Nadav Shashar

Polarization vision helps detect transparent prey

Transparency enables aquatic organisms to avoid detection by visual predators. But we have found that this camouflage can be broken using a visual mode evolved by several predators, such as squid. Under partially linearly polarized lighting, squid detect zooplankton prey at a distance 70% greater than those achieved under non-polarized illumination. The role of polarization sensitivity in predation is confirmed by squids preference for transparent, yet polarization-active, targets that mimic their prey.

Cuttlefish use polarization sensitivity in predation on silvery fish

Cephalopods are sensitive to the linear polarization characteristics of light. To examine if this polarization sensitivity plays a role in the predatory behavior of cuttlefish, we examined the preference of Sepia officinalis when presented with fish whose polarization reflection was greatly reduced versus fish whose polarization reflection was not affected. Cuttlefish preyed preferably on fish with normal polarization reflection over fish that did not reflect linearly polarized light (n = 24, chi 2 = 17.3, P < 0.0001), implying that polarization sensitivity is used during predation. We suggest that polarization vision is used to break the countershading camouflage of light-reflecting silvery fish.

Behavioural evidence for polarisation vision in stomatopods reveals a potential channel for communication.

Polarisation sensitivity (PS) - the ability to detect the orientation of polarised light - occurs in a wide variety of invertebrates [1] [2] and vertebrates [3] [4] [5], many of which are marine species [1]. Of these, the crustacea are particularly well documented in terms of their structural [6] and neural [7] [8] adaptations for PS. The few behavioural studies conducted on crustaceans demonstrate orientation to, or local navigation with, polarised sky patterns [9]. Aside from this, the function of PS in crustaceans, and indeed in most animals, remains obscure. Where PS can be shown to allow perception of polarised light as a special sensory quality [1], separate from intensity or colour, it has been termed polarisation vision (PV). Here, within the remarkable visual system of the stomatopod crustaceans (mantis shrimps) [10], we provide the first demonstration of PV in the crustacea and the first convincing evidence for learning the orientation of polarised light in any animal. Using new polarimetric [11] and photographic methods to examine stomatopods, we found striking patterns of polarisation on their antennae and telson, suggesting that one function of PV in stomatopods may be communication [12]. PV may also be used for tasks such as navigation [5] [9] [13], location of reflective water surfaces [14] and contrast enhancement [1] [15] [16] [17] [18]. It is possible that the stomatopod PV system also contributes to some of these functions.

Behavioral evidence for intraspecific signaling with achromatic and polarized light by cuttlefish (Mollusca: Cephalopoda)

Intraspecific visual communication was studied quantitatively by testing the behavior of Sepia officinalis, the common cuttlefish, as senders and receivers of body pattern signals. These signals can be achromatic and/or linearly polarized and are produced by specialized dermal cells. Experiment 1 addressed whether the presence of conspecifics affected the visible, achromatic body patterns of males. These patterns tended to vary among conditions, suggesting limited sensitivity to audience. One set of body patterns varied with the number of conspecifics viewed while an uncorrelated set of body patterns varied with the sex of conspecifics viewed. Males showed high-contrast zebra banding when viewing another male, supporting previous studies of body patterns in male-male agonistic behavior. Experiment 2 addressed the relationship of polarized light patterns with visible body patterns of males and females, and tested whether senders modified their polarization patterns in response to conspecifics. Polarization patterning was only weakly associated with visible body patterns. Females showed more polarized body patterns than did males, but polarized patterns did not differ among conditions; thus, no sensitivity to audience by senders was found. Experiment 3 addressed whether conspecific receivers used information from polarized body patterns. Limited evidence was found for changes in the behavior of female but not male observers, suggesting that female receivers may use polarized patterns as a source of information about conspecifics. The information contained in polarization patterning may complement that contained in zebra patterning such that both males and females advertise their species, sex, location, and size to conspecific receivers.

Early familiarization overrides innate prey preference in newly hatched Sepia officinalis cuttlefish

In the cuttlefish Sepia officinalis, hatchlings do not benefit from parental care and have to search independently for their own food. We investigated the effect of exposing newly hatched cuttlefish to a nonpreferred prey on their subsequent choice of prey. We tested the choice of food between crabs (nonpreferred) and shrimps (preferred) made by 3-day-old cuttlefish that had been exposed visually and chemically to crabs at hatching, or had been exposed visually only to crabs, or had had no exposure to crabs. Juveniles that had been exposed to crabs significantly preferred crabs, whereas hatchlings that had had no crab exposure preferred shrimps. These results show that a simple visual exposure to a naturally nonpreferred prey immediately on hatching is sufficient to change the juvenile cuttlefishs innate preference.

