The ontogeny of phototaxis by larvae of the crab Rhithropanopeus harrisii

Phototaxis by each zoeal stage of the crab Rhithropanopeus harrisii (Gould) was quantitatively measured by means of a microscope closed-circuit television system. The megalopa stage is indifferent to light stimulation and was thus not tested. The action spectrum for positive phototaxis is similar at each zoeal stage, having the most pronounced maximum at about 500 nm and smaller maxima at 400 and 280 nm. Responsiveness to various intensities of 500 nm light is also similar at each stage. After dark-adaptation strongest positive phototaxis occurs at intensities between 1.0 and 3.0x10^-4 W/m², with no negative phototaxis to lower intensities. After light-adaptation, the positive response occurs to higher intensities between 20 and 0.01 W/m², with a pronounced negative response to lower intensities. Generally, mean swimming speeds during positive phototaxis do not change with stimulation intensity, although during negative phototaxis mean speeds do vary with intensity. At the highest intensities which initiate a negative response, mean values are significantly greater than values for positive phototaxis. Based on the pattern of phototaxis at each developmental stage, a prediction of events during diurnal vertical migration is possible. The negative response after light-adaptation might also function as a shadow reflex.     

Behavioral responses of larvae of the crab Rhithropanopeus harrisii (Brachyura: Xanthidae) during diel vertical migration

Larval Rhithropanopeus harrisii (Gould) show nocturnal vertical migration. Larval behavioral responses to different rates of increase and decrease in light intensity were measured in an apparatus with a natural angular light distribution. A central objective was to establish whether phototaxis actually participates in vertical migration. At sunset the level of light adaptation controlled the readiness of the larvae to migrate, while an ascent was initiated by a preductable relative decrease in intensity (e.g. 4.0x10^-3s^-1). Rates of relative decrease around sunset would evoke continuous upward swimming. Gravity was the orienting cue and there was no change in swimming speed during the ascent. At sunrise, the larval descent was initiated by exposure to an absolute light intensity of about 0.23 log unit above the lower visual threshold. Light served as an orienting cue, as larvae descended by a negative phototaxis. Thus, phototaxis is not a laboratory artifact and does participate in vertical migration. A consideration of behavioral responses of other crustacean zooplankton indicates there is considerable variation in the initiating and directing cues for vertical movements. The variety of behavioral responses of R. harrisii suggests that a synthesis of hypotheses about migration may provide the proper basis for explaining the mechanisms underlying diel vertical migration.     

Comparative study of crustacean larval photoresponses

Ovigerous females of four brachyuran (Cancer gracilis, Lophopanopeus bellus bellus, Hemigrapsus oregonensis and Scyra acutiforns) and two anomuran (Pagurus beringanus and P. granosimanus) species, which live as adults in coastal areas, were collected near Friday Harbor, Washington, USA, in 1985, and spectral sensitivity and phototactic pattern of their larvae were measured. Responses were compared with previous measurements on estuarine species to determine whether responsiveness varies with adult habitat. Estuarine and coastal species have similar photoresponses. Spectral sensitivity of the test brachyran species had two maxima, one near 400 to 420 nm and another around 500 nm. The anomuran species were similar, but had an additional peak in the region of 580 to 620 nm. This sensitivity is adapted to daytime light conditions in the adult environment, and available spectra at the time of larval movement during diel vertical migration. Upon light adaptation and stimulation with a narrow light field, all six species showed positive phototaxis to high light intensities and a pronounced negative response to low intensities. Only the negative response will occur in natural underwater light conditions, and it is part of a predator-avoidance shadow-response which operates in areas of higher light intensity. The same phototactic pattern is observed in all species when darkadapted except H. oregonensis. If nocturnal vertical migration occurs, this negative response may be responsible for the descent at sunrise and depth maintenance during the day.     

