Phototactic Response Research Articles

Negative phototactic response to UVR in three cosmopolitan rotifers: a video analysis approach

Ultraviolet radiation (UVR) is an environmental stressor in several ecosystems and can affect organisms’ survival and reproduction, and community structure. Rotifers cope with UVR stress adopting preventive behavioral and metabolic mechanisms. However, the demonstration of an immediate behavioral response in rotifers is missing. We investigated the short-term response of rotifers to UVR, by combining video analysis and movement ecology methods, in three common species: Brachionus calyciflorus, Keratella cochlearis, and Keratella quadrata. We recorded the behavior of B. calyciflorus (both sexes), K. quadrata, and K. cochlearis (females) exposed to white light, and to intermittent cycles of UVR (30:30 s). Individual trajectories were extracted from videos with open-source software. We found that B. calyciflorus females exposed to UVR exhibited strong negative phototaxis with increased swimming speed, and a weak positive phototaxis in males. Keratella cochlearis and K. quadrata showed a weaker response. Our study reveals a species-specific behavioral response to UVR in rotifers. Furthermore, we highlight how sexual dimorphism in B. calyciflorus does not only occur in morphology and movement, but also in behavioral traits. Our results help to understand zooplankton community dynamics by providing a mechanistic explanation of UVR response in one major zooplankton taxonomic group.

Decreased capture of natural enemies of pests in light traps with light‐emitting diode technology

Light traps, containing a fluorescent lamp, are commonly used to control pests in Chinese tea gardens. However, owing to them emitting light with a wide wavelength range, they can also attract and kill numerous natural enemies of such pests, which can potentially threaten the ecological balance of tea gardens. The light source in traps is a key factor determining the species of trapped insects. Based on interspecific differences in the phototactic response to diverse light of narrow wavelength among insects, a light source friendly to natural enemies of pests was designed in this study using light‐emitting diode chips. First, the wavelengths corresponding to the peaks of phototactic responses of two major tea pests, Ectropis obliqua (Lepidoptera: Geometridae) (385 nm) and Empoasca onukii (Hemiptera: Cicadellidae) (420 nm), and of 10 dominant natural enemies (380 nm) in tea gardens, were confirmed by indoor bioassays, and then 385‐nm and 420‐nm light‐emitting diode chips were selected to construct the light source for traps. Furthermore, a light trap combining a conical light source with suction fan equipment was designed to increase the quantity and lethality of catches. Finally, evaluation of the performance of the designed light trap was conducted in the field, revealing that it had the desired effect, namely, trapping more tea pests as well as fewer natural enemies compared with light traps with a fluorescent lamp.

Phototactic response of Frankliniella occidentalis to sticky traps with blue light emitting diodes in herb and Alstroemeria greenhouses

Blue and yellow sticky traps equipped with blue light emitting diodes (LEDs) were evaluated for their attractiveness to the western flower thrips (Frankliniella occidentalis Pergande) and compared to similar traps without light in two greenhouses with commercial production of either mixed herbs or Alstroemeria cut flowers. Blue traps were more attractive to F. occidentalis than the yellow traps in both crops, regardless of whether they were equipped with light or not. In herbs, the blue light equipped traps caught 1.7 to 2.5 times more thrips compared to blue traps without light, and 1.7 to 3.0 times more thrips than yellow traps with light. Blue light on both blue and yellow traps increased thrips catches in one out of two experiments in Alstroemeria. The blue light equipped traps caught 3.4 and 4.0 times more thrips than blue traps without light in coloured and white Alstroemeria cultivars, respectively, whereas yellow light equipped traps increased thrips catches 4.5 times compared to yellow traps without light in both coloured and white cultivars. The yellow light equipped traps caught, however, only equal to or only slightly more thrips than blue traps without light, and caught fewer thrips than the light equipped blue traps. The relative trapping efficiency of the different combinations of trap colour and light varied with experiment, crop and Alstroemeria cultivars. This suggests that factors other than merely the addition of light influenced the thrips' phototactic response to the traps. Such factors could be differences in the relative strength of the competition between attractive signals from traps and plants between the two crops and Alstroemeria cultivars, thrips density, seasonal lighting conditions or different pest management strategies and other operational procedures in the greenhouses. The light from the traps did not increase the thrips population on the plants below the traps. The implications of the results for thrips control and suggestions for further studies are discussed.

