Direct and indirect trophic interactions of <Emphasis Type="Italic">Aurelia </Emphasis>sp. (Scyphozoa) in a stratified marine environment (Mljet Lakes,Adriatic Sea)

The pattern of diel vertical migration and the trophic interactions of moon jelly (Aurelia sp.) were investigated in the sea lakes of Mljet Island (Adriatic Sea) where this scyphomedusa is present throughout the year. Water column characteristics, plankton and in situ behaviour of Aurelia were followed over several 24-h cycles (6–8 times during each cycle) from the surface to the bottom (44 m). Aurelia exhibited a consistent pattern of diel vertical migration. Most of the time Aurelia were located at the bottom of the thermocline layer at temperatures lower than 19°C. Aurelia migrated towards the surface at dusk when the majority was found within the thermocline or just above it. During the night the medusae sank into the deepest layers below 25 m. The main medusa food items inferred from stomach contents were small adult copepods like Oithona nana and Paracalanus parvus and copepodites of small calanoids and cyclopids. In addition, in situ feeding experiments indicated high clearance rates for nauplii and naked ciliates and clear response of bacterial populations pointing to indirect cascade effects of Aurelia on microbial in addition to classical food web.     

Night irradiance and synchronization of lunar release of planula larvae in the reef coral Pocillopora damicornis

Pocillopora damicornis (Linnaeus), which is known to release planula larvae on a monthly cycle, was grown in full daytime solar irradiance, but with four treatments of night irradiance: (1) natural night irradiance, (2) shifted-phase (total darkness during nights of full moon with artificial irradiance at lunar intensity on nights of new moon), (3) constant full moon (full lunar irradiance every night), and (4) constant new moon (total darkness every night). The reproductive cycle of the corals held in the shifted-phase treatment moved out of synchrony with the cycle of corals exposed to a natural lunar cycle of night irradiance. Two previously described types of P. damicornis were tested. The Type Y normally start releasing larvae at full moon, with peak production at third quarter. In the shifted-phase treatment they began releasing planulae at new moon (artificial full moon), with peak production at first quarter. The Type B corals, that normally start releasing planulae at new moon with peak production at first quarter, began to release planulae at full moon (artificial new moon), with peak production at third quarter. Populations of corals grown either in the constant full moon or constant new moon treatment quickly lost synchronization of monthly larva production, although production of planulae continued. Thus spawning is synchronized by night irradiance.Contribution No. 702 of the Hawaii Institute of Marine Biology     

Diel migration of phytoplankton and spectral light field in the Ría de Vigo (NW Spain)

Diel migration of Mesodinium rubrum, Eutreptiella sp., Scrippsiella trochoidea, Dinophysis acuminata and Ceratium furca throughout a 24 h cycle is described for a stable, well-stratified estuary (Ría de Vigo, NW Spain). Daily changes in light quantity and in spectral light ratios i.e. red:far-red, blue:red, green:red and blue:green have been analysed. The spectral light ratios changed at twilight and around noon at various depths. Some of the downward migrations were well predicted by Stokes' law, while other migrations were faster and deeper than calculated. The coincidence of these movements with abrupt changes in red:far-red, green:red and blue:green light ratios is discussed. Some species are able to migrate through the pycnocline, whereas others do not seem to be able to do so. Several species are present in maximum numbers at depth at night, while others display upward migration independent of light, suggesting the existence of endogenous rhythms. Upward migration at dusk began with dispersal of populations, with renewed aggregation at the sea surface coincident with an increase in the red:far-red ratio at 6 m and the green:red ratio at 6 and 10 m. Based on direct evidence for the control of flagellar mobility by light quality reported by other authors from laboratory studies, it is suggested that, together with other cues, spectral light ratios of different light qualities modulate vertical phytoplanktonic migration. Received: 18 July 1997 / Accepted: 17 October 1997     

