 16 h period in January 1990, in order to study feeding of four copepod species at an Antarctic oceanic site near South Georgia. Vertical distributions of their life stages, as well as those of dominant competitors and predators, are described in relation to the feeding cycles of Calanoides acutus CV, Calanus simillimus CV, Calanus propinquus CV and Rhincalanus gigas CIII, CV and CVI. Comparisons with vertical ring-net catches, which were used for concomitant gutevacuation experiments, demonstrated the suitability of the LHPR for these fine-scale studies. Planktonic predators, with the exception of the diel migrant Themisto gaudichaudii, resided deeper than the herbivores. During the day and around midnight, when feeding rates were low, species and stages reached their maximum vertical separation. At these times, new generation copepodites of the four species lived progressively deeper and the overwintered generation (i.e., R. gigas Stages CIV, CV, CVI) were progressively shallower. During the afternoon or evening (depending on species), all stages older than CII, as well as Euphausia frigida and T. gaudichaudii, migrated upwards, to amass in the surface mixed layer. Feeding was restricted to darkness, although R. gigas commenced several hours before dusk. In detail their migration and feeding differed widely, with combinations of unimodal and apparent bimodal cycles. As a whole, the results suggest that (1) feeding could occur during sinking as well as during upward migrations, (2) upward migrations were not always associated with feeding increases, and (3) individuals appeared to descend after filling their guts.     

Group orientation in Neomysis mirabilis (Mysidacea: Crustacea)

The orientational behaviour of the neritic mysid Neomysis mirabilis in a swarm is considered under natural and experimental conditions. In nature, the swarms exhibit elements of inner integration, such as homogeneous age structure, collective behaviour, and relative constancy of inter-individual spacing. The mysids are guided by combined optomotor and vibrotactile cues, determining their disposition in aquaria of different shapes. Maximum accuracy of spatial arrangement is observed in round vessels. It is suggested that the stress encountered in an aquarium produces responses comparable to those elicited by the presence of a predator. The rigid spatial arrangement at the supra-individual level is interpreted as evolutionally-optimized prey strategy. These elements in the geometrical distribution of individuals in mysid swarms are inherent in other pelagic crustaceans also, and are not controlled by random taxes stimulated by hydrological factors.     

Growth and moulting of Neomysis integer (Crustacea: Mysidacea)

The growth and moulting of Neomysis integer (Leach) was investigated in the field and the laboratory. In the Ythan estuary, Aberdeenshire, Scotland, monthly samples taken from November 1976 to October 1978 revealed that the summer generation juveniles and mature individuals grew at a rate of 4 to 5 mm and 1 to 2 mm monthly, respectively. The winter generation had a growth rate of 3 to 4 mm monthly for juveniles and about 1 mm for mature individuals; during the winter there was a period of 3 mo when growth was almost completely stopped. Mysids reared in the laboratory on Artemia sp. nauplii had an average daily growth rate of 0.06 mm at 9°C and 0.09 mm at 16°C. The growth factors of N. integer ranged from 3 to 17% for mature and immature individuals, respectively. Intermoult periods ranged from 3 to 7 d in immature mysids to 12 to 18 d in mature mysids. Average laboratory growth curves calculated from information on growth factors and intermoult periods indicate that at 9°C (winter generation) it takes N. integer 277 d to grow to be a 15 mm mature individual, whereas at 16°C (summer generation) it takes 188 d. N. integer moults 24 times as it grows from a juvenile to a mature individual.     

Microhabitat segregation of the amphipod genus Gammarus (Crustacea: Amphipoda) in the Northern Baltic Sea

Closely related species may occupy very similar niches but are often found to diverge by one or more traits when they share the same habitat. Five indigenous and sympatric Gammarus species are characteristic for the Baltic rocky littoral ecosystem. Yet, the species-specific distribution of these sympatric gammarids has not been well studied in the northern Baltic Sea. This study was undertaken to assess the spatial distribution of gammarid amphipods along wave exposure and depth gradients to study whether they show segregation in their microhabitat use. We sampled 12 rocky sublittoral shores along a wave exposure gradient over a period of 5 years. Samples differed with respect to depth and macroalgal type. Three of the five gammarid species occurred mainly in different depth zones and among different macroalgae at the exposed shores. In contrast, on protected shores, where algal zonation is weaker, a link to zonation and macroalgal type was almost absent. Moreover, the microhabitat use was strongest during the reproductive seasons of the species. The observed microhabitats of the three gammarid species fit well to their species-specific mean body sizes.     