Occurrence of a shadow response among brachyuran larvae

A closed-circuit television system was used to study phototaxis and behavioral responses upon sudden decreases in light intensity in light-adapted Stage I zoeae from 7 species of Brachyura (Panopeus herbstii, Menippe mercenaria, Uca pugilator, Callinectes sapidus, Sesarma cinereum, Pinnotheres maculatum, and Libinia emarginata). All species except one show positive phototaxis to high intensities of 500 nm light and negative phototaxis to low intensities. Upon a sudden light intensity decrease, larvae show a shadow response, which depends upon the initial and final intensities. If the initial intensity is sufficient to induce positive phototaxis, and the light is extinguished, the larvae of 6 species stop swimming and passively sink (sinking response). Reducing the intensity to an absolute level that causes negative phototaxis, induces a sinking response followed by negative phototaxis. For intensity reductions that occur in the intensity range above that for negative phototaxis, a sinking responseis observed. The intensity decrease that initiates the sinking response is independent of initial stimulus intensity and duration, is consistent between species, and is equivalent to a decrease by 0.5 OD (optical density) units. The sinking response is greatest at intensity decreases of at least 1.0 to 1.1 OD units. It is concluded that the shadow response is common in brachyuran larvae inhabiting coastal-estuarine areas.     

Responses of cirripede larvae to light. I. Experiments with white light

The phototactic response of the nauplius larva of Balanus balanoides, B. crenatus and Elminius modestus shows darkadaptation; the response of the cyprid of B. balanoides shows both phototaxis and low photokinesis. The phototactic responses and the orientation of the cyprid to white light at settlement require an intensity of illumination slightly above 10^-5 lux. The ability to select a shaded position by cyprids of R. balanoides requires a higher intensity of 10^-2 to 10^-4 lux; hence a different mechanism may be involved. Barnacle larvae are sufficiently sensitive to be able to respond to light beneath the sea surface, even on cloudy, moonless nights.     

Phototaxis in Fungiidae corals (Scleractinia)

Corals within the scleractinian family Fungiidae were observed to move toward light (positive phototaxis). Negative phototactic movement was not observed in any of the specimens tested. On sandy substratum, Diaseris distorta moved faster (max. speed of 3 cm h^-1) than other species. Although D. distorta has symbiotic algae, phototactic movement also was observed both in bleached corals and in those treated with a specific inhibitor (dichlorophenyl dimethyl urea) of photosynthesis. D. distorta was phototactic even on a glass plate, and climbed up a steep slope (up to 30°). Based on experiments with Fungia fungites and D. distorta, soft tissues at the peripheral region of the dise seem to be responsible for movement via peristalsis. It is suggested that positive phototaxis in symbiontbearing fungiid corals is an important trait for selection of favorable habitats.     

Depth regulation of larval marine decapod crustaceans: test of an hypothesis

This study tested the hypothesis that the dimensions and symmetry of the depth regulatory window of crustacean larvae are controlled by the level of light adaptation. Responses of first and last zoeal stages of the crab Rhithropanopeus harrisii (Gould) to different rates of pressure change were analyzed with a video system. Crabs were collected from the Neuse River estuary (North Carolina, USA) from May to September 1988. Responses were measured when larvae were adapted to light having an angular light distribution similar to that underwater at intensities ranging from one log unit above the lower phototaxis threshold to four log units higher. For both zoeal stages in darkness and at 10^-6 W m^-2, the distance larvae descend before responding to a pressure increase was much shorter than the distance they would ascend before responding to a pressure decrease. When adapted to a light level of 10^-4 W m^-2 both zoeal stages descended and ascended approximately equal distances before responding to an increase or decrease in pressure, respectively. Finally at the highest test light intensity (10^-2 W m^-2), the ascent distance was much shorter than the descent distance. These results support the hypothesis. The depth regulatory window dimensions predict an ascent in the water column upon adaptation to low light intensities and descent at high light levels. Thus Sulkin's negative feedback model provides the general mechanism of depth regulation. The effects of light adaptation on the limits of the depth regulatory window provide an additional component that negates the requirement for depth regulation at an absolute depth. The composite model can be termed the light-dependent negative feedback model of depth regulation.     