MODIFIED LABORATORY CULTURE TECHNIQUES FOR THE EUROPEAN CUTTLEFISH SEPIA OFFICINALIS

The cuttlefish Sepia officinalis Linnaeus, 1758, is an important model for a variety of biological and biomedical investigations (1). To introduce this organism to the North American research communities, and make it readily available, various methods have been used to maintain or culture the species. The most intensive efforts and successes have been achieved by Forsythe and colleagues at the University of Texas Medical Branch in Galveston (2,3). Their most noteworthy achievement was to culture seven consecutive generations in large-scale recirculating seawater systems. Recently, cuttlefish were brought to the Marine Resources Center (MRC) of the Marine Biological Laboratory, where they have also been cultured successfully through their life cycle. Presently the third laboratory generation is under culture. Unlike previous work on Sepia culture in the United States, our focus has been to use a mostly open (flow-through) seawater system modified to function well through a northern winter, and to develop feeding methods that are suitable for our locale.

Fouling reefal communities on artificial reefs: Does age matter?

Man-made submerged structures, including shipwrecks, offering substrata for fouling organisms and fish, have been classified secondarily as artificial reefs (ARs). The current approach in AR design is that of low-profile structures placed on the seabed and attempting to mimic natural reef (NR) communities with the aim of mitigating degraded marine ecosystems. To examine the validity of this concept, a long-term comparison of the developing AR fouling communities to those of nearby NRs is required. A survey of the fouling reefal organisms was conducted on seven shipwrecks (Red Sea, Egypt), comprising three young (ca 20 years old) and four old (u200a>u200a100 years old) unplanned ARs, in comparison to nearby NR communities. The hypothesis tested was that the age of the ARs shapes the structure of their fouling coral communities. The results demonstrated distinct differences between ARs and NRs and between young and old ARs. While the species composition on ARs may resemble that of NRs after approximately 20 years, obtaining a similar extent of coral cover may require a full century. Moreover, differences in structural features between ARs and NRs may lead to differences in species composition that persist even after 100 years.

Experimental and theoretical study of skylight polarization transmitted through Snell's window of a flat water surface

The celestial polarization pattern may be scrambled by refraction at the air-water interface. This polarization pattern was examined in shallow waters with a submersible polarimeter, and it was calculated by using land measurements (semiempirical predictions) and models of the skylight polarization. Semiempirically predicted and measured e-vector orientations were significantly similar. Conversely, predicted percent polarization was correlated but lower than measurements. Percent polarization depended on wavelength, where at high sun altitudes maximal percent polarization generally appeared in the UV and red spectral regions. The wavelength dependency of polarization may lead to differential spectral sensitivity in polarization-sensitive animals according to time and type of activity.

Modular organization of adaptive colouration in flounder and cuttlefish revealed by independent component analysis

Flounders and cuttlefish have an impressive ability to change colouration, for camouflage and, in the case of cuttlefish, for communication. We pursue the hypothesis that these diverse patterns are created by combining a small number of distinct pattern modules. Independent component analysis (ICA) is a powerful tool for identifying independent sources of variation in linear mixtures of signals. Two versions of ICA are used, one assuming that sources have independence over time, and the other over space. These reveal the modularity of the skin colouration system, and suggest how the pattern modules are combined in specific behavioural contexts. ICA may therefore be a useful tool for studying animal camouflage and communication.

Polarization contrast of zooplankton: a model for polarization-based sighting distance.

Transparency is commonly used by zooplankton for camouflage in open waters. Polarization vision allows planktivorous animals to increase their preys detectability. Polarization properties of zooplankton were analyzed by measuring changes in the transmitted light. The transmitted light was subjected to depolarization and phase retardance, resulting in a species-specific polarization contrast between animal and background; from 5% in Corycaeus sp. to 92% in Undinula vulgaris (Copepoda). This contrast diminishes exponentially with distance, reaching 50% of the inherent value at 1 and 2m, for moderately turbid and clear waters, respectively. However, at reactive distances of planktivorous fishes this contrast is reduced by less than 20%.