Diel variability of feeding activity in haddock (Melanogrammus aeglifinus) larvae in the East Shetland area, North Sea

Investigations of factors affecting feeding success in fish larvae require knowledge of the scales of variability of the feeding process itself and the indices used to assess this variability. In this study, we measured short-term (diel) variability in feeding rates of wild haddock (Melanogrammus aeglifinus) larvae four times per day during a 10-d cruise in the northern North Sea. Feeding activity was evaluated using indices of gut fullness, prey digestive state and biochemical measurements (tryptic enzyme activity). The gut fullness and the enzyme activity indices indicated moderate to high rates of food consumption throughout the cruise. Time series analysis of the three indices showed significant diel variability in all indices and enabled identification of significant lags between food uptake and peak digestive enzyme activity. The typical pattern of food consumption and digestion was characterized by maximal ingestion of prey early in the evening (19:00 hrs) and peak digestive enzyme activity at 01:00  hrs. The time scale over which enzyme activities reacted to prey ingestion was ca. 6 h, and is consistent with expectations from controlled laboratory experiments with other larval fish species. Significant diel variability in tryptic enzyme activity suggests that attempts to relate this measure of feeding success to other variables (e.g. food concentrations) should take care to accommodate natural cycles in feeding activity before making statistical comparisons. Received: 29 October 1998 / Accepted: 18 June 1999     

Diel patterns of planktonic bioluminescence in the northern Sargasso Sea

Day-night changes in the vertical distribution, intensity, and size of bioluminescence flashes were investigated during a series of cruises to the northern Sargasso Sea in 1987 and 1988. Overall, depth integrated bioluminescence potential and flash density estimated from in situ measurements with a pumping bathyphotometer were 2 to 5 x higher at midnight than at midday. Depths from 50 to 100 m exhibited the most substantial day to night increases in bioluminescence potential and flash density. When classified by flash size (photon output per flash event), the increase from day to night was significant for all flash sizes, but was most dramatic for small flashes producing <7 x 10⁸ photons flash^-1. Bioluminescence potential and flash density increased 2 to 3 x during bathyphotometer measurements made at dusk. Bioluminescent light budgets estimated from day and night net collections in May and August 1987 also predicted 2.5 x higher nighttime than daytime mesoplankton bioluminescence. However, large bioluminescent taxa (mesoplankton) capable of significant vertical migrations only contributed on the order of 15% of the total bioluminescence in surface waters. Our results do not support the idea that most of the nightly increase in bioluminescence potential and flash density are due to vertical migration of bioluminescent organisms; rather they are consistent with an alternate view that photoinhibition of bioluminescent flashing by dinoflagellates may be primarily responsible for the diel patterns.     

Distributional patterns of the eulitoral fauna near the isle of Hiddensee (Baltic Sea,Rugia)

In the eulitoral zone near the isle of Hiddensee (Baltic Sea) aggregated distribution is typical for the macrofauna. Only mussels are randomly distributed; they occur in low individual abundance. In contrast, Clark and Milne (1955) found random distribution to be the general rule for animals of the sublitoral zone.     

Temporal variability in functional responses and prey selectivity of the pelagic mysid, Mysis mixta, in natural prey assemblages

Predation rates and prey selection of the pelagic mysid shrimp, Mysis mixta, were studied experimentally in the northern Baltic Sea in 1998 during their most intensive growth period, from June to October. Functional responses during 5 months were determined by providing the mysids with a natural zooplankton assemblage, diluted to several different concentrations. The results show that ingestion rate increased, along with mysid growth, from early summer to autumn and that saturation level was reached between 400 and 500 μg C l⁻¹. Ingestion rates increased with increasing prey concentration, and sigmoidal curves explained mostly the variation in ingestion rates (explanatory levels of 86–97%). Prey selection was evident in June, July and August, though weaker during the latter 2 months. Selection differed between the studied months but, generally, copepods were more positively selected than cladocerans. Rotifers were the main prey during June and July, when mysids were small, while larger mysids fed on copepods and cladocerans. Of the copepods, Eurytemora affinis was a truly selected species. This study shows that mysids feed on many zooplankton taxa and that they undergo ontogenetic diet shifts. Received: 19 July 2000 / Accepted: 19 October 2000     

Diel vertical migration and timing of metamorphosis of larvae of the Dungeness crab Cancer magister

