Evidence for behavioral sensitivity to near-UV light in the deep-sea crustacean Systellaspis debilis

The role of UV light in the deep-sea environment has been discounted in the past, due to the assumptions that (1) there is insufficient UV light available for vision and, therefore (2) deep-sea organisms would not be sensitive to these wavelengths. A recent study that employed electrophysiological techniques on dark-captured deep-sea crustaceans demonstrated that several species of deep-sea crustaceans possess very high sensitivity to near-UV light. The current study was undertaken to determine if near-UV light would also elicit a behavioral response from these species. The species studied was Systellaspis debilis, an oplophorid shrimp whose daytime depth ranges from 600 to 700 m. A method for tethering shrimp was developed which allowed them to freely orient in response to changes in the ambient light field. Behavioral responses to changes in ambient light included changes in body tilt with respect to the horizontal plane, changes in swimming speed, and movement of the feeding appendages. These experiments, the first of their kind on a deep-sea organism, demonstrate that behaviorally, S. debilis is equally sensitive to very low intensities of blue-green and near-UV light.     

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.     

Photosensitivity of the calanoid copepod Acartia tonsa

The light intensity and spectral sensitivities of the calanoid copepod Acartia tonsa Dana were determined by measuring phototactic responses. Adult females displayed only positive phototaxis. The dark-adapted copepod, which possesses a single naupliar eye, perceived light at intensities as low as 2.8x10¹¹ photons m^-2 s^-1. The action spectrum for positive phototaxis had no clear maxima but rather showed a broad range of greatest sensitivity from 453 to 620 nm. This sensitivity encompassed those wavelengths that are maximally available at the depth where the copepod is found during the day. This spectral overlap, coupled with the finding that the copepod requires light cues for nocturnal vertical miration, suggests that broad spectral sensitivity is an adaptive mechanism to maximize light intensity sensitivity during migration.     

Spectral composition of bioluminescence of epipelagic organisms from the Sargasso Sea

The spectral characteristics of single identified epipelagic sources of bioluminescence from the western Sargasso Sea were measured with an optical multichannel analyzer (OMA) system during the April, 1985, Biowatt cruise. The emission spectra of specimens representing 45 species from 8 phyla were measured. Peak bioluminescence emissions typically occurred between 440 and 500 nm, in the blue region of the visible spectrum. Three exceptions involved emission in the green, yellow, and red spectral regions. Intraspectific variability in spectra, was noted in several species. One shrimp species exhibited two modes of light emission, each with different emission spectra. Other cases involved dynamic color shifts of 10 to 14 nm; the source of the spectral variability is unknown, but may involve optical filtering or differences in the color of luminescence from multiple sites of light emission. Measurements from independent samples of unsorted plankton revealed different spectral distributions. This suggests that the spectral emissions of bioluminescence in the upper water column will vary, based on species assemblage.     

Photopigment and spectral sensitivity in the bioluminescent fish Porichthys notatus

Digitonin extracts from retinas of the marine fish Porichthys notatus, from California and Puget Sound (USA), contain a single photopigment with maximum absorbance at 498 nm. In the presence of hydroxylamine the final product of bleaching absorbs maximally at 367 nm, indicating that the photopigment is based on retinal. Spectral sensitivity measurements performed on intact eyes and eye-cup preparations in darkadapted and selectively light-adapted states yield curves which suggest that the retina of P. notatus contains at least 3 different types of receptors. In the darkadapted state, the spectral sensitivity curve has a broad peak, with a maximum between 480 and 500 nm. In the blue and red light-adapted states, in addition to depressing the sensitivity, the shape of the sensitivity curve also changes. Under these conditions new peaks appear which suggest the presence of at least two different types of cones, with sensitivity maxima in the blue and the green regions of the spectrum, respectively. Comparison of the spectral sensitivity curves with the in vivo bioluminescence emission spectrum of the fish gives an especially good correspondence and suggests that the fish is able to perceive the bioluminescence of its own species.     

Green and blue fluorescing dinoflagellates in Bahamian waters

At three stations in Bahamas waters, in 1989, 15 to 30% of all the dinoflagellates >20µm diameter observed in near-surface waters fluoresced green under blue excitation light, 55 to 66% fluoresced red, and the remainder did not fluoresce at all. The abundance of these green-fluorescing dinoflagellates ranged from ca 5 to 10 cells l^–1 at the study sites. Under UV excitation, however, the dinoflagellates had a blue to blue-green appearance. Almost all the blue-green fluorescing dinoflagellates appeared to be heterotrophic, except for one species,Phalacroma rapa Stein, which also contained red-fluorescing (under blue light) chlorophylla. The emission spectra from all species examined were of three basic types. Type 1 typically had two fluorescence emission peaks (ca 440 and ca 510 nm). Type 2 spectra possessed one sharp peak at 495 nm. Spectra belonging to Type 3 had a broad peak around 470 to 480 nm. The green fluorescence thus is likely caused by different substances in individual species. The attempt to reconstitute observed spectra with nicotinamide adenine dinucleotide (NADH) and riboflavin 5-phosphate (FMN) solutions was unsuccessful.     

