Vertical Migration Rhythms Research Articles

Adverse effects of strong light and nitrogen deficiency on cell viability, photosynthesis, and motility of the red-tide dinoflagellate Karenia mikimotoi

:The dinoflagellate Karenia mikimotoi is a red tide–forming alga that causes serious damage to aquaculture in coastal areas around the world. Its eco-physiological characteristics have been investigated extensively but conditions associated with declines of red tides of K. mikimotoi remain unknown. In the present study, we investigated the effects of light and nutrients on the viability, activity of photosystem II (PSII), and motility of K. mikimotoi. Cells were incubated under low (100 μmol photons m−2 s−1 on a 12:12 light:dark cycle of illumination) or high irradiance (1000 μmol photons m−2 s−1) for 4 h in the middle of the photoperiod, in different media. Under low light, most cells survived for about 1 month in media depleted of nitrogen (N) and phosphorus (P), similar to their survival pattern in complete medium. However, when cells that had been pre-incubated in N-depleted medium for 7 d were incubated under high light in an N-depleted medium, cell density and PSII activity declined within a few days. In P-depleted and complete media, there were no significant differences in cell density and PSII activity between low and high light, at least for 10 d. Furthermore, PSII activity was much more sensitive to photoinhibition in N-depleted than in complete medium. The rhythm of diurnal vertical migration of K. mikimotoi was observed in very low light (10 μmol photons m−2 s−1) in different media. The rhythm became obscure in N- and P-depleted media; in fact, most cells stayed in the surface layer all day in N-depleted medium. Moreover, cells accumulating in surface layers in N- and P-depleted media descended very little, even when irradiated with high light. These observations suggest that N-deficiency might trap cells in the surface layer and increase their susceptibility to photoinhibition of PSII, possibly leading to cell death. The combination of N-deficiency and high light thus appears to be an important factor that determines the decline of red tides of K. mikimotoi.

Diurnal vertical migration rhythms and their photoresponse in four phytoflagellates causing harmful algal blooms

AbstractWe observed diurnal vertical migrations (DVMs) of four noxious phytoflagellates: Chattonella antiqua, Heterosigma akashiwo (raphidophyte), Karenia mikimotoi, and Heterocapsa circularisquama (dinoflagellate) under various light conditions such as fluence rate, day length, light regime, and wavelength in the laboratory. For all flagellates cyclic DVMs were observed at an irradiance of 30 μmol quanta m−2 s−1 on a 12 h : 12 h light : dark (L : D) cycle. Start times of upward and downward migrations under the light condition differed among species, indicating that each flagellate has its own original DVM cycle. The ascent period of C. antiqua during the photoperiod was much longer than the other flagellates, whereas K. mikimotoi remained submerged longer. No flagellate displayed cyclic DVM in the dark, and only K. mikimotoi did even under weak light of 1 μmol quanta m−2 s−1. C. antiqua and K. mikimotoi extended their surface‐accumulation periods at longer photoperiods, but the other species scarcely changed theirs. For all the flagellates, a DVM rhythm was entrained by shifting the L : D cycle; this occurred under white or blue light, but not under red light. These results indicate that all flagellates require (blue) light for maintenance and adjustment of DVM cycle but responses to change in light intensity and day length differ among species. The present study suggests that the species‐specific DVM cycle and its photoresponse property can change time staying in surface or bottom layers in the natural environment, and therefore, may affect photosynthesis and nutrient uptake for survival and growth in four phytoflagellates.

Proximate Control of Diel Vertical Migration In Phyllosoma Larvae of the Caribbean Spiny Lobster Panulirus argus