Effect of several factors on the phototactic response of the oriental armyworm, Mythimna separata (Lepidoptera: Noctuidae)

Abstract Recently, light traps equipped with light-emitting diode (LED) lights have been used to monitor and control nocturnal insect pests. The oriental armyworm, Mythimna separata Walker, is an important cosmopolitan agriculture pest. The phototactic responses of nocturnal insects, including moths are influenced by abiotic and biological factors. The present study aimed to evaluate the phototactic response to different wavelength LED lights to determine the wavelength to which M. separata adults are most sensitive, and then identify the effects of several factors on the phototactic response. Our experiments revealed that M. separata adult moths had a significantly higher attraction rate to the green (520 nm) LED light compared with other LED lights, the green LED light was 1.28 times more attractive the moths than the commercial double-wave light used as a control. The phototactic response of the adult moths was significantly influenced by several factors, including adaption time in darkness, age, mating status and sexuality. These findings suggest that using a green LED light as a light source in trap could be effective in monitoring or controlling M. separata moths. Our results may provide a theoretical and scientific basis for improving the light trap technique for M. separata moths.

Investigating fish behavioural responses to LED lights in trawls and potential applications for bycatch reduction in the Nephrops-directed fishery

AbstractLight-emitting diodes (LEDs) have been tested in trawl fisheries to reduce the bycatch of unwanted species through behavioural stimulation. Previous studies used LED lights to either highlight escaping routes or increase the contact rate with square-mesh panels. However, phototactic responses (moving towards or away from light sources) to LED lights could also be exploited to separate species during the catching process. We investigated if either positive or negative phototaxis can be used to improve fish vertical separation from Nephrops (Nephrops norvegicus) in the aft section of a horizontally separated trawl codend. The aim was to increase the proportion of fish entering the upper compartment. We conducted two different experiments in front of the separation into compartments, inserting green LED lights in the upper and lower netting panel, respectively. Species vertical separation was analysed and compared in two identical trawls towed in parallel, one equipped with lights and one without. We obtained significant changes in vertical separation, but no clear species-specific phototactic response was identified. Neither of the light positions improved fish separation from Nephrops. However, the potential of LED lights as behavioural stimulators is confirmed, and a more mechanistic understanding of light and fish vision may improve the results of future applications.

Centers for Disease Control-type light traps equipped with high-intensity light-emitting diodes as light sources for monitoring Anopheles mosquitoes

In this study the phototactic response of anopheline mosquitoes to different luminous intensity light-emitting diodes (LEDs) was investigated. Centers for Disease Control-type light traps were changed by replacement of the incandescent lamps by 5 mm round type green (520 nm) and blue (470 nm) LEDs of different luminous intensities: green-LED traps with luminous intensities of 10,000, 15,000 and 20,000 millicandela (mcd) and the blue-LED traps with luminous intensities of 4000, 12,000 and 15,000 mcd. Our data showed that increasing luminous intensity has an effect on the attraction of anopheline mosquitoes to light traps, highlighting the importance of taking LEDs and light sources of high luminous intensity into account when using light-trap collections in monitoring populations of Anopheles species.

Photogravitactic Microswimmers

AbstractPhototactic microorganisms are commonly observed to respond to natural sunlight by swimming upward against gravity. This study demonstrates that synthetic photochemically active microswimmers can also swim against gravity. The particles initially sediment and, when illuminated at low light intensities exhibit wall‐bound states of motion near the bottom surface. Upon increasing the intensity of light, the artificial swimmers lift off from the wall and swim against gravity and away from the light source. This motion in the bulk has been further confirmed using holographic microscopy. A theoretical model is presented within the framework of self‐diffusiophoresis, which allows to unequivocally identify the photochemical activity and the phototactic response as key mechanisms in the observed phenomenology. Since the lift‐off threshold intensity depends on the particle size, it can be exploited to selectively address particles with the same density from a polydisperse mixture of active particles and move them in or out of the boundary region. This study provides a simple design strategy to fabricate artificial microswimmers whose two‐ or three‐dimensional swimming behavior can be controlled with light.