Behavioral responses of a larval crustacean to hydrostatic pressure: Rhithropanopeus harrisii (Brachyura: Xanthidae)

Responses of the four zoeal stages of the crab Rhithropanopeus harrisii (Gould) to step and continuous changes in hydrostatic pressure were analyzed with a video system. Crabs were collected from the Neuse River estuary (North Carolina, USA) from June to August, 1987. The lower thresholds for step increases and decreases in pressure were 3 and 8 to 10 mbar, respectively. There was little change in sensitivity with zoeal development. Tests of larval responses in a light field that simulated the underwater angular light distribution indicated positive phototaxis does not occur upon pressure changes. In darkness, rates of pressure increase at and above 0.175 mbar s^-1 induced high barokinesis and negative geotaxis in all but Stage IV zoeae, which had a threshold of 1.19 mbar s^-1. Since larval sinking and descent swimming speeds exceed these threshold rates, larvae can move rapidly enough to produce suprathreshold changes in pressure which evoke behavioral responses. Slow rates of pressure decrease induced passive sinking while rapid rates caused an active ascent. This ascent response upon a pressure decrease is unreported among crustaceans, and is hypothesized to function for avoidance of feeding and respiratory currents of benthic invertebrates. The descent response occurs in all zoeal stages, except IV, at rates of pressure decrease (0.4 to 0.71 mbar s^-1) that are within the range of ascent swimming speeds. These results support Sulkin's negative feedback depth regulation model. The absolute distances moved before corrective vertical responses to threshold rates of pressure change are initated delimit the depth regulatory window. In darkness, the asymmetry of the window would lead to an ascent. It is hypothesized that light is an additional component in depth regulation, and that the limits and symmetry of the depth regulatory window may be controlled by the level of light adaptation.     

Vision in the lanternfish Stenobrachius leucopsarus (Myctophidae)

The visual system of the midwater fish Stenobrachius leucopsarus (Myctophidae) has been studied by biochemical, anatomical, and electrophysiological methods. Partial bleaching analysis in the presence of hydroxylamine showed that the eye contains a single extractable photopigment, based on retinal and absorbing maximally at 492 nm. The photoreceptor population consists entirely of rods, approximately 5.0×10⁵ rods per mm² and 1.8×10⁷ per retina. Visual sensitivity is enhanced by the lack of pigment granules in the pigment epithelium and the presence of a choroidal tapetum lucidum. Influenced by the high concentration of visual pigment, the spectral sensitivity has a broad plateau between 460 and 540 nm, which suggests that the eye retains high sensitivity to a diversity of bioluminescent stimuli and depth-attenuated solar light.     

Kinetics of light-intensity adaptation in a marine planktonic diatom

The marine planktonic diatom Thalassiosira weisflogii was grown in turbidostat culture under both continuous and 12 hL: 12 hD illumination regimes in order to study the kinetics of adaptation to growth-irradiance levels. In both illumination regimes adaptation to a higher growth-irradiance level was accompanied by an increase in cell division rates and a decrease in chlorophyll a cell^-1. The rates of adaptation for both processes, derived from first order kinetic analysis, equaled each other in each experiment. The results suggest that during the transition from low-to-high growth-irradiance levels chlorophyll a is diluted by cell division and is not actively degraded. Introduction of a light/dark cycle lowered the rate of adaptation. In transitions from high-to-low growth-irradiance levels there was a sharp drop in growth rates and a slow increase in chlorophyll a cell^-1 under both continuous and intermittent illumination. In the 12 hL:12hD cycle there was a circadian rhythm in chlorophyll a cell^-1, where cellular chlorophyll contents increased during the light cycle and decreased during the dark cycle. This circadian rhythm was distinctly different from light intensity adaptation. For kinetic analysis of light intensity adaptation in a 12 hL: 12 hD cycle, the circadian periodicity was separated from the light intensity response by subjecting the data to a Kaiser window optimization digital filter. Kinetic parameters for light-intensity adaptation were resolved from the filtered data. The kinetics of lightintensity adaptation of marine phytoplankton are discussed in relation to their spatial variations and time scales of mixing.This research was performed at Brookhaven National Laboratory under the auspices of the United States Department of Energy under Contract No. DE-AC02-76 CH00016     