Phyllosoma larvae of the spiny lobster Panulirus argus undergo diel vertical migration (DVM), in which they are at depth during the day and nearer the surface at night. This study determined the visual spectral sensitivity of Stage I larvae and investigated whether light plays a proximate role in DVM as an exogenous cue and as an entrainment cue for an endogenous rhythm in vertical migration. Under constant conditions, larvae have a circadian rhythm (24.5-h period) in vertical swimming that resulted in a twilight DVM pattern. The behavioral response spectrum and electroretinogram recording indicated two photoreceptor spectral classes with maxima at 360 and 486 nm. When stimulated in an apparatus that simulated the underwater angular light distribution, dark-adapted larvae showed only positive phototaxis, with a threshold intensity of 1.8 × 10(13) photons m(-2) s(-1) (3.0 × 10(-5) μmoles photons m(-2) s(-1)). They have an avoidance response to predator shadows in which they descend upon sudden decreases in light intensity of more than 69%. When stimulated with relative rates of decrease in light intensity as occur at sunset they ascended, whereas they descended upon relative rates of light intensity increase as occur at sunrise. Thus, the DVM pattern is controlled by both an endogenous circadian rhythm in swimming and behavioral responses to light at sunrise and sunset.

Larval Biology of the CrabRhithropanopeus harrisii(Gould): A Synthesis

This synthesis reviews the physiological ecology and behavior of larvae of the benthic crab Rhithropanopeus harrisii, which occurs in low-salinity areas of estuaries. Larvae are released rhythmically around the time of high tide in tidal estuaries and in the 2-h interval after sunset in nontidal estuaries. As in most subtidal crustaceans, the timing of larval release is controlled by the developing embryos, which release peptide pheromones that stimulate larval release behavior by the female to synchronize the time of egg hatching. Larvae pass through four zoeal stages and a postlarval or megalopal stage that are planktonic before metamorphosis. They are retained near the adult population by means of an endogenous tidal rhythm in vertical migration. Larvae have several safeguards against predation: they undergo nocturnal diel vertical migration (DVM) and have a shadow response to avoid encountering predators, and they bear long spines as a deterrent. Photoresponses during DVM and the shadow response are enhanced by exposure to chemical cues from the mucus of predator fishes and ctenophores. The primary visual pigment has a spectral sensitivity maximum at about 500 nm, which is typical for zooplankton and matches the ambient spectrum at twilight. Larvae can detect vertical gradients in temperature, salinity, and hydrostatic pressure, which are used for depth regulation and avoidance of adverse environmental conditions. Characteristics that are related to the larval habitat and are common to other crab larval species are considered.

Adaptations of microphytobenthos assemblages to sediment type and tidal position

Tagus intertidal microphytobenthos (MPB) assemblages were characterized over a wide range of sediment type and tidal height and the possible effects of these variables on MPB spatial distribution and photo-adaptation mechanisms were investigated. Two transects with six different sediment type and different tidal height sites were sampled once every two months from 2002 to 2004. Upper shore and sandy sites showed higher chlorophyll a (chl a) content, with sandy sediments showing a biomass peak in late winter–early spring, and muddy sites showing no obvious seasonal pattern. Stepwise multiple linear regressions showed that only SiO 2, tidal height and sediment particle size <63 μm were significant variables ( p<0.05), explaining 50% of MPB biomass spatial–temporal variability. However, when data were separated by transect, only tidal height remained significant at both transects. Sandier sediments exhibited higher zeaxanthin/chl a and lower fucoxanthin/chl a ratios characteristic of a mixed cyanobacteria/diatom assemblages, showing an alternate seasonality with cyanobacteria increasing in summer and diatoms dominating in spring. Diatom biofilms showed contrasting features depending on the sediment type. Epipsammic diatoms were small with an average length of around 10 μm, while epipelic diatoms showed a wider size range with size distribution peaks at 10–15 μm, 25–35 μm and >60 μm. Epipelic biofilms showed evidence of being low light-acclimated (high fucoxanthin/chl a) and of photo-regulating by vertical migration movements (presence of endogenous vertical rhythms and lower diatoxanthin/diadinoxanthin). Epipsammic biofilms showed higher diatoxanthin/diadinoxanthin ratios and no vertical migration rhythms. Thus, the two diatom biofilm types had distinct strategies to photo-regulate: epipelic diatoms using vertical migration to position themselves at the sediment depth of optimum light conditions, and epipsammic diatoms using the xanthophyll cycle to photo-regulate. Further studies comparing epipsammic and epipelic assemblages are necessary to better understand MPB photo-regulation mechanisms.