Joint USA/USSR ichthyoplankton surveys off the coasts of Washington, Oregon and northern California during the years 1981 to 1985 sampled more than 120 stations each year, from 5 to 360 km offshore and between Latitude 40° and 48° N, providing information on ontogeny and diel migration of larvae of the Dungeness crab Cancer magister on a scale not studied previously. We developed a maximum likelihood method for estimating abundance and fraction in the neuston at each station from a neuston tow and an oblique bongo tow. Latestage megalopae migrate vertically on a diel basis, with the fraction in the neuston being (on average) 62% at night (19.00 to 08.00 hrs Pacific Standard Time, PST) and 8% during the day (08.00 to 19.00 hrs PST). The hourly pattern of this migration includes a peak in the early evening, possibly another in the early morning, and an intermediate level in the late afternoon. We detected no dependence of vertical migration on cloud cover or sea state. Early-stage megalopae were present in much lower fractions in the neuston, but weakly displayed the same diel pattern of migration. Zoeae appeared to be below the neuston at all times, except for 2 or 3 h in the evening. From an abrupt change in larval stage in samples from a north-south cruise, we concluded that the majority of the larvae metamorphose from zoeae to megalopae over a fairly short time span (2 to 4 wk) at a given latitude. In later cruises, 95% of the larvae were megalopae, indicating that metamorphosis over the study area either occurs at the same time or proceeds from south to north over a time span of less than a month in early spring.     

Interrelationships of the two families constituting the Lophogastrida (Crustacea: Mysidacea) inferred from morphological and molecular data

The Lophogastrida are primitive Mysidacea and comprise only six genera. One of these, Eucopia is considered as highly specialized and constitutes the family Eucopiidae; the other genera constitute the Lophogastridae. Among the latter family, the genus Gnathophausia is closely related to Eucopia, with two species (G. gracilis and E. sculpticauda) sharing similar morphological characteristics [i.e. ornamentation (spines) of the uropods, and the gastric mill]. This indicates that these species are phylogenetically related. To test this hypothesis, the partial 16S mitochondrial ribosomal RNA gene from various representative species of Gnathophausia and Eucopia were compared. The resulting phylogenetic tree suggests that each genus is monophyletic, and that Gnathophausia, which is the deepest-branching genus, is the most primitive, with the Eucopiidae originating from the Lophogastridae. The molecular results support the morphological hypothesis, and suggest an early separation of the two genera or a rapid divergence of Eucopia due to morphological specialization. Received: 29 June 1997 / Accepted: 16 March 1998     

Oxygen consumption inNeomysis americana (Crustacea: Mysidacea), and the effects of naphthalene exposure

Oxygen uptake ( ) of individual mysids was measured in a novel continuous flow respirometer for 24-h periods, and in a sealed chamber respirometer for several hours. Mysids were acclimated 30–100d under conditions which allowed complete life-cycle cultivation. was normalized for mysid size using an unconventional linear regression: log ( ) vs log (body water). Compared to the conventional regression, log ( ) vs log (body mass), the new approach showed two advantages: significantly greater coefficient of determination,r ², and removal of differences in between sexes. Minimum rates of oxygen uptake measured in the continuous flow respirometer were lower than sealed chamber measurements, and may more accurately reflect standard metabolic rate. Hourly data also showed a daily cycle when adjusted to represent a standard size mysid. Exposure to ca 1.1 mg l^-1 naphthalene increased and the range of spontaneous shown by the mysids; exposure to ca 0.1 mgl^-1 naphthalene depressed the range of . Both exposures caused aberrations in the daily cycle.     

Locomotor activity patterns and feeding habits in the prawn Palaemon xiphias (Crustacea: Decapoda: Palaemonidae) in Alfacs Bay,Ebro Delta (northwest Mediterranean)

Diel activity rhythms of the prawn Palaemon xiphias Risso, 1818 from Alfacs Bay, Ebro Delta, were studied by time-lapse videorecordings. Activity displayed an endogenous circadian rhythm, with maximum activity at night. Feeding habits were studied by frequency of occurrence and by the points method. The food of P. xiphias consisted mainly of crustaceans; the remains of amphipods, isopods, mysids, copepods, decapods and ostracods were identified. The remaining items consisted of molluscs, polychaetes, ophiuroids, plant material, sand, and unidentified organic debris. The results indicate that P. xiphias is a predator of benthic invertebrates rather than a scavenger or detritus feeder. Diet composition changes with increasing size of the prawn.     

Vertical distribution and nocturnal migration of Nyctiphanes couchi (Crustacea: Euphausiacea) in relation to the summer thermocline in the Celtic Sea

Vertical distributions and nocturnal migrations of the developmental stages of Nyctiphanes couchi (Bell) in relation to the summer thermocline in the Celtic Sea, 25 to 26 August 1982, have been investigated using the Longhurst-Hardy Plankton Recorder (LHPR). The vertical distributions of the metanauplii and adult females suggest that N. couchi liberates its young within the euphotic zone as mature metanauplii which, in a matter of hours, moult into the first feeding stage (Calyptopis I). The ascent migration by adult females took a maximum of 3 h (17.10 to 20.05 hrs) and had an amplitude of 50m (54 to 4 m) from below to above the thermocline. A 7C° thermocline occurred between 20 to 30 m in these profiles. The nocturnal migrations by the females were for the purpose of breeding as well as feeding within the euphotic zone and were not influenced by the presence of the thermocline. The majority of the calyptopes and furciliae remained above the thermocline over the sampling period. The post-larval males and females migrated; their vertical distributions showed a pattern similar to those of the adult females. The larger the developmental stage, the deeper was the mode of its vertical distribution. The zooplankton dry weight in the profiles ranged from 3.74 to 6.91 g per haul (=1.85 to 3.45 g C m^-2, 0 to 100 m). The euphausiids represented 35% of total zooplankton dry weight and their migrations removed a large percentage of the total zooplankton biomass from the euphotic zone for 18 h d^-1. Such a large displacement of biomass would have a major impact on the biological interactions within the ecosystem.     