Nocturnal feeding migrations of Nerita plicata,N. undata and N. textilis (Prosobranchia: Neritacea) on the rocky shores at Mkomani,Mombasa, Kenya

The vertical zonation of the three common rocky shore neritids at Mkomani, Mombasa, Kenya, Nerita plicata Linnaeus, N. undata Linnaeus, and N. textilis Dillwyn, as a function of feeding migrations and of size, was studied from 28 February to 24 March 1983. These snails perform feeding migrations at night starting at around mid-ebb tide and return to their resting positions with the flood tide. They remain in their resting positions throughout the day until the next nocturnal ebb tide. The direction of migration is sizerelated, with the larger snails of each species moving in the opposite vertical direction to the smaller ones, so that the populations as a whole exhibit no statistically significant net vertical displacement. The larger individuals of two of the species, N. plicata and N. undata, invariably move downwards to their feeding levels, while the smaller individuals move upwards; the larger individuals of N. textilis display a different pattern of migration, moving downwards on and around spring-tide days and upwards on and around neap tide days, while the smaller individuals move in the opposite directions. N. textilis rest above their feeding level around spring tides, and below that level around neap tides. It is demonstrated how these nocturnal migratory feeding rhythms are integrated into the spring-neap and seasonal cycles of the snails' daytime resting positions. The adaptive significance of these migrations is also discussed.     

Laboratory experiments on the diurnal vertical migration of marine dinoflagellates through temperature gradients

The diurnal vertical migrations of 4 marine dinoflagellates (Cachonina niei, Ceratium furca, Gymnodinium splendens, and Prorocentrum micans) were studied in a thermally-stratified Plexiglas column (1.70 m deep, 0.61 m diameter). All species migrated through temperature gradients exceeding 7 C°. Some species exhibited altered migration patterns at different salinities either between experiments or compared to observations by other investigators. P. micans exhibited complex changes in swimming speed that depended both on temperature and on the light cycle. These changes provide insight into the possible time course of a field organism's diurnal exposure to physical variability in the water column. P. micans exhibited compositional changes in response to the light cycle, but not to the cross-thermocline temperature differential. A multi-parameter response matrix is required to model coupling between organisms and biologically-active physical mechanisms realistically.     

Mechanisms to initiate a Heterosigma akashiwo red tide in Osaka bay

The ecological role of diel vertical migration of Heterosigma akashiwo Hada to initiate red tide was investigated in Tanigawa Fishing Port and Sano Harbor, Osaka Bay, Japan during red tide seasons in 1979 and 1980. This species migrated toward the surface early in the morning at a velocity of 1.0 to 1.3 m h^-1. Downward migration was found in the afternoon, and more than 2.0×10³ cells ml^-1 aggregated in the bottom layer at night. The upward migration started before sunrise and downward shifting occurred prior to sunset. The movement is presumably associated with circadian rhythm, which is known as one of the biological periodicities. H. akashiwo crossed steep temperature and salinity gradients (6.5°C and 5.7 S) during the diel vertical migration. High values of particulate organic carbon and nitrogen concentrations were obtained in dialysis bags suspended in situ at identical layers with high cell concentration, while the values for surface and bottom bags were comparatively low. The results reveal that H. akashiwo migrates toward the sea surface to carry out photosynthesis effectively, and to the bottom to utilize nutrients efficiently.     