Developmental shift in the selective tidal-stream transport behavior of larvae of the fiddler crab Uca minax (LeConte)

Following hatching, larvae of the fiddler crab Uca minax (La Conte) are exported from the adult habitat in estuaries to coastal and shelf waters where they undergo development prior to re-entering estuaries as postlarvae (megalopae). Studies of the spatial distribution of both newly hatched zoeae (Stage I) and megalopae indicate they undergo rhythmic vertical migrations associated with the tides for dispersal and unidirectional transport (selective tidal-stream transport) both within estuaries and between estuaries and the nearshore coastal ocean. We tested the hypothesis that U. minax zoeae possess a circatidal rhythm in vertical migration that facilitates offshore transport in ebb tidal flows, while postlarvae (megalopae) return to estuaries using a similar flood-phased endogenous rhythm. We also determined if the expression of the rhythm was influenced by the salinity conditions zoeae and megalopae experience as they transition between low-salinity regions of estuaries and high-salinity coastal waters. Stage I zoeae were collected by holding ovigerous female crabs in the lab until hatching. Megalopae were collected from the plankton and identified to species using molecular techniques (PCR-RFLP). Under constant laboratory conditions, both zoeae and megalopae exhibited endogenous circatidal rhythms in swimming that matched the principal harmonic constituent of the local tides (12.39 ± 0.07 h; X¯ ± SE). Upward swimming in Stage I zoeae occurred 2.5-4 h after high tide near the time of expected maximum ebb currents in the field. Rhythmic swimming of megalopae occurred slightly earlier in the tide (2.5 ± 0.09 h after high tide; X¯ ± SE) but was not entirely synchronized with flood currents, as expected. Salinity conditions had no apparent effect on the expression or pattern of the rhythms. Results indicate that this circatidal rhythm forms the behavioral basis of selective tidal-stream transport (STST) in early stage U. minax zoeae, but does not undergo a sufficient phase shift to account for vertical distribution patterns exhibited by megalopae in the field.

The ontogeny of the endogenous rhythm in vertical migration of the blue crab Callinectes sapidus at metamorphosis

Postlarvae or megalopae of the blue crab Callinectes sapidus are transported from offshore areas into estuaries where they settle and metamorphose in areas of submerged vegetation. Separate previous studies found that both intermolt megalopae and juvenile crabs have circadian rhythms in vertical migration with different phasing. Intermolt megalopae vertically migrate during the day phase of the entrainment cycle whereas juvenile crabs vertically migrate during the night phase. The present study considered (1) whether the switch in vertical migration activity from the day to night phase at metamorphosis occurred in individual animals and (2) whether the rhythm continued after metamorphosis without re-entrainment to the light:dark cycle and (3) whether exposure to offshore or estuarine water affected the rhythm. Vertical migration of individual animals was monitored under constant conditions with a video system. Re-entrainment was not necessary because the circadian rhythm in individual animals continued and the switch in phase occurred when C. sapidus was held under constant condition from the intermolt megalopa stage through metamorphosis to the first crab stage. Offshore and estuarine waters only affected the phasing of the rhythm around the time of metamorphosis but did not affect the ontogeny of the rhythm. The continuation of the rhythm and change in phase at metamorphosis contribute to horizontal transport in estuaries.

Automated Recording of Vertical Negative Phototactic Behaviour in Daphnia magna Straus (Crustacea)

Diurnal vertical migration is a well-known phenomenon in the circadian activity rhythms of zooplankton. Our goal was to test whether negative phototaxis in Daphnia magna clone BEAK (provoked by artificially induced light stress, alternating light and dark phases in 2 h intervals), and its interference with the endogenous rhythm of diurnal vertical migration, can be automatically registered with a biomonitor. For the first time the vertical swimming behaviour of D. magna was recorded quantitatively based on nonoptical data recording in a fully automated biotest system, the Multispecies Freshwater Biomonitor in a new experimental setup consisting of a column of three recording units (3-level chambers). Circadian vertical migration was clearly recorded in the 3-level chambers and the rhythm was more clear with 5 than with 1 organism per chamber. The organisms clearly responded to induced light stress with negative phototaxis, however best in larger chambers. The artificially induced rhythm was influenced by the endogenous rhythm. This approach may facilitate long-term observations of vertical swimming activity of zooplankton in the future.

