Flat targets made of colored, transparent plastic cemented on a solid white background were placed at the end of each choice arm. Four targets were used during the experiment: a red rectangle, a green rectangle, a red triangle and a green triangle Fig.
Because stomatopods in previous behavioral experiments successfully learned to discriminate red and green colored targets Marshall et al. A cylindrical holding chamber was centered at the far end of the entrance arm. The holding chamber was designed to be rotated on its side by a researcher, allowing an animal placed inside the chamber access to the rest of the Y -maze.
Layout of experimental setup. A The four targets used during the experiment: a red triangle, a green triangle, a red rectangle and a green rectangle. C A Y -maze was placed in a cylindrical tank with an incandescent light source centered above it.
A diffusing filter was placed above the arena. The filter had a centered hole, where the lens of a camera was fitted to record each trial. The Y -maze contained an entrance arm and two choice arms oriented 90 deg from one another. A cylindrical holding chamber was centered at the end of the entrance arm.
At the end of each choice arm laid a hole set below the floor of the Y -maze. A food reward could have been placed in either hole. One of the targets in A was placed at the end of each choice arm as indicated. The dashed lines in the choice arms of the arena indicate the point at which an animal was recorded as having made a choice.
Radiance measurements of the light source were taken using the same spectrometer. Each N. During training trials, the focal target e. The target of opposite shape and color e. A stomatopod was placed in the holding chamber before a trial and allowed 5 min to adjust to its surroundings. After this time, the holding chamber was turned, allowing the animal to enter the arena, initiating the experiment.
Once a stomatopod entered the arena, the first choice arm it traveled down was noted once it passed the choice boundary of the arm, at two-thirds of the length of the arm. If it found the food, the experimental animal was allowed 5 min to eat as a reward before being removed from the arena. Each animal experienced the training procedure twice per week. After each individual training session, the water in the arena was mixed to prevent olfactory cues from influencing the choice of subsequent training sessions.
At the end of each week, the percentage of correct choices each individual made since the start of training was calculated. Individuals were required to have been trained for at least 1 month eight training trials before being considered for testing. The procedure of the testing phases was identical to that of the training phase except that no food reward was offered during testing sessions.
Trained stomatopods were subjected to three distinct tests: 1 a shape recognition test, 2 a color recognition test and 3 a conflicting cues test see Fig. Initially, only the conflicting cues test was conducted. Once it became clear that animals would indeed perform well in the conflicting cues test, we continued testing using all three test types. At this time, individuals experienced these three types of tests in a randomized order. Once testing began, training or testing occurred twice per week with two training sessions administered between each testing session to facilitate reward seeking between tests.
In order to test whether N. The cue of the opposite shape and the same color of that to which the individual had been trained was placed at the end of the other arm e. A correct choice was recorded if the stomatopod chose the arm displaying the cue with the trained color and shape.
The cue of the same shape and the opposite color was placed at the end of the other arm e. The cue of the opposite shape and the same color was placed at the end of the other arm e. Neither cue was of the shape and color combination identical to the one which the animal was trained to recognize.
All statistical analyses were run in R v3. Generalized linear mixed modeling GLMM was used to analyze the data for each of the three tests. Because individual stomatopods were tested more than once, the models for each test included individual ID as a random term.
As we used both males and females for our study, sex was also included as a random term for our full models; however, because sex did not significantly increase the explanatory power of our models, it was removed from our final models.
Individual ID did not significantly increase the explanatory power of our models, but was left in the final models to account for repeated measures. All statistical outcomes are reported in Table S1. On average, animals responded to the paradigm i. From these choices, animals learned to associate food with their respective trained targets over time Fig. Training was successful for animals trained to all possible target color and shape combinations Table S2.
Training results. A Neogonodactylus oerstedii located food during the training procedure approximately half of the time. The solid line represents the line of best fit. B Neogonodactylus oerstedii associated food with their respective trained targets over time. Each point represents the percentage of animals that correctly chose the target they were training to from all animals who made choices during that training session. C Sample size per point in B.
The number of animals undergoing training decreased over time because animals either progressed to the testing procedure or died during the course of the study.
Once animals reached the performance criteria to enter the testing phase, they were tested in three separate procedures: a shape recognition test, a color recognition test and a conflicting cues test. During the shape recognition test, both arms contained targets of the color to which an animal had been trained, but the target in each arm was of a different shape.
Neogonodactylus oerstedii recognized the target by its shape, not its color. Blue and red bars represent proportions of choices during testing that were for the target of the correct shape and color, respectively. Grey bars represent proportions of choices during testing that were for the incorrect target. Dark grey lines represent standard errors of the means.
Examples of targets placed in either arm of each test for an individual that was trained to associate food with a red triangle are found on the left of each bar. During the color recognition test, both arms contained targets of the shape they were trained to but the color of the target differed per arm. During the conflicting cues test, one arm contained a target with the same shape but opposite color to the target to which they were trained, while the other arm had a target with the same color but alternate shape to the trained target.
This result reflected the results of the shape and color discrimination tests, implying that the shape of the trained target was more important than the target's color to N. Choices made during each test were not influenced by the shape or color of the trained target Table S3. Our study demonstrates that mantis shrimp are able to recognize objects of distinct shapes.
We found that mantis shrimp tended to recognize the trained object by its shape rather than its color Fig. Because mantis shrimp use landmarks during navigation Patel and Cronin, c , the findings in our study suggest that the shape of a landmark may be more important than its color when being identified by a mantis shrimp during navigation.
Similarly, the shapes of prey, predators and body structures used for signaling may be useful for recognition and for generating appropriate behavioral responses. Identifying an object by its shape might be more effective than recognizing its color when the object is viewed underwater. In water, contrast attenuates with distance and depth owing to the absorption and scattering of light.
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