Observations on the diurnal vertical migrations of an oceanic animal community

Diurnal changes in abundance caused by vertical migrations have been examined in populations of copepods, ostracods, euphausiids, amphipods, decapods, chaetognaths, siphonophores and fish. The animals were taken in a series of hauls made over a 24 h period with an opening-closing midwater trawl system (RMT 1+8), consisting of a net of 1 m² mouth area combined in the same frame as one of 8 m² mouth area. The samples were taken at 250 m depth in a position 30°N; 23°W on 7/8 April 1972. The specific composition of the community and the numbers of individuals changed continuously with time. The numbers of fish, decapods and chaetognaths increased at night, but those of copepods, ostracods and euphausiids decreased. More species of fish, decapods and copepods were present by night than by day, whereas the numbers of species per haul for other groups remained fairly constant. The relative abundances of groups caught by the RMT 1 have been analysed, but similar treatment of the RMT 8 samples was impossible as only 3 groups were taken from this net. Non-migrants were a minority in every group except chaetognaths. Migrant species have been put into one of 6 transitory categories according to their patterns of abundance and hence migrations. Within each category, migratory behaviour varied both inter- and intraspecifically. The patterns of abundance of many species were smooth and continuous, suggesting slow migratory cycles of small amplitude. Conversely, extensive migrants had discontinuous patterns and presumably more rapid movements. Few migrants had a steady numerical plateau between their upward and downward migrations, and most apparently moved up or down continuously. The presence of migratory species in the sampled layer depended upon the time of day or night. It is concluded that, in a vertical series of hauls, the depths of occurrence of migrants will vary with the sampling time. Further-more, a vertical series will show a species minimum migration range but not necessarily its maximum. Individuals of some species were out of phase with the migrations of their main populations. There is evidence that the distributions and migrations of some species of decapods, euphausiids, copepods and fish could be related to the distribution of underwater light. Three pairs of congeneric copepod species were both spatially and temporally segregated for at least part of their diurnal cycles. Such an orderly arrangement could provide a means of reducing competition between species. Some species, however, overtook others on their migrations and the pattern of underwater light cannot, therefore, regulate the distribution of all species in the same way.     

Vertical distribution and feeding patterns of midwater fish in the central equatorial Atlantic

Fishes and zooplankton were obtained (March–April 1979 and partly in August 1974) from 45 hauls taken during the day and at night in the central equatorial Atlantic between Latitude 3°N and 2°S from the surface to 1250-m depth, using the RMT 1+8, a combined opening-closing plankton and micronekton trawl. The vertical distribution of 30 myctophid species is described. All species migrate in a diel pattern, Ceratoscopelus warmingii and Lampanyctus photonotus down to at least 1250 m. During daytime most species aggregated at 400-to 700-m depth, therefore only partly occupying the depth of the Deep Scattering Layer (400 to 500 m at 15 kHz). The feeding patterns of seven of the most abundant species were compared, with a total of 1 905 stomach contents being analysed. All seven species are regarded as opportunistic predators, which feed predominantly during the night on calanoid copepods. A total of 66 species of calanoid copepods were identified among the prey items, with smaller species definitely being in the minority. Stomachs of C. warmingii (700 to 1 250 m depth) and Lepidophanes guentheri (500 to 900 m depth) from daytime samples contained copepod species restricted to the upper 150 m of the water column, including Undinula vulgaris, Nannocalanus minor, and Euchaeta marina, thereby confirming an extended vertical migration of predators. Differences in diet and preferences between species in their total food spectrum are described.     

Vertical habitat-partitioning by copepodites and adults of subtropical oceanic copepods

Vertical distributions of the abundant larger copepods, both adults and late copepodites, were observed day and night in the upper 500 m of the North Pacific central gyre in early November, 1971. Densities of the copepodites usually equalled or exceeded those of the adults. Copepod species with maximum densities at or above 100 m (Calanus spp., Nannocalanus minor f. major, Undinula darwini, and Euchaeta rimana) usually had no ontogenetic or diel migration. Neocalanus spp. and Haloptilus longicornis exhibited ontogenetic but not diel migrations. Nannocalanus minor f. minor, Aetideus acutus, Euchaeta media, Scolecithrix spp. and Pleuromamma spp., had both ontogenetic and diel migrations. Adults and copepodites of E. media and Pleuromamma spp. usually had their night modes at the same depth, but the daytime modes were at progressively deeper depths for progressively older stages. Daytime modes for adults and copepodites of A. acutus and Scolecithrix bradyi were at the same depth, but the nighttime modes were at shallower depths for progressively older stages. Night modes of all these migrators were usually in the mixed layer (75 m), where primary production rates were maximal. Congeners usually had similar migratory behavior, but competition probably has been a significant determinant of vertical distribution, since congeners, particularly sibling species, consistently had different depths of maximum occurrence during both day and night.     