Orientation and navigation in coastal and estuarine zooplankton

The axiom that zooplankton species exhibit upwards migration behaviour at night is challenged by recent findings. Such behaviour is not universal, may vary during ontogeny, and is occasionally reversed. Moreover, in some estuarine and coastal zooplankton species vertical migration rhythms are of tidal, not diel, periodicity. There is evidence for several species that vertical migrations are endogenously controlled, occurring in constant conditions in the laboratory, suggesting that they have arisen under considerable selection pressure. They appear to play a significant role in orientation and navigation of coastal and estuarine zooplankton, predicated on the selective advantage of closure of life cycles. Vertical migrations between water masses moving in different directions at tidal, diel or longer timescales permit dispersal and recruitment, or retention, of planktonic larvae and adults in favourable ecological locations. Exogenous factors serving as cues for, or directly controlling, vertical migration rhythms include light, hydrostatic pressure, salinity, temperature, water movements and gravity responses, besides biological factors such as the timing of larval release, duration of larval life and predator/prey interactions. Behavioural characteristics should be quantified and factored into dispersal models which assume that zooplankton adults and larvae are advected as passive particles.

THE CIRCATIDAL RHYTHM IN VERTICAL SWIMMING OF FEMALE BLUE CRABS, CALLINECTES SAPIDUS, DURING THEIR SPAWNING MIGRATION: A RECONSIDERATION

Ovigerous blue crabs, Callinectes sapidus Rathbun, undergo ebb-tide transport (ETT) during the spawning migration from estuaries to coastal areas where they release larvae. An initial field study found that only females with mature embryos underwent the spawning migration and that postspawned females switched to flood-tide transport for movement back into and up estuaries. Alternatively, a more recent field study found ETT occurred in females throughout embryo development and continued after larval release. An endogenous rhythm in vertical migration underlies ETT during the spawning migration. A past study found the circatidal rhythm was only present in females with mature embryos but not in females with immature embryos or after larval release. The present study reinvestigates this rhythm in vertical migration. Both females with early embryos and postlarval release females entrained in the field have circatidal rhythms, in which they swim vertically at the time of consecutive ebb tides at the collection site or on alternate ebb tides. These results support the field study that ovigerous females undergo ETT throughout embryo development and after larval release. Thus, females will be transported seaward and release subsequent larval clutches in coastal areas that increase the likelihood that larvae will be transported to offshore developmental areas.

Diel vertical migration of the marine copepod Calanopia americana. II. Proximate role of exogenous light cues and endogenous rhythms

The marine copepod Calanopia americana Dahl undergoes twilight diel vertical migration (DVM) in the Newport River estuary, North Carolina, USA, in synchrony with the light:dark cycle. Copepods ascend to the surface at sunset, descend to the bottom around midnight, and make a second ascent and descent before sunrise. Behavioral assays with C. americana in the laboratory during fall 2002/2003 and summer 2004 investigated aspects of three hypotheses for the proximate role of light in DVM: (1) preferendum hypothesis (absolute irradiance), (2) rate of change hypothesis (relative rates of irradiance change), and (3) endogenous rhythm hypothesis. Results suggest that C. americana responds to exogenous light cues consistent with its DVM pattern; changes in absolute irradiance evoked swimming responses that would result in an ascent at sunset and descent at sunrise, while relative rates of irradiance decrease at sunset (−0.0046 s−1) evoked an ascent response, and relative rates of irradiance increase at sunrise (0.0042 s−1) evoked a descent response. Furthermore, C. americana expressed an endogenous rhythm in vertical migration that was positively correlated with field observations of twilight DVM. Collectively, these results indicate that both exogenous light cues and endogenous rhythms play a proximate role in twilight DVM of C. americana, providing redundancy in the causes of its vertical migration.

Is the rhythm of vertical migration of Daphnia longispina circadian or simply nycthemeral ?