Identification of marine organisms using kinetic and spectral properties of their bioluminescence

We have used a polychromator consisting of six photomultiplier tubes, each filtered to a different wavelength with narrow band-pass interference-filters, to study bioluminescence. Spectral and kinetic data collected from ten marine species in the laboratory describe their luminous flashes. These data suggest that the concept of luminous signatures, within the limits of our studies, is a valid one, with potential uses for future biological studies both in the laboratory and in situ. The kinetic parameters considered were rise time (RT), decay time (DT) and total time (TT), while the spectral parameters consisted of ratios of light intensities at 480 and 520 nm to the intensity at 500nm. Oneway analysis of variance (ANOVA) demonstrated significant heterogeneity among species for all variables. A posteriori analysis performed with the ANOVA for TT indicated that mean TT for most species is significantly different from all other species. Canonical discriminant analysis was performed to estimate the value of kinetic and spectral parameters for species' identification. Kinetic data were somewhat more valuable in species' classification than spectral data. Discriminant analysis with RT and DT alone gave 83.1% correct species-classifications. Classification success based only on relative intensities at 480 and 520 nm was 77.5%. When all four variables were included, classification success was 100%.     

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.     

Bioluminescence spectra of shallow and deep-sea gelatinous zooplankton: ctenophores, medusae and siphonophores

We have examined the variability and potential adaptive significance of the wavelengths of light produced by gelatinous zooplankton. Bioluminescence spectra were measured from 100 species of planktonic cnidarians and ctenophores collected between 1 and 3500 m depth. Species averages of maximal wavelengths for all groups ranged from 440 to 506 nm. Ctenophores (41 species) had characteristically longer wavelengths than medusae (34 species), and the wavelengths from siphonophores (25 species) had a bimodal distribution across species. Four species each produced two different wavelengths of light, and in the siphonophore Abylopsistetragona these differences were associated with specific body regions. Light from deep-dwelling species had significantly shorter wavelengths than light from shallow species in both ctenophores (p = 0.010) and medusae (p = 0.009). Although light production in these organisms was limited to the blue-green wavelengths, it appears that within this range, colors are well-adapted to the particular environment which the species inhabit. Received: 27 April 1998 / Accepted: 27 October 1998     

Spectral sensitivity and visual pigment of the compound eye of the galatheid crab Pleuroncodes planipes

The compound eye of the galatheid crab Pleuroncodes planipes has been studied by means of visual pigment extractions and spectral sensitivity measurements under various conditions of adaptation. A photosensitive pigment with maximum absorption at 503 nm was identified. Its chromophore is retinal. The spectral sensitivity maximum is near 523 nm in the dark-adapted eye. Selective adaptation experiments suggest that the apical region of the eye has a homogeneous receptor population. The spectral sensitivity characteristics of P. planipes seem to be an adaptation to its mainly benthic life.     

Microplankton of the Gulf of Elat: Aspects of seasonal and bathymetric distribution

The components of 4 major groups of microplankton were identified and their numerical abundance determined in net samples collected at depth intervals down to 600 m at a permanent station off the H. Steinitz Marine Biological Laboratory, Elat. The samples analyzed were collected once a fortnight over a period of 1 year beginning in June 1974. The groups studied were the Cyanophyta, Bacillariophyta and Pyrrhophyta of the phytoplankton and the Tintinnina of the microzooplankton. The pattern of vertical distribution of the phytoplankton as a whole showed a general decrease in cell numbers with increasing depth. The blue-green algae, consisting mainly of Trichodesmium sp. trichomes, were confined primarily to the upper 100 m. The diatoms were unevenly distributed, with one species, a minute centric diatom, Thalassiosira subtilis, associated with a massive bloom during March 1975 between 300 and 400 m. The peridinians, the group with the largest number of species, included forms which were evenly distributed throughout the whole water column and forms limited either to the upper or deeper water strata in accordance with their light intensity preferences. The mass occurrence of newly-emerged dinoflagellate cysts of Pyrophacus horologicum, a weakly-armoured dinoflagellate, in the 200 to 300 m depth interval during April 1975, suggests that reproductive processes in dinoflagellates may also be light-controlled. The tintinnids, like the phytoplankton groups, were most abundant in the upper 100 m with a gradual decrease in numbers of individuals, though not in species, in the deeper water strata. The overall yearly pattern of microplankton distribution indicates 3 peaks: late fall and early summer peaks consisting primarily of blue-green algae and one in early spring consisting of several species of diatoms and peridinians and of species of tintinnids which thrive in the same niches as the phytoplankton. Both phytoplankton and tintinnid production was lowest during the summer months.     