Diel vertical migrations and nocturnal feeding of a dense coastal krill scattering layer (Thysanoessa raschi and Meganyctiphanes norvegica) in stratified surface waters

To study the nocturnal feeding of euphausiids during vertical migrations and its impact on the phytoplankton, a phytoplankton-rich water mass (drogue marked) drifting over a dense krill scattering layer (acoustic 104kHz) in the lower St. Lawrence estuary was monitored over 46 h in July 1982. Phytoplankton >20 m was abundant and mostly concentrated at the bottom of the photic layer above the pycnocline. Less than 42% of the particulate carbon was due to phytoplankton. The krill scattering layer was about 2 to 3 km in width, elongated along the 100-m bathymetric contour, and absent when the bottom was shallower than 50 m. Its upper day depth was 50 m. At deeper depths, its vertical distribution frequently changed from unimodal to polymodal shapes and viceversa, often with large concentrations of zooplankton just above the bottom. Typical vertical migrations were observed on both days. At night the scattering layer had a lower scattering strength. Most of it was below the thermocline but net catches showed that large concentrations of euphausiids (up to 57 individuals m^-3) crossed it. Stomach pigment content of Thysanoessa raschi was generally low, but mean stomach fullness was always high. They were more opportunistic than herbivorous. From stomach fullness and the presence of a food bolus in mouth parts, feeding in surface waters appeared to be intensive, but gut content indicated that food was not processed there. It is therefore suggested that individuals underwent vertical interchanges across the thermocline while feeding during the night. Meganyctiphanes norvegica had significant herbivorous activity during the night. The grazing pressure impact of the scattering layer on phytoplankton was negligible.Contribution to the program of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec)     

Seasonal population structure,vertical distribution and migration of the chaetognath Sagitta elegans in the Celtic Sea

The biology of the chaetognath Sagitta elegans Verrill has been much researched, but detailed studies of population structure have generally been conducted in coastal water where dynamic tidal conditions may cause difficulty in interpretation of data. The resolution of sampling examining vertical distribution and diurnal migration has also been rather coarse. During a series of eight cruises to a seasonally thermally stratified sampling site in the Celtic Sea in 1978 and 1979, detailed vertical zooplankton profiles were taken to study the seasonal population structure, vertical distribution and migration of this species. The overwintering stock of S. elegans (22 to 52 individuals m^-2, 0 to 90 m) had a wide range of lengths (5 to 20 mm) and matured in 1978 from early March, spawning several times before dying out by late July. Young produced by the overwintering stock started to mature in July and population numbers reached their highest in August (2483 m^-2, 132.8 mg C m^-2) when sea temperature peaked (17.1°C). By October, the population of S. elegans declined (284 m^-2), which was thought to be due to a combination of lower sea-water temperature, competition for and availability of food, and predation. Because of the length range of the overwintering population (5 to 20 mm), it is assumed that reproduction continued at a low level over the winter, although eggs were not found in January and February, the coldest months of the year. In summer, the smallest S. elegans (2 to 6 mm) were found in the near-surface waters and did not migrate, but as their lengths increased they occupied deeper depth ranges and a portion of the population started to migrate diurnally. Individuals which did not migrate and stayed in the warmer surface waters, or those which migrated into it, matured faster than those remaining in the colder water below the thermocline. Migration to surface waters by mature individuals seemed to be stopped by high surface temperatures (17°C) and a sharp thermocline (3 C°). As sea temperature increased during the year from the winter minimum of 7.7°C, S. elegans matured at a progressively shorter length (14 mm in March 1978 to 10 mm in August). There are probably only three generations of S. elegans a year in the Celtic Sea.     

Carotenoids in the hermit crab Clibanarius misanthropus (Crustacea,Paguridae) from the Black Sea

The authors investigated the carotenoids in the hermit crab, Clibanarius misanthropus (Risso), from the Black Sea. By means of columnar and thin-layer chromatography the following carotenoids were found to be present in that species; -carotene, canthaxanthin, lutein, astaxanthin and some kind of polioxyxanthophill.