From an ecological viewpoint, diel vertical migration (DVM) of zooplankton is an adapted response to environmental factors. DVM coincides with changes in light intensity, but persists also under constant illumination conditions. This fact suggests the existence of an internal clock. In Daphnia longispina, it could be located in the eye. Indeed, the spatial distribution of the pigments containing in the eye ommatidia exhibits significant changes over time when Daphnia are kept under light/dark (LD) conditions. These variations coincide with displacements of the organisms. Furthermore, it is observed that the blue/dark conditions, which inhibit DVM, also stop changes in the distribution of pigments in the ommatidia. The modifications of the eye observed under LD white light represent either the internal clock or its manifestation.

83 Comparative analysis of vertically migrating euglena viridis populations in tidal and non‐tidal benthic environments

Benthic populations of Euglena viridis exhibit vertical migration behavior on high energy intertidal beaches and along the sand banks of freshwater streams. This study examines similarities and differences in the migratory behavior and cell morphology of populations of E. viridis inhabiting Scripps Beach, La Jolla, California and Coble Brook, Burlington, North Carolina. The timing of migration was measured by counting the number of cells in samples collected from the sediment surface throughout the day. Sediment cores were extracted and sectioned to determine the vertical distribution of the population. Neutral density filters and opaque canisters were used to shade the substratum to 56%, 22%, 2%, and 0% of incident irradiance (Io) to examine the effect of light on cell morphology and migratory behavior. On intertidal beaches, E. viridis exhibited a tidal rhythm in vertical migration with cells migrating below the sediment surface at night (&gt;15 cm) and during daytime high tides. In this habitat, the upward migration response was enhanced at irradiances lower than 100% Io but cell morphology was not altered by shading. On the banks of freshwater streams, E. viridis exhibited a diurnal migratory rhythm with both tear‐drop and spherical morphologies observed throughout the day. The population was most concentrated at the surface around solar noon and at night it was located between 1 and 2 cm below the surface. Shading did not enhance upward migration but it did affect cell morphology. These results will be interpreted in the context of the dominant selection pressures in each environment.

Circatidal activity rhythms in ovigerous blue crabs, Callinectes sapidus: implications for ebb-tide transport during the spawning migration

Female blue crabs (Callinectessapidus Rathbun) with mature embryos have a spawning migration in which they: (1) undergo ebb-tide transport for movement seaward from estuaries, (2) release their larvae, and (3) reverse direction by undergoing flood-tide transport for up-estuary movement. The study determined whether ebb-tide transport during the spawning migration is based upon an endogenous rhythm in vertical migration. Under constant conditions in a rectangular container, which limited horizontal and vertical movements, females with young and mature embryos had circatidal rhythms (periods=12.11–12.95 h) in migratory restlessness (swimming activity) and egg maintenance behavior (abdominal pumping). However, the rhythms were out of phase, as migratory restlessness occurred during the expected time of ebb tide in the field, and egg maintenance behavior, during the time of flood tide. Under constant conditions in vertical columns (1.32 m high), crabs with mature embryos had a circatidal rhythm (periods=12.2–13.7 h) in which they had frequent bouts of swimming to the surface of the column during the expected time of ebb tide in the field and remained on the bottom during the time of flood tide. This rhythm was not present in crabs with young embryos and disappeared after larval release. Thus, an endogenous rhythm in vertical migration does underlie the ebb-tide transport behavior of ovigerous blue crabs with mature embryos during their spawning migration.

EFFECT OF SUBSURFACE NUTRIENT SUPPLIES ON THE VERTICAL MIGRATION OF EUGLENA PROXIMA (EUGLENOPHYTA)1

Euglena proxima Dangeard inhabits intertidal sand flats and displays a tidal rhythm in vertical migration. During daytime low tides when the sand flat is emersed, millions of cells are visible on the sediment surface, but the population remains below the surface at all other times. An earlier study demonstrated that the extent of downward migration of E. proxima is reinforced by the presence of a subsurface layer of black sediment. The present study was designed to test the hypothesis that the higher availability of inorganic nutrients or organic substrates in or above the black layer is responsible for the enhancement of downward migration in E. proxima. This hypothesis was tested experimentally by manipulating the bottom water in 24 mesocosm containers in a tidal tank. Six replicates of each of the following nutrient treatments were tested: seawater control; deep porewater collected from 70 cm below the sediment surface; seawater enriched with ammonium, nitrate, and phosphate; and seawater enriched with acetate, glucose, and the preceding inorganic nutrients. Multivariate analysis of variance revealed that the chl a biomass and chl a‐weighted mean depth of the population at high tide were significantly greater for replicates receiving inorganic nutrients. There was no difference between those receiving only inorganic nutrients and those enriched with inorganic nutrients, acetate, and glucose. These findings represent the first experimental evidence that subsurface nutrients are an important resource that reinforces the maintenance of vertical migration behavior in benthic microalgae.