Impact of copepod grazing on the red-tide flagellate Chattonella antiqua

Although planktonic copepods are major suspension feeders in the sea, the impact of their grazing pressure upon red-tide flagellates has not been fully investigated. In the present study, the grazing of adult females of several copepod species is examined using three food types: viz. natural suspended particles, natural suspended particles mixed with cultured Chattonella antiqua, and cultured C. antiqua. The functional response on C. antiqua was investigated for five species of copepods (Acartia erythraea, Calanus sinicus, Centropages yamadai, Paracalanus parvus and Pseudodiaptomus marinus). Ingestion rates increased linearly with increasing cell concentrations until a maximum level was reached, beyond which the rates were constant. This cell concentration was higher for larger copepods. The weight-specific maximum ingestion rates were higher in the small species. In general, copepods tended to feed selectively on larger particles when feeding on natural particles. This tendency was strongest in a simulated red-tide environment. Thus, it can be surmised that copepods may selectively graze on C. antiqua during the outbreak of a red tide. Grazing pressure by the natural copepod community in Harima Nada, the Inland Sea of Japan, was calculated by integration of the laboratory determined feeding rates and field measurements of zooplankton biomass. The daily removal rate was 3.4 to 30.8% (mean: 12.3%) of C. antiqua biomass at 20 cells ml^-1 and decreased to 0.6–4.3% (mean: 1.8%) at 500 cells ml^-1. Therefore, the grazing pressure by the copepod community is important at the initial stage of the red tide.     

Rapid response to changing light environments of the calanoid copepod<Emphasis Type="Italic"> Calanus sinicus</Emphasis>

Calanus sinicus is a large calanoid copepod and a dominant species in the coastal waters of Japan. During a research cruise in Sagami Bay on 18 June 1996, we found C. sinicus performing an unusual diel vertical migration (DVM), a behavior that has not been reported in previous studies on this species. This study examined the DVM of C. sinicus under different light environments and revealed the copepods characteristic response to light. Field and laboratory results show that the DVM of C. sinicus is flexible and also confirmed its sensitivity and its rapid response to changing light environments. It is suggested that C. sinicus reacts to changes in absolute light intensity. This feature may be common in oceanic copepod species. The copepods quick reaction to light variation provides decreased predation risks and increased feeding opportunities, which make them a dominant survivor in coastal water habitats.Communicated by T. Ikeda, Hakodate     

Analysis of phytoplankton pigments by HPLC before,during and after mass occurrence of the microflagellate Corymbellus aureus during the spring bloom in the open northern North Sea in 1983

Daily observations in April and May 1983 near a subsurface drifter launched in the Fladen Grounds area of the North Sea revealed that a large crop of the microflagellate Corymbellus aureus Green, producing up to 3 g C m^-2 d^-1, succeeded the diatoms dominating during the earlier phase of the spring bloom. The concentration of fucoxanthin was highest during the first half of May, while pigment fingerprints of high-performance liquid chromatograms of suspended matter sampled during the second half of May were dominated by 19-hexanoyloxyfucoxanthin, the main carotenoid of C. aureus (0.8x10^-9 mg cell^-1). This colonial Prymnesiophycean was described for the first time less than 10 years ago and has nerver been reported to be present in the North Sea. After 16 May the C. aureus population rapidly lost its growth potential, but the peak of the bloom in terms of cell number was found several days later (up to 9x10⁶ cells per dm³ on 19 May). The population then started to decline; senescence was associated with decreasing pigment contents of cells. The low growth rate of copepods registered during the second half of May was probably related to the poor quality of C. aureus as copepod food: the concentration of a phaeophorbide a typically found in copepod fecal pellets was highest during the diatom phase of the spring bloom preceding the C. aureus bloom.     