Vertical distribution and nocturnal migration of zooplankton in relation to the development of the seasonal thermocline in Patraikos Gulf

Vertical distribution and nocturnal migration of zooplankton species in relation to the development of the seasonal thermocline in the shallow waters (90 m) of Patraikos Gulf (Ionian Sea, Greece) were investigated using a WP-2 closing net. Juvenile and adult copepods accounted for a mean of 91% of the total collected in three sampling periods, i.e. May, July and September 1985.Ctenocalananus vanus, Paracalanus parvus andOithona plumifera were the dominant copepods. The majority of the zooplankton tend to aggregate at the thermocline layer. Among copepods the two congeneric speciesClausocalanus pergens andC. furcatus exhibited different migratory responses to the development of the thermocline.C. pergens occurred in the lower part of the thermocline andC. furcatus in the upper region or above. The diel vertical migration of all species could be divided into four types: (1) no vertical migration; (2) upward migration at night; (3) occasional migration; and (4) reverse migration (down at night). In July when the strongest thermocline developed, most zooplankters rose close to the surface at night. For most species, temperature discontinuity did not limit their diel migration.Please address all correspondence and requests for reprints to Dr J.J. Lykakis     

Growth rhythms in the shrimp Penaeus vannamei and P. schmitti

Growth increments of the penaeid shrimp Penaeus vannamel (Boone) and P. schmitti (Burkenroad) grown in commercial production ponds in Ecuador and Colombia were measured over a 4 mo period between October 1987 and January 1988. P. vannamel grown from wild caught seed in Ecuadorian ponds exhibited a growth rhythm which significantly coincided with lunar phases. Growth increments of at least 1 gwk^-1 occurred around a new and full moon, while growth increments of less than 1 gwk^-1 occurred during first and last quarters. P. vannamel transported from Ecuador as nauplii, raised in Colombian hatcheries and on grown in Colombian ponds exhibited a cyclic pattern in wet weight increments but not correlation to lunar cycles was observed. P. schmitti showed no cyclic pattern in growth. The two species under consideration are indigenous to two different, and quite unique areas displaying different environmental characteristics. The link between the growth pattern, biological and lunar cycles is discussed.     

Cyclical mucous sheet formation on poritid corals in the San Blas Islands,Panama

Mucous sheets are common on poritid corals as well as other reef coelenterates, but the frequency, cause and function of these sheets have rarely been investigated.Porites furcata Lamarck andPorites astreoides Lamarck colonies at three sites in the San Blas Islands, Panama, were monitored for the presence or absence of mucous sheets every 2 d between 11 May and 27 August 1983. Concurrent measurements of polyp expansion, particulate matter concentrations, salinity, water temperature and sedimentation rates were also made at each site. Mucous sheet formation was not correlated with any of the environmental parameters measured. Mucous sheet formation by both species had a lunar periodicity.P. furcata formed mucous sheets around the time of the full moon, whileP. astreoides formed mucous sheets during the first quarter (around a week before full moon). Mucous sheet formation was correlated with polyp contraction behavior and may be a secondary effect of a lunar cycle in polyp activity.     