How blind are they? Phototactic responses in stygobiont diving beetles (Coleoptera: Dytiscidae) from calcrete aquifers of Western Australia

AbstractSubterranean water beetles endemic to groundwater calcretes of Western Australia exhibit convergent traits typical of troglomorphic arthropods, including loss of eyes, pigmentation and wings. As these dytiscid species are estimated to have been isolated underground in permanent darkness for over three million years, it is predicted that they will completely lack phototactic responses. We tested this hypothesis by analysing the behaviour of six subterranean beetle species within an observational arena with dark and light hemispheres. Scan samples at 1 min intervals and total time spent on each hemisphere were recorded over a 20 min period, testing at least 15 individuals per species. We quantified behaviour as an index (dark ratio) so that individual species in this, and future, studies can be consistently compared. Results analysed as both categorical and absolute proportion of time spent in each hemisphere suggest negative phototaxis in Paroster macrosturtensis. The remaining five species did not display any preference for either light or dark hemispheres. These results raise the possibility that some ancestral Paroster species may have exhibited negative phototactic behaviour prior to subterranean colonization. The retention of such a behavioural trait in lightless environments could represent the maintenance for some unknown pleiotropic function. Alternatively, it is possible that insufficient time has passed for neutral processes to render photoreception genes and phototactic behaviours non‐functional. Our study adds to a growing body of evidence that implies highly troglomorphic animals may have evolved from ancestral species that exhibited negative phototaxis as a preadaptation to living in permanent darkness.

Ontogenetic variation in photosensitivity of developing echinoderm propagules

Swimming behaviours and sensory abilities of early pelagic stages play a prominent role in the life history and ecology of sessile/sedentary benthic species, with implications for settlement, recruitment and dispersal. Light is a particularly important driver of navigational behaviour in the ocean, as a signal of key habitat characteristics (e.g., depth, shelter). Work to date on phototaxis has largely focused on planktotrophic larvae that feed during development, and much less on the larger lecithotrophic larvae that rely on maternal provisions (yolk). It remains unclear how responses to light might differ among ciliated propagules of different sizes and nutritional modes. The present study explored if/how phototactic responses are modulated by ontogeny (from embryo to larva), nutritional mode and light colour (wavelength) in ciliated propagules using four co-occurring species of echinoderms: the sea stars Asterias rubens and Crossaster papposus, the sea urchin Strongylocentrotus droebachiensis and the sea cucumber Cucumaria frondosa. Two types of behavioural responses to stimuli (white, red, and blue light) were examined: 1) taxis when the light stimulus was placed at one end of the chamber (net movement towards or away from the light stimulus) and 2) activity level, using a suite of swimming metrics, under uniform illumination. All four species consistently displayed some level of photosensitivity to white light and responses varied intra- and interspecifically. When the stimulus was red or blue light, planktotrophs modified their phototactic responses in a species- and stage-specific manner, while lecithotrophs generally displayed responses without a clear net direction. Swimming speeds displayed stage and species-specific variation under constant red or blue light, but swimming trajectories were consistently straighter under red light, resulting in greater displacement. Taken together, the results suggest that propagules of species with different life-history strategies respond to light stimuli in distinctive stage-wise manners. Interestingly, ontogenetic patterns were not conserved within nutritional modes or taxa. Further investigations of species-specific responses to light might help clarify its roles, in combination with factors such as buoyancy and gravity, in the ecology of propagules of benthic invertebrates.

Is there correlation between photophobia and troglomorphism in Neotropical cave millipedes (Spirostreptida, Pseudonannolenidae)?

Absence of light, typical of the subterranean environment can lead to the loss of synchronizers of physiological and behavioral activities, mainly those related to photoperiods and light perception. Cave-dwelling species present phototactic responses that vary from photonegative to photopositive ones. There are few studies focusing on reaction to light in millipedes and the most studied species are considered photophobic. In this study, we analyzed the phototactic response of cave-dwelling and epigean millipedes of the genus Pseudonannolene (Spirostreptida, Pseudonannolenidae). For this, the chamber choice method was used. We related the phototatic reactions to morphological characters that indicate troglomorphisms (e.g., eyes and melanic pigmentation). The eight-studied operational taxonomic unit (OTU) showed photophobic reaction to all light intensities tested (150, 500 and 1000 lx) and photophobia can be considered a plesiomorphic character in the group. The morphological analysis showed that the light response is negatively correlated to the number of ommatidia and ocular area, that is, the larger the area and the number of ommatidia, the more negative is the light response (photonegative), suggesting that the eyes are the main sensory organs in the perception of light and, apparently, the integument of Pseudonannolene is not related to light sensitivity, opposite to that observed for other millipede species.