Diel variation in chlorophyll a,carbohydrate and protein content of the marine diatom Skeletonema costatum

Skeletonema costatum (Greville) Cleve isolated from Narragansett Bay, USA, was incubated at 3 light intensities (ca. 0.008, 0.040 and 0.075 ly min^-1) under a 12 h light: 12 h dark (12L:12D) photoperiod at 2°, 10° and 20°C. Cellular chlorophyll a increased at intensities less than ca. 0.040 ly min^-1; increases occured within one photoperiod at temperatures above 10°C. Cellular carbohydrate increased with light intensity at all temperatures; increases during the photophase were due to net production of the dilute acid-soluble fraction. Cellular protein increased during the photoperiod at 10° and 20°C; there was little difference in cellular protein among all cultures after one photoperiod. The rate at which cellular chlorophyll a increased in response to a decrease in light suggests that diel variation in cellular chlorophyll a is temperature-dependent in S. costatum. Protein: carbohydrate ratios ranged from ca. 0.5 to 2.0 over a diel cycle; ratios increased at lower intensities and higher temperatures. The diel range in protein:carbohydrate ratios equals that in cultures developing nitrogen deficiency; thus, use of this ratio as an index to phytoplankton physiological state must account for diel light effects.     

Omnivorous feeding behavior of the Antarctic krill Euphausia superba

Feeding experiments were conducted at Palmer Station from December 1985 to February 1986 to examine the potential role of copepod prey as an alternative food source for Euphausia superba. Copepod concentration, copepod size, phytoplankton concentration, the duration of krill starvation and the volume of experimental vessels were altered to determine effects on ingestion and clearance rates. Krill allowed to feed on phytoplankton and copepods in 50-litre tubs showed greatly increased feeding rates relative to animals feeding in the much smaller volumes of water traditionally used for krill-feeding studies. Clearance rates on copepods remained constant over the range of concentrations offered, but clearance rates on phytoplankton increased linearly with phytoplankton concentration. Feeding rates increased when larger copepods were offered and when krill were starved for two weeks prior to experiments. Clearance rates of krill feeding on copepods were higher than, but not correlated with, their clearance rates on phytoplankton in the same vessel. E. superba may have a distinct mechanism for capturing copepods, perhaps through mechanoreception. Although our observed clearance rate of 1055 ml krill^-1 h^-1 indicates that krill can feed very efficiently on copepod prey, such feeding would meet less than 10% of their minimum metabolic requirements at the typical copepod concentrations reported for Antarctic waters. However, substantial energy could be gained if krill fed on the patches of high copepod concentrations occasionally reported during the austral summer, or if krill and copepods were concentrated beneath the sea ice during the winter or spring months. Our results, indicating efficient feeding on zooplankton and higher clearance rates on phytoplankton than previously believed, represent a step towards balancing the energy budget of E. superba in Antarctic waters.     

Growth and competition of the marine diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana. II. Light limitation

The marine diatoms Phaeodactylum tricornutum (Bohlin) and Thalassiosira pseudonana (Hasle and Heimdal) were grown under both continous illumination and a 14 h light: 10 h dark cycle at light intensities ranging from 1.53×10^-4 to 2.95×10^-1 ly min^-1. Under both photoperiods, T. pseudonana exhibited higher division rates than P. tricornutum at high light intensities, but the reverse was true at all light intensities <3×10^-3 ly min^-1. Comparison of these results with available data on light-limited growth of other planktonic algae suggests that P. tricornutum may be unusually efficient at maintaining its cell division rate at low light intensity. This efficiency may contribute substantially to its success in turbid, nutrient-enriched mass algal culture systems, the only environments in which it is known to attain great numbers.Contribution No. 4086 from the Woods Hole Oceanographic Institution.     