Microbial activities as a function of water depth in the Ligurian Sea: an autoradiographic study

Fourteen species of sergestid shrimps were collected in the Sargasso Sea between the surface and 1500 m near Bermuda on 4 cruises. The vertical distribution and feeding activity of the most abundant species are discussed in relation to interspecific competition and the adaptive significance of vertical migration. Each species lives within a narrow depth range and exhibits a diel vertical migration. Sergestes splendens migrated as much as 825 m, while S. japonicus migrated less than 100 m. Neither the seasonal nor permanent thermocline influenced the migration range. The only species which occurred together both day and night were S. pectinatus with S. vigilax and S. pectinatus with S. sargassi. Morphological differences in the third maxillipeds of these species suggest differences in feeding. Although most species eat a variety of organisms, the foreguts of S. grandis, S. corniculum, and S. splendens contained euphausiids more often than those of other species, and S. grandis and S. robustus fed more frequently on fishes. In contrast, S. japonicus appears to feed on detritus. Food was found in the foreguts of most species less frequently during the day than night, but no species fed only at night. S. sargassi and S. pectinatus fed equally day and night.     

Effects of nitrate on the diurnal vertical migration,carbon to nitrogen ratio,and the photosynthetic capacity of the dinoflagellate Gymnodinium splendens

A non-thecate dinoflagellate, Gymnodinium splendens, was studied in a 12 d laboratory experiment in 2.0x0.25 m containers in which light, temperature, and nutrients could be manipulated. Under a 12 h light: 12 h dark cycle, the dinoflagellates exhibited diurnal vertical migrations, swimming downward before the dark period began and upward before the end of the dark period. This vertical migration probably involved geotaxis and a diel rhythm, as well as light-mediated behavior. The vertical distribution of nitrate affected the behavior and physiology of the dinoflagellate. When nitrate was present throughout the container, the organisms resembled those in exponential batch culture both in C:N ratios and photosynthetic capacity (P_max); moreover, they migrated to the surface during the day. In contrast, when nitrate was depleted, C:N ratios increased, P_max decreased, and the organisms formed a subsurface layer at a depth corresponding to the light level at which photosynthesis saturated. When nitrate was present only at the bottom of the tank, C:N ratios of the population decreased until similar to those of nutrient-saturated cells and P_max increased; however, the dinoflagellates behaved the same as nutrient-depleted cells, forming a subsurface layer during the light period. Field measurements revealed a migratory subsurface chlorophyll maximum layer dominated by G. splendens. It was just above the nitracline during the day, and in the nitracline during the night, which concurs with our laboratory observations.     

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.     

Reproduction and development of the blue coralHeliopora coerulea (Alcyonaria: Coenothecalia)

Up to the present time, the reproductive cycle and larval development ofHeliopora coerulea, living fossil and sole member of the alcyonarian order Coenothecalia, have been undescribed. Populations ofH. coerulea studied on the Great Barrier Reef between 1984 and 1989 were found to have separate sexes and an annual gametogenic cycle. Mature eggs were large (>800µm diam), and their development was initiated internally. There appeared to be a lunar or semi-lunar periodicity in reproductive behaviour, with brooding of larvae commencing around either full or new moon. Development proceeded via superficial cleavage and a solid stereoblastula. Subsequent development took place on the surface of the colony, within a temporary brood-space created by protracted distension of the polyps. Larvae lacked zooxanthellae and were brooded for between 6 and 14 d prior to leaving the colony. Reproduction and development reflected the taxonomic affinities ofH. coerulea, but brooding and embryological development also appeared to be constrained by its morphology. Reproductive and larval characteristics ofH. coerulea may offer a partial explanation of the current biogeographic range ofH. coerulea.     

Ontogenetic diel vertical migration of the planktonic copepod Calanus sinicus in the Inland Sea of Japan