Developmental ethanol exposure impairs locomotor movement in Japanese medaka (Oryzias latipes) larvae targeting epigenome

Evidence indicated ethanol exposure during development disrupts brain functions that induces fetal alcohol spectrum disorder (FASD) phenotypes with behavioral abnormalities. We aimed to investigate whether prenatal ethanol exposure has any potential impact on behavior of a FASD fish model. Fertilized Japanese medaka (Oryzias latipes) eggs were exposed to 100–300 mM ethanol or 2 mM 5-azacytidine (5-azaC), 0–2 day post fertilization (dpf), in embryo-rearing medium (ERM). Survived embryos were maintained in clean ERM and used either for gene expression analysis on 2- and 6-dpf or allowed to hatch for behavioral study. Photomotor response of 3–4 day post hatch larvae were tracked for 3 h with light-dark transitions. It was observed that larval swimming was phototactic; enhanced in presence of light, declined in dark. Phototactic response was also observed in larvae prenatally exposed to ethanol or 5-azaC; however, the total distance swum by these larvae compared to controls declined. Further analysis indicated that, in light phases, total swimming activity and average swimming speed were reduced in larvae prenatally exposed to ethanol (300 mM) or 5-azaC. Expression analysis of baz1a and baz2a in embryos indicated developmental regulation. Ethanol (100–300 mM) or 5-azaC (2 mM) were able to modulate downregulation of both baz1a and baz2a mRNAs only in 6 dpf embryos of 300 mM ethanol and 5-azaC (2 mM) groups. These studies indicated that prenatal exposure to ethanol or 5-azaC was able to disrupt movements and thus swimming behavior in FASD phenotypes probably due to delayed remodeling of genome and epigenome.

The spawning and early life-history of a New Zealand endemic amphidromous eleotrid, bluegill bully (Gobiomorphus hubbsi)

ABSTRACTThe larval stage of fishes is critical in determining their dispersal, survival and recruitment, but little is known of the larval behaviours and tolerances of amphidromous fishes, particularly in New Zealand. We report the results of a series of observational and experimental studies on bluegill bully (Gobiomorphus hubbsi), including spawning sites and behaviours, larval characteristics at hatch, phototactic responses of larvae, and larval survival at different salinity levels. Spawning primarily occurred in the lower reaches of the river, and larvae from different nests exhibited marked differences in, and trade-offs between, larval characteristics at hatch, potentially affecting larval success. Larvae were positively phototactic to intense light, an unexpected result based on diel drift patterns and international research. Finally, larvae exhibited markedly higher survival rates when reared at intermediate salinities compared to freshwater or seawater, suggesting estuaries may play an important role as nursery grounds for bluegill bully and other amphidromous fish.

Phototaxis beyond turning: persistent accumulation and response acclimation of the microalga Chlamydomonas reinhardtii

Phototaxis is an important reaction to light displayed by a wide range of motile microorganisms. Flagellated eukaryotic microalgae in particular, like the model organism Chlamydomonas reinhardtii, steer either towards or away from light by a rapid and precisely timed modulation of their flagellar activity. Cell steering, however, is only the beginning of a much longer process which ultimately allows cells to determine their light exposure history. This process is not well understood. Here we present a first quantitative study of the long timescale phototactic motility of Chlamydomonas at both single cell and population levels. Our results reveal that the phototactic strategy adopted by these microorganisms leads to an efficient exposure to light, and that the phototactic response is modulated over typical timescales of tens of seconds. The adaptation dynamics for phototaxis and chlorophyll fluorescence show a striking quantitative agreement, suggesting that photosynthesis controls quantitatively how cells navigate a light field.

Negative phototaxis in M. incognita

Phototaxis is a well-known behavior in many animals. However, little is known whether nematodes living in soil recognize light. Meloidogyne incognita is one of the most important root-knot nematode pests in the world. It has a very wide host range and can infect almost all plant species. Here, we examined the phototactic response in M. incognita. Negative phototactic behavior was observed in M. incognita on an agar plate. Nematodes responded by avoiding white light and blue light more than red light on agar plates. Light avoidance behavior of the nematodes was examined by microscopic analysis, and the nematodes escaped from near ultraviolet light (365 nm) in only approximately 3.2 seconds. They also avoided blue (470 nm), blue-green (500 nm), and green (550 nm) light in approximately 4.1, 7.4, and 10.0 seconds, respectively. These results suggest that M. incognita exhibits negative phototaxis, in particular, responding efficiently to shorter wavelength of light.