Vertical migration behaviour in the larvae of the shore crab Carcinus maenas from a microtidal system (Gullmarsfjord, Sweden)

Tide-related vertical migration behaviours which assist in horizontal transport of marine larvae have been documented in many species from meso-tidal systems; however, little is known about larval behaviour in areas where the tides do not produce predictable changes in water cur- rents. We assessed the vertical migration behaviour in larvae of the shore crab Carcinus maenas (L.) from the Swedish west coast, to explore possible transport mechanisms in a micro-tidal system. Repli- cated plankton samples were collected at a fixed station from 2 depths during flood and ebb tides, and analysed for effects of tides and light on temporal and vertical patterns in abundance. In a com- plementary experiment, first-stage zoeae were hatched in the laboratory from field-collected females and studied in constant darkness for endogenous circa-tidal or circa-diel rhythms. The abundance of different larval stages decreased from the first to the fourth zoeal stage by a factor 10, whereas mega- lopal densities were similar to those of the first zoea. This pattern in abundance indicates an off- shore/onshore transportation of larvae during ontogeny, suggesting a behavioural component in larval dispersal. However, contrary to what has been demonstrated in larvae from meso-tidal areas, none of the larval stages displayed a vertical or temporal pattern in abundance related to the tide, although a clear semi-diurnal tidal change of approximately 0.3 m was registered throughout the sampling periods. Instead, the highest densities of all larval stages were consistently sampled during low light conditions, indicating a diel pattern in swimming behaviour. This behaviour appears to be controlled by exogenous cues, since the laboratory experiment with first zoeae in constant darkness did not indicate any consistent tidal or diel rhythm in vertical migration. The lack of an endogenous circa-tidal vertical migration rhythm suggests that Swedish shore crabs are reproductively isolated from the British Isles populations, where this behaviour has been demonstrated to be inherited. The possibility that C. maenas larvae in micro-tidal systems may use predictable sea and land breezes as a mechanism for cross-shelf transport is discussed.

Use of an intelligent CCD camera for the study of endogenous vertical migration rhythms in first zoeae of the crab Carcinus maenas

This paper describes an actographic system, based on video camera and image analysis software, which can detect small planktonic organisms and automatically determine their position in a bi-dimensional plane. This actograph was used to study swimming behaviour, under constant dark conditions in the laboratory, of the first zoea of the crab Carcinus maenas (L.), hatched from ovigerous females collected in the Ria de Aveiro, Portugal. An endogenous rhythm of vertical migration with a circatidal period was detected, in accordance with previous laboratory and field observations made on the species' first zoea from Wales and the Ria de Aveiro, respectively. The system also enabled the computation of dispersion indexes that supported the conclusion that crab zoeae use passive sinking during the descent phase of vertical migration. Mean swimming speed in the absence of light, irrespective of direction, was 0.0666 cm s–1. Vertical upward and downward components of swimming were 0.0437 and 0.0599 cm s–1, respectively. Although lower than values reported in the literature, these velocities are high enough to allow swimming throughout the water column of most estuaries during vertical migration in natural conditions at night.