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.     

The speed of an isolume: a shrimp's eye view

Using data from electroretinogram recordings, we designed a submersible light meter with a spectral luminous efficiency comparable to that of a vertically migrating shrimp. Using this light meter on the “Johnson Sea Link” submersible, we were able to ascend with the isolumes (preferred light levels) associated with the sergestid and euphausiid shrimp layers. The speed of the submersible's movement was recorded and correlated with simultaneous measurements of surface irradiance. In situ measurements of the downwelling spectral distribution were also made with a fiber optic spectrometer. The average measured speed of the sergestid isolume was 8.8 cm s⁻¹, while that of the shallower euphausiid isolume was 6.7 cm s⁻¹. These values are different from those calculated using average diffuse attenuation coefficients and surface light measurements. This difference was due to the broadening of the spectral distribution of downwelling light above 120 m and to the variability of the diffuse attenuation coefficients with depth. These results clearly demonstrate that when investigating the relationship between light and vertical migration patterns, it is important that: (1) the light measurements be made in situ and (2) such measurements take into account the spectral sensitivity of the eyes of the migrators. Received: 7 June 2000 / Accepted: 18 November 2000     

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.     

Evaluating risks associated with transport of the ghost shrimp <Emphasis Type="Italic">Neotrypaea californiensis</Emphasis> as live bait

The ghost shrimp Neotrypaea californiensis is imported into southern California from Oregon and Washington for use as live bait in recreational marine fisheries. We studied the population genetic structure of N. californiensis across much its range to assess the possibility that the transport of ghost shrimp across phylogeographic boundaries poses a risk of homogenizing existing genetic variation in the species. Analyses of two mitochondrial DNA markers showed little phylogeographic structure across the sampled range, suggesting that this risk is low. Unexpectedly, mitochondrial DNA analyses revealed that a second putative species of ghost shrimp frequently coexisted with N. californiensis in southern California intertidal habitats; almost all previous studies of soft-sediment communities in the region report the presence of N. californiensis only. We also assessed the possibility that the import of ghost shrimp might pose a risk of introduction of a parasitic castrator, the bopyrid isopod Ione cornuta, into southern California waters, where it does not appear to be native. Prevalence of living I. cornuta in samples purchased from bait shops was high (5.8%), suggesting that this is a real risk that merits further study.     

Impact of time since collection on avian eggshell color: a comparison of museum and fresh egg specimens

Studies of avian eggshell coloration have been a long-standing research focus in behavioral evolutionary ecology. Museum collections have provided a widely used resource because they allow efficient sampling across broad temporal, geographical, and taxonomic ranges, even for species that are rare and for which sampling in the wild is ethically or practically unwarranted. We used reflectance spectrophotometry across the avian visual spectrum to compare eggshell color of specimens of the song thrush (Turdus philomelos) in two museums (Natural History Museum, UK and Auckland Museum, New Zealand) with each other and with eggshells collected fresh in New Zealand. These data enabled us to test the effects of source and storage in different museums, as well as time since collection, across four metrics of eggshell coloration: blue-green and ultraviolet chroma, overall brightness, and the spectral coefficient of variation. Variation within an egg, within a clutch, and among clutches, was similar between the two museum datasets but different from those of fresh eggs. We found significant differences in all four metrics between the collections, and that fresh eggshells reflected stronger in the blue-green wavelength than in museum eggs. This difference is most likely due to different preservation techniques and storage histories. Furthermore, an effect of time since collection was only apparent in the blue-green chroma and was higher in more recent museum eggshell samples. Our results support the use of historic museum samples in intraspecific studies of shell coloration providing that efforts are made to compare specimens, which were collected during similar periods.     

Vertical distributions and photosynthetic action spectre of two oceanic picophytopianlcters,Prochlorococcus marinus andSynechococcus sp.

Two picophytoplankters,Prochlorococcus marinus andSynechococcus sp., were isolated from the bottom of the euphotic zone (150 m depth) in the western Pacifie Ocean. The concentration ofP. marinus at this depth was more than 10⁴ cells ml^–1 while that ofSynechococcus sp. was less than 10² cells ml^–1. TheP. marinus isolate has a high divinyl-chlorophylla:b ratio similar to that of the Mediterranean strain, while theSynechococcus sp. isolate is of the phycourobilinrich type. The growth rate ofP. marinus was higher thanSynechococcus sp. when both were cultured under weak blue-green to blue-violet light (ca. 2 E m^–2 s^–1). While the chlorophyll-specific absorption spectra showed higher values inSynechococcus sp., the photosynthetic action spectre revealed thatP. marinus was able to use blue-violet light, whereasSynechococcus sp. was able to use blue-green light, more efficiently for photosynthesis. The photosynthetic quantum yield ofP. marinus was higher than that ofSynechococcus sp. at any wavelength between 400 and 700 nm. The calculated in situ photosynthesis rates per Gell volume forP. marinus were estimated to be higher than forSynechococcus sp. at 50 and 150 m depth. These results indicate thatP. marinus photosynthetically surpassesSynechococcus sp. in the blue-light-rieh environment of the oceanic euphotic zone. This may be why the former predominates at depths in temperate to tropical open ocean waters.     