The ontogenetic diel vertical migration of the planktonic copepod Calanus sinicus was investigated in the Inland Sea of Japan in June 1989, when the water column was thermally weakly stratified. Because of fewer eggs and less variation in their abundance, nocturnal spawning was not apparent. A pronounced upward migration occurred in NIII. NIII to CIII resided in the upper 20 m layer throughout the day, and from CIV on their median depths descended. CV and adult females underwent significant diel vertical migration, whereas adult males did not migrate. By integrating the results from the present study and those from our previous investigations (in August–September 1988, November 1988 and March 1989), we review seasonal variation in the ontogenetic diel vertical migration of C. sinicus. Spawning was largely nocturnal, reaching its maximum level around dawn, but spawning depth and fecundity changed seasonally. The distribution of pre-feeding stages, NI and NII, was similar to that of eggs. A pronounced upward migration always occurred in the first feeding stage, NIII, and late nauplii and early copepodites always resided in the food-rich upper layer, indicating that upward migration by NIII is feeding migration. As the stages progressed, they extended their vertical distribution range, and CV and adult females usually underwent diel vertical migration. However, the pattern and strength of this migration differed seasonally. Their day depths increased with the increase of relative biomass of planktivorous fish, indicating that predator avoidance induces their diurnal downward migration. High chlorophyll a concentrations in the upper layer (<15 m deep) relative to the lower layer (>20 m deep) amplified their diel vertical migrations. Diel vertical migration of C. sinicus is a phenotypic behavior.     

Diurnal and lunar patterns of larval recruitment of Brachyura into a mangrove estuary system in Ranong Province, Thailand

The present study followed the temporal recruitment pattern of brachyuran larvae in a mangrove tidal creek on the Andaman Sea coast of Ranong Province, Thailand, based on the assumption that the processes governing recruitment are important for the overall population dynamics of mangrove brachyuran crabs. Plankton net samples were taken on five occasions: on two new moon spring tides, one waxing moon neap tide, one full moon spring tide and one waning moon neap tide during October and November 1997. In addition collectors for larval crab megalopae were employed every 3 days through one dry season and one wet season (March–October 1998). Both the plankton net samples and collector samples revealed four major brachyuran groups in three families: Ocypodidae, Grapsidae and Portunidae. The grapsid group was further separated into two morphotypes which were identified as Metaplax and sesarmid species. Identified group mean numbers per cubic metre were ocypodids 3.0, sesarmids 0.8 and Metaplax 0.5, while portunid megalopae were very scarce (≪0.1 m⁻³). Further analysis of plankton net samples showed that when considering the parameters date, depth, current direction and the diel cycle, Metaplax and ocypodids distribute differently in the tidal and lunar cycle. Metaplax recruitment dominates on flood tides and on bottom layers, followed by middle and surface layers. Conversely, ocypodid abundance varied significantly with date only. Notably recruitment was not dependent on the diel cycle for either group. The collector samples of megalopae showed that recruitment of ocypodids, Metaplax and sesarmids occurred on full and new moon spring tides, while portunid megalopae preferred to settle on full moon spring tides. Since tidal currents were related to the lunar cycle megalopa groups are also cross-correlated with tidal amplitude, except for the portunid group. It is concluded that megalopae recruit in a similar manner to what has been found in other regions of the world, except that the abundance of ocypodids and Metaplax is not influenced by the diel cycle. Received: 14 February 2000 / Accepted: 24 November 2000     

Respiration and chemical composition of the bathypelagic euphausiid Bentheuphausia amblyops

Bentheuphausia amblyops is a cosmopolitan bathypelagic euphausiid with a vertical range of from 280 to 7 000 m. Determinations of proximate and elemental composition show that B. amblyops has a more robust structure (lower water content and higher protein content) than is typical of other bathypelagic Crustacea. B. amblyops is a strong swimmer and is capable of employing its thoracic legs in raptorial fashion. Discrete depth trawls taken between depths of 400 and 2 500 m on a diel basis show no evidence of vertical migration. There was no significant difference in oxygen consumption within the species environmental temperature range (1.5° to 7.5°C), which may be partially ascribed to a limited temperature effect and partially to variability in spontaneous activity at the different temperatures. Between 7.5° and 9.5°C there was a marked temperature effect on metabolism (Q₁₀=6.4). The observed vertical distribution and metabolic response to elevated temperature preclude the migrations to the surface that have been postulated for B. amblyops. The robust composition of the species, its behavioral characteristics, and published diet information strongly suggest that the species is omnivorous with a strong predatory component in its foraging strategy.