One pulse, one light curve: Fast characterization of the light response of microphytobenthos biofilms using chlorophyll fluorescence

Research on microphytobenthos (MPB) photosynthesis has increasingly relied on pulse amplitude modulation fluorometry, often through the generation of light response curves of the relative electron transport of PSII (rETR), generated through the sequential exposure to a range of actinic irradiances. However, fast, vertical phototactic responses by the motile diatoms that dominate these biofilms occur during the generation of sequential light curves. Light-induced vertical migration is known to confound the characterization of the inherent physiological response of the cells forming the biofilm. The alternative approach of shortening the duration of the light curve protocol prevents the reaching of a physiological steady state and results in light curves heavily dependent on initial light conditions. This study tested the application to MPB of a recently introduced method allowing for the fast generation of non-sequential light curves, based on steady state and temporally independent measurements (“single-pulse light curves,” SPLC). This method was compared to the commonly used sequential protocols regarding its ability to (1) measure a steady-state light response, (2) minimize the effects of vertical migration, and (3) minimize the effects of photoacclimation state induced by initial conditions. The results indicate that the SPLC method can be optimized by applying one light exposure period of just 60 s prior to the generation of the light curve, resulting in a clearly advantageous alternative to the sequential protocols. It effectively allows characterizing the steady-state light response of MPB samples while avoiding the effects of light-induced vertical migration and of the initial light conditions on light curve parameters.

Emergent Phototactic Responses of Cyanobacteria under Complex Light Regimes.

ABSTRACTEnvironmental cues can stimulate a variety of single-cell responses, as well as collective behaviors that emerge within a bacterial community. These responses require signal integration and transduction, which can occur on a variety of time scales and often involve feedback between processes, for example, between growth and motility. Here, we investigate the dynamics of responses of the phototactic, unicellular cyanobacterium Synechocystis sp. PCC6803 to complex light inputs that simulate the natural environments that cells typically encounter. We quantified single-cell motility characteristics in response to light of different wavelengths and intensities. We found that red and green light primarily affected motility bias rather than speed, while blue light inhibited motility altogether. When light signals were simultaneously presented from different directions, cells exhibited phototaxis along the vector sum of the light directions, indicating that cells can sense and combine multiple signals into an integrated motility response. Under a combination of antagonistic light signal regimes (phototaxis-promoting green light and phototaxis-inhibiting blue light), the ensuing bias was continuously tuned by competition between the wavelengths, and the community response was dependent on both bias and cell growth. The phototactic dynamics upon a rapid light shift revealed a wavelength dependence on the time scales of photoreceptor activation/deactivation. Thus, Synechocystis cells achieve exquisite integration of light inputs at the cellular scale through continuous tuning of motility, and the pattern of collective behavior depends on single-cell motility and population growth.

Phototactic behavioral response of agricultural insects and stored-product insects to light-emitting diodes (LEDs)

Agricultural insects and stored-product insects are influenced by luminance intensities, exposure times, and wavelengths of light-emitting diodes (LEDs). Based on the phototactic behaviors of the agricultural insects, green or blue LEDs are most attractive for Bemisia tabaci, Trialeurodes vaporariorum, Myzus persicae, Liriomyza trifolii, Spodoptera exigua, and Spodoptera litura. Green LED attracts Plutella xylostella and Frankliniella occidentalis. Similarly, green or blue LEDs are more attractive to agricultural insects, such as Liriomyza sativae, Sogatella furcifera, and Nilaparvata lugens, than other wavelength LEDs. Concerning the phototactic behaviors of the stored-product insects, red LED is attractive for, in descending order Tribolium castaneum, Sitophilus zeamais, Lasioderma serricorne, and Tyrophagus putrescentiae. Blue LED captures most Sitophilus oryzae and Sitotroga cerealella. Red and blue LEDs are more attractive for stored-product insect pests rate than ultraviolet LED and green, yellow, white, and infrared LEDs. Based on the attraction rate of the stored-product insects on granary, red LED is most attractive for S. cerealella and Plodia interpunctella. These light sources are effective in controlling agricultural and stored-product insects. Applying LED technology for greenhouses and granaries along with conventional traps reduces crop loss due to moths, beetles, aphids, and weevils. LEDs have potential value in integrated pest management.

