Abstract:

Larval fish orientation remains a poorly understood mechanism. It is critical for studying larval dispersal but has, surprisingly, never been tested in any pelagic species. This laboratory experiment investigates the ability of Coryphaena hippurus L. larvae, hereafter ‘mahi-mahi’, to orient in relation to a sun-like light source through ontogeny. Larvae were tested using the Drifting In Situ Chamber (DISC), which was initially developed to tackle larval fish orientation in situ. It is equipped, among others, with a GoPro that captures the position of the larvae every second during a 10-minute observation period, and three compasses to track their directionality. Here, the DISC was adapted to conduct laboratory in a large (3 m3) circular aquarium filled with seawater. As such, the stimulus perceived by the larvae could be thoroughly controlled and the larvae’s behavioral response identified at both the individual, age and population levels. A total of 82 individual larvae, ranging from 7 days to 23 days post-hatching, were exposed to a directional and alternating (by 180°) light, meant to mimic the sun’s changing position in the sky. The results show that larvae respond to the light over any other directional stimuli and that this ability is absent until the flexion stage, and prominent afterwards. This strong phototaxism implies that early in their development, mahi-mahi larvae may be able to use the sun as a compass to orient themselves in the open ocean, providing them with an efficient mechanism to optimize foraging.

Suggested Citation:

Robin Faillettaz, Eve Johnson, Patrick Dahlmann Sacha Syunkova, John Stieglitz, Daniel Benetti, Martin Grosell, Claire B. Paris. 2020. Ontogeny of phototaxis in early life stages of mahi-mahi (Coryphaena hippurus). Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/HZB203BM

Purpose:

These data were collected to describe the ontogeny of phototaxism in mahi-mahi early life stages, since no data quantitative were available to date. Here, we focused the light stimuli, since mahi-mahi larvae were suspected to respond to light sources in hatchery tanks, and because several pelagic stages of larvae posses a sun-compass that may be the only direction cue available to swim directionally in the open-ocean (Faillettaz et al. 2015, PLoS ONE). The presence and the accuracy of an orientation behavior through ontogeny also provides proxies of the sensorial abilities of the species.

Data Parameters and Units:

Raw and rotate larval tracks (°), swimming speed (cm/s), turning angles (°), and many more statistics computed with the 'discr' R package. In addition, the age of the larva (variable 'age', in days post-hatching), the standard length (variable 'length.mm', in mm), whether the larva oriented toward a cue or toward the DISC structure (variable 'orient', TRUE=orientation toward a cue, FALSE=orientation toward the DISC) and the side of the light for each experiment (variable 'light', left is -65° from the North and right +115° from the North) are added. Description of the variables: "deploy_id": unique identifier of the deployment; "trackNb", identifier of the track, "rotation": the degrees of rotation that the DISC experienced during the deployment; "n": the number of images used to compute the mean and precision of orientation, "mean": the mean bearing of the larva; "se.mean": the standard error (SE) of the mean, "kappa": statistics of the concentration of the bearing throughout the track, "se.kappa": SE of the kappa, "variance": variance of the kappa, "r": directionality of the larva; "p.value": p-value of the directionality, "signif": is the p-value significant (<0.05), "dir.n": number of positions during the track, "dir.mean": mean direction of the positions, "dir.se.mean": SE of the positions' direction; "dir.kappa": kappa of the positions' direction; "dir.se.kappa": SE of the kappa of the positions' direction,"dir.variance": variance of the positions' direction, "dir.r": directionality (r) of the positions' directions, "dir.p.value": p-value of the directionality of the positions' direction, "dir.signif": is the dir.p.value <0.05 (logical), "turn.n": number of turning points used, "turn.abs.mean": absolute mean of the turning points’ direction, "turn.freq.gt45": frequency of turning points >45°; "speed.n": number of points used to compute the swimming speed, "speed.mean": mean swimming speed (cm s-1), "speed.sd": Standard deviation of the swimming speed, "speed.median": median swimming speed, "speed.mad": median absolute deviance of the swimming speed, "bin.angle": angle bin used to compute the orientation (°),"age": in days post hatching,"[length.mm|http://length.mm/]": standard length in mm, "stage": development stage (preflexion, flexion, post-flexion, juvenile),"light": side if the ceiling light used for the experiment for each deployment (right or left), "orient": larva orientation is stronger regarding the light’s direction than regarding the frame of the DISC (logical).

Methods:

The data were collected in the laboratory with the Drifting In Situ Chamber (described in Paris et al. 2013, PLoS ONE), placed in a large circular tank. Mahi-mahi larvae were reared at the hatchery facilities of the University of Miami, and provided on a daily basis, in the morning of experiment days. There were placed in 30 L buckets with plenty of food. DISC data were processed with the open-source software package discr (https://github. com/jiho/discr), modified from Irisson et al. 2008 (L&O: Methods). The camera produced 7 megapixels images of the fish larva in the chamber at 1-second intervals. The position of the larva was recorded on each image by clicking on it through a graphical user interface. Each position was converted to polar coordinates relative to the axis from the center of the chamber to the top of the picture. The angular part of the coordinate was converted to a bearing relative to the North by subtracting the bearing of the top of the picture, recorded by the analog compass. These bearings were the base data to detect cardinal orientation.

Instruments:

The Drifting In Situ Chamber is described in Irisson et al. 2008, Paris et al. 2013 and Faillettaz et al. 2015.

Provenance and Historical References:

Faillettaz R, Blandin A, Paris CB, Koubbi P, Irisson J-O (2015) Sun-Compass Orientation in Mediterranean Fish Larvae. PLoS ONE 10(8): e0135213. doi:10.1371/journal.pone.0135213 Irisson JO, Guigand C, Paris CB. Detection and quantification of marine larvae orientation in the pelagic environment. Limnology and Oceanography: Methods. 2009; 7:664–672. doi: 10.4319/lom.2009.7.664 Paris CB, Atema J, Irisson JO, Kingsford M, Gerlach G, Guigand CM. Reef odor: a wake up call for nav- igation in reef fish larvae. PloS one. 2013; 8(8):e72808. doi: 10.1371/journal.pone.0072808 PMID: 24015278

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