Ultraviolet radiation effects on the behavior and recruitment of larvae from the reef coral Porites astreoides

We tested the rarely considered hypothesis that the ultraviolet portion (UVR, 280–400 nm) of the light spectrum affects patterns of recruitment in reef-building corals. The premise for this hypothesis rests in the fact that biologically relevant intensities of UVR penetrate to considerable depths (>24 m) in the clear waters surrounding many coral reefs, and that reef organisms allocate substantial resources to prevent and repair UVR damage. The ability of larvae spawned by the brown morph of the Caribbean coral, Porites astreoides, to detect and avoid UVR was assessed in petri dishes where one-half of the dish was shielded from UVR and the other exposed. Observations made every 30 min between 10:30 and 13:30 h showed significantly higher densities of larvae swimming in regions shielded from UVR. To determine how this behavior affects settlement patterns, larvae collected from P. astreoides adults at 18 m depth were released into chambers deployed at 17 m depth where they were given a choice of three different light regions in which to settle: PAR (PAR=400–700 nm), PAR+UVAR (UVAR=320–400 nm), and PAR+UVAR+UVBR (UVBR=280–320 nm). At the end of the experiment, greater numbers of P. astreoides larvae had settled in the region of the tube where UVR was reduced than would be expected if dispersion were random. To our knowledge, this is the first demonstration in any reef-building coral species that planula larvae can detect UVR and that it affects their choice of a settlement site. These results indicate that the capacity to detect and avoid habitats with biologically damaging levels of UVR may be one factor contributing to the successful recruitment of coral larvae.     

Thresholds in Songbird Occurrence in Relation to Landscape Structure

Abstract:  Theory predicts the occurrence of threshold levels of habitat in landscapes, below which ecological processes change abruptly. Simulation models indicate that below critical thresholds, fragmentation of habitat influences patch occupancy by decreasing colonization rates and increasing rates of local extinction. Uncovering such putative relationships is important for understanding the demography of species and in developing sound conservation strategies. Using segmented logistic regression, we tested for thresholds in occurrence of 15 bird species as a function of the amount of suitable habitat at multiple scales (150–2000-m radii). Suitable habitat was defined quantitatively based on previously derived, spatially explicit distribution models for each species. The occurrence of 10 out of 15 species was influenced by the amount of habitat at a landscape scale (≥500-m radius). Of these species all but one were best predicted by threshold models. Six out of nine species exhibited asymptotic thresholds; the effects of habitat loss intensified at low amounts of habitat in a landscape. Landscape thresholds ranged from 8.6% habitat to 28.7% (     = 18.5 ± 2.6%[95% CI]). For two species landscape thresholds coincided with sensitivity to fragmentation; both species were more likely to occur in large patches, but only when the amount of habitat in a landscape was low. This supports the fragmentation threshold hypothesis. Nevertheless, the occurrence of most species appeared to be unaffected by fragmentation, regardless of the amount of habitat present at landscape extents. The thresholds we identified may be useful to managers in establishing conservation targets. Our results indicate that findings of landscape-scale studies conducted in regions with relatively high proportions of habitat and low fragmentation may not be applicable in regions with low habitat proportions and high fragmentation.     

Temporal adaptations in visual systems of deep-sea crustaceans

The temporal characteristics of the visual systems of deep-sea crustaceans were examined at varying light levels. Experimental organisms were collected off Hawaii and southern California in 1991 and 1992, and continually maintained in the dark. At the University of California Santa Barbara Marine Laboratory, the temporal components of both visual interneuron activity and electroretinograms (ERGs) of adult Gnathophausia ingens were tested at threshold and with background light levels up to 6 log units above threshold. To compare these responses with those of mid-water organisms inhabiting shallower depths, the temporal characteristics of the ERGs of Oplophorus spinosus and juvenile G. ingens were assessed. Adults of both species showed little or no change in the time components of their responses when light-adapted, except for the response latency. There was also no evidence of a circadian rhythm in visual sensitivity. O. spinosus, a vertical migrator, and juvenile G. ingens, which inhabit a shallower depth than the adults, exhibited more rapid responses than adult G. ingens. These varying responses are considered adaptations to the differences in light levels at the characteristic depths of the organisms.