Visual reaction effects induced and stimulated by different lights on phototactic bio-behaviors in Locusta migratoria manilensis

Control of locusts traditionally relies on chemical insecticides. However, chemical control had led to serious environment problems, while light trap techniques based on manipulating insect phototactic responses, are proposed to use for plague locusts control. The key to high efficiency of the trap is to decide the photoelectric reaction intensities on the positive phototaxis of insects. The study compared the visual reaction effects of locusts with their phototactic photosensitivity through the use of spectrometry and behavior investigations, to obtain the influences of the visual spectrum characteristics on locusts positive phototaxis. The results showed that the visual absorption of photons of mixed spectral light and the visual glimmer reaction of the locusts with migratory spectrum characteristics of the peak wavelength of LED lights, reflected the degree of the visual photosensitivity to light photons absorbed by the eye. The visual photosensitive response intensity and the phototactic aggregation effect stimulated by violet light were the most effective, showing that violet light of locusts visual trending sensitivity was consistent with the short sensitive spectrum light of insects visual electrophysiology, whereas the timeliness of the visual photosensitivity reaction and the induction effect was superior under orange light, showing that the biological behavior of locusts visual physiology reaction caused by orange light regulating locusts light rhythm was complicated. Moreover, weak light resulted in stronger visuospatial orientation in terms of the phototaxis of locusts and stronger light resulted in the better visual orientation towards the light source. The results indicated that optical wave properties could affect the visual absorption of spectral light photons, which resulted in different visual responses to different optical frequency energies and restricted the visual bio-photosensitive induction activity of locusts. The study provides new understanding of positive and negative phototaxis of locusts and useful data for the further development of photoelectric induction techniques for the control of locusts. Keywords: locusta migratoria, visual reaction effect, phototactic bio-behavior, light property, bio-control DOI: 10. 25165/j.ijabe.20171004.2357 Citation: Liu Q H, Xin Z, Zhou Q. Influences of visual reaction effect induced and stimulated by different lights on phototactic bio-behaviors in Locusta migratoria manilensis. Int J Agric & Biol Eng, 2017; 10(4): 173–181.

Phototactic Behavior of Scleroderma guani (Hymenoptera: Bethylidae) - Parasitoid of Pissodes punctatus (Coleoptera: Curculionidae)

Although light trap can be used to control pest populations, they can also kill the natural enemies of pests. Scleroderma guani (Hymenoptera: Bethylidae) is a parasitoid of a bark-weevil Pissodes punctatus (Coleoptera: Curculionidae). To understand the phototactic behavior of S. guani, we investigated its diurnal and nocturnal behavior, then examined its phototactic response to nine monochromatic lights and to five intensities of the two most attractive lights. Our results showed that S. guani is most active during the day, while remain still in a dark room or at night. S. guani showed a positive response both to a broad spectrum of monochromatic light and total light (natural light), which implies a broad sensitivity to the light spectrum. S. guani was most sensitive to blue (450 nm) and green (549 nm) lights, suggesting its visual system composed of blue and green receptors. S. guani was least sensitive to ultraviolet (340 nm) light, which may be caused by long-term mass rearing and propagating under artificial conditions. Furthermore, low intensities elicited a positive phototactic response, while high intensities showed a decreased trend under both blue and green lights. Thus, S. guani is a phototactic insect which shows preferences for light in both color and intensity. This study suggests that light trap can only be utilized to control the adult P. punctatus during and after its peak emergence, due to the overlap in the spectral sensitivity of both pest and parasitoid adults.

A true test of colour effects on marine invertebrate larval settlement

Past tests of settlement by marine invertebrate larvae on different colours were not designed to distinguish between responses to substrate colour and substrate brightness. Using colour vision testing methods, the design of this study provided a true test for responses to colour by including both coloured and grayscale settlement plates. The dominant fouling taxa at the test sites (the solitary ascidian, Ciona intestinalis; the colonial ascidians Botryllus schlosseri or Botrylloides violaceous; and a bryozoan Bugula sp.) showed no significant differences in settlement between blue, red or green plates. In contrast, the ascidians responded to substrate brightness with significantly lower settlement on lighter plates, while the bryozoans showed no significant preference relative to substrate brightness. The methods used here are a model for future tests of responses to colour during settlement. In addition, the contrasting responses to brightness are at odds with previous laboratory experiments that showed larvae of both ascidians and bryozoans are negatively phototactic at settlement. These results suggest that other cues must supersede any phototactic responses in the bryozoans, or alternatively that the range of light intensities near darker and lighter substrata were not part of the tests in the earlier reports.