The retention of fish larvae in estuaries: among-tide variability at Beaufort Inlet, North Carolina, USA

Sixty-eight tides were sampled consecutively to study the retention of fish larvae transported into estuaries. The experiment was conducted at Beaufort Inlet, North Carolina, USA, in March 1996. The inlet is primarily tide-driven and meteorological forcing has a minor influence. Four species of winter-spawning fish were abundant in the samples: Atlantic menhaden, Brevoortia tyrannus; Atlantic croaker, Micropogonias undulatus; spot, Leiostomus xanthurus; and pinfish, Lagodon rhomboides. Across-shore winds significantly affected flow and physico-chemical characteristics of the water. Across-shore winds also positively influenced the larval retention provided larvae did not present a clear tidal rhythm of vertical migration within the water column and that winds pushed in the water during flood tides. The retention of larvae migrating vertically in respect to the direction of the flow was independent of meteorological forcing of the water.

Emergence patterns of small subtidal arthropods in relation to day/night, tidal, and surface/bottom factors: Investigations in the Boreal sea, Japan (Akkeshi, Hokkaido)

The emergence of small arthropods was studied in the boreal sea, Japan (Akkeshi, Hokkaido). In the shallow subtidal zone, two impeller pumps were set in the surface and bottom waters. The pumps ran continuously for 25 days (22 August-16 September, 1998), and invertebrates were sampled using a nylon net (300 to 500 μm in mesh size). The small arthropods collected with the net belonged to 16 orders comprising about 60 taxa. In the dominant 23 taxa, a two-way ANOVA was applied to determine whether there was any significant difference in abundance between day and night and between surface and bottom. While emergence of 15 taxa (65%) was significantly different with regard to the day/night factor, that of the other 8 taxa (35%) was not. As to the difference between the two depths, the distribution of 15 taxa (65%) was significantly different. Furthermore, in 25 taxa for which over 100 specimens were collected in each of the two depths of water, emergence patterns were examined with regard to the synchrony with day/night and tidal cycles. There were various nocturnal patterns, and the strength of the synchrony with the day/night cycle was different for each species or group. Within the same taxa, nocturnal patterns were more clearly manifested in the surface water than in the bottom water. A tidal rhythm of emergence was only seen in zoeas of shrimp. Variations of the emergence patterns of benthic crustaceans are accounted for by a hypothesis that the frequency of swimming during day versus at night is different in each species. On the other hand, the emergence patterns of some zooplankton reflect by the daily rhythm of vertical migration or dispersal in the water column.

Behaviour and sensory physiology of Atlantic menhaden larvae, Brevoortia tyrannus, during horizontal transport

Atlantic menhaden, Brevoortia tyrannus, spawn on the continental shelf off North Carolina in the late fall and winter. Larvae are transported shoreward where they enter and migrate up estuaries to the juvenile habitat. In this paper, we synthesize behavioural and sensory physiological studies of the ontogeny of biological rhythms, responses to temperature, salinity and light, and swimbladder inflation and deflation of this species, to develop descriptive models of larval behaviour in ocean and estuarine areas that contribute to horizontal transport. Young (postyoke sac, &lt;9 mm total length, TL) larvae occur primarily offshore, have an endogenous rhythm in nocturnal diel vertical migration, and are predicted to remain at depths above the pycnocline, if present, due to responses to light, temperature and salinity. They ascend in response to a salinity increase and temperature decrease, which frequently occur at the pycnocline. Intermediate age (9–17 mm TL) and older (18–27 mm TL) larvae are found in inshore and estuarine areas. They have an endogenous activity rhythm that suggests they will remain at moderate depths during the day in the ocean. At sunset, they swim up to surface to engulf air to fill their swimbladders and then sink due to inactivity and negative buoyancy. Thus, at night the majority of larvae are deeper than during the day. The swimbladder is deflated at sunrise in response to light and larvae are predicted to swim up to their daytime depth. They have the same responses to a temperature decrease and salinity increase as younger larvae, which should keep them above a pycnocline. The ascent to fill the swimbladder is not impeded by responses to temperature or salinity change. Chemical cues in ocean and estuarine waters induce different behavioural responses. Larvae use selective tidal stream transport for up‐estuary movement in which they are in the water column on rising tides at night and not abundant at other times. Larvae lack an endogenous tidal rhythm in vertical migration upon entering an estuary, and swimming during the day is reduced by light. The ascent into the water column during flood tides may be cued by the rate of increase in salinity at this time. Collectively, these behaviours contribute to feeding, predator avoidance and transport from the spawning area to the juvenile habitat.