Photoadaption in marine phytoplankton: Response of the photosynthetic unit

Some species of phytoplankton adapt to low light intensities by increasing the size of the photosynthetic unit (PSU), which is the ratio of light-harvesting pigments to P700 (reaction-center chlorophyll of Photosystem I). PSU size was determined for 7 species of marine phytoplankton grown at 2 light intensities: high (300 E m^-2 s^-1) and low (4 E m^-2 s^-1); PSU size was also determined for 3 species grown at only high light intensity. PSU size varied among species grown at high light from 380 for Dunaliella euchlora to 915 for Chaetoceros danicus. For most species grown at low light intensity, PSU size increased, while the percentage increase varied among species from 13 to 130%. No change in PSU size was observed for D. euchlora. Photosynthetic efficiency per chlorophyll a (determined from the initial slope of a curve relating photosynthetic rate to light intensity) varied inversely with PSU size. In contrast, photosynthetic efficiency per P700 was enhanced at larger PSU sizes. Therefore, phytoplankton species with intrinsically large PSU sizes probably respond more readily to the rapid fluctuations in light intensity that such organisms experience in the mixed layer.Contribution No. 1180 from the Department of Oceanography, University of Washington, Seattle, Washington, USA     

Copepod photobehavior in a simulated natural light environment and its relation to nocturnal vertical migration

To investigate the roles of light in initiating, controlling and directing nocturnal vertical migration, photoresponses of the adult, female copepod Acartia tonsa Dana were measured under simulated natural underwater light conditions using a video system. Copepods were adapted to a range of background light levels and tested with the following stimuli: absolute quantal intensity, absolute change in quantal intensity and relative (%) change in quantal intensity. The stimulus initiating vertical movements was relative change in quantal intensity, while responsiveness was controlled by the level of light adaptation. A. tonsa swam upward in response to decreases. Response with minimal stimulation occurred at an adaptation intensity close to that in the copepod's natural habitat at the time of the migratory ascent (near the bottom of the Newport River estuary, North Carolina, near sunset). Analysis of the angles of upward movement showed that light is not a directional cue. Relative increases in intensity resulted in sinking, with minimal stimulation required at an adaptation intensity close to that in the field when the migratory descent occurs near sunrise. These results offer a reasonable explanation of how light cues may shape nocturnal vertical migratory patterns.     

Effect of light on apical length growth of Fucus distichus spp. edentatus (Phaeophyta)

The length growth of Fucus distichus L. ssp. edentatus (Pyl.) Powell in continuous light is compared to growth in natural daylight. In continuous light the length growth of vegetative and fertile apices were saturated at 25 to 40 Wm^-2 and 15 to 30 Wm^-2, respectively. In natural daylight vegetative apices reached saturation at about 300 Wm^-2 (in May), while fertile apices were not saturated at 180 Wm^-2 (in March). This shows that saturation levels are dependent on the light regime and that they may be greatly underestimated when continuous light is used. At 20 Wm^-2 vegetative and fertile apices were daylength saturated at 8 and 12 to 16 h, respectively. Maximum growth in vegetative apices occurred in July, but growth rates were high also during wintertime (45% of July maximum in March at 4.0°–4.2°C). This corresponds to the experimental results which indicate that F. d. edentatus grows well at low irradiances and during short photoperiods.     

Food concentration and stocking density effects on survival and growth of laboratory-reared larvae of bay anchovy Anchoa mitchilli and lined sole Achirus lineatus

Bay anchovy (Anchoa mitchilli) eggs were stocked at densities from 0.5 to 32.0 l^-1 and larvae were fed on wild plankton (copepod nauplii) in concentrations that ranged from 50 to 5000 prey l^-1. Lined sole (Achirus lineatus) eggs were stocked at 0.5 to 16.0 l^-1 and larvae were fed wild plankton at concentrations from 50 to 1000 prey l^-1. Some larvae of each species survived at all stock and food levels to the transformation stage at 16 days after hatching. Survival rates for both species exceeded 40% when food concentration was 1000 l^-1 or higher. Growth and dry weight yields also increased significantly at the higher food concentrations. Effects of initial stocking density were not well defined, but both survival and growth decreased at the highest stocking rates. Standardized culture of bay anchovy and lined sole larvae can be based on a food concentration of 1000 copepod nauplii l^-1 to routinely produce healthy larvae.