Differences in chemosensory response between eyed and eyeless Astyanax mexicanus of the Rio Subterráneo cave
- Jonathan Bibliowicz†1,
- Alexandre Alié†1,
- Luis Espinasa2,
- Masato Yoshizawa3,
- Maryline Blin1,
- Hélène Hinaux1,
- Laurent Legendre1,
- Stéphane Père1 and
- Sylvie Rétaux1Email author
© Bibliowicz et al.; licensee BioMed Central Ltd. 2013
Received: 1 July 2013
Accepted: 26 July 2013
Published: 4 September 2013
In blind cave-dwelling populations of Astyanax mexicanus, several morphological and behavioral shifts occurred during evolution in caves characterized by total and permanent darkness. Previous studies have shown that sensory systems such as the lateral line (mechanosensory) and taste buds (chemosensory) are modified in cavefish. It has long been hypothesized that another chemosensory modality, the olfactory system, might have evolved as well to provide an additional mechanism for food-searching in troglomorphic Astyanax populations.
During a March 2013 cave expedition to the Sierra de El Abra region of San Luís Potosi, Mexico, we tested chemosensory capabilities of the Astyanax mexicanus of the Rio Subterráneo cave. This cave hosts a hybrid population presenting a wide range of troglomorphic and epigean mixed phenotypes. During a behavioral test performed in situ in the cave, a striking correlation was observed between the absence of eyes and an increased attraction to food extract. In addition, eyeless troglomorphic fish possessed significantly larger naris size than their eyed, nontroglomorphic counterparts.
Our findings suggest that chemosensory capabilities might have evolved in cave-dwelling Astyanax mexicanus and that modulation of naris size might at least partially underlie this likely adaptive change.
Cave-dwelling animals have long been recognized as excellent models for evolutionary biology. Comparative studies of cave-dwelling (cavefish, CF) and surface-dwelling (surface fish, SF) morphs of the teleost fish Astyanax mexicanus have revealed several adaptations to life in dark cave environments[2–5], which have occurred during a few million years of evolution from a common SF-like ancestor. In CF, temporal and spatial modulations in early developmental signaling pathways appear to influence brain development and organization[7–9]. Lateral line neuromasts on the head are more numerous and mediate a special mechanosensory behavior in response to vibrations on the water surface. Chemosensory structures are also modified, as CF possess a higher number of taste buds than their SF counterparts[4, 10], and studies in a laboratory setting suggest that laboratory-raised CF originating from the Pachón cave might possess higher olfactory capabilities. Thus, sensory modalities of CF might have evolved as an adaptation to life in dark caves for navigation, mate recognition and food-searching. However, the possibility that these changes might provide a sensory compensation for finding food in the dark has yet to be tested directly in natural cave environments.
Additional file 1: Recording behavior in the Rio Subterráneo cave. A one minute movie (infrared recording) shows attraction of “CF-like” but not “SF-like” fish to food-related odors. The “food” tubing is at the top left, and the “control water” tubing is at the top right of the plastic pool. (MP4 17 MB)
Our findings reveal an apparent correlation between troglomorphic characteristics and chemosensory-dependent feeding behavior in wild Astyanax fish of the Subterráneo cave. Previous studies in a laboratory setting suggested that adult Astyanax Pachón cavefish are significantly better than their SF counterparts at finding food in the dark. It has recently been suggested that one of the mechanisms that may contribute to this feeding success in cavefish is their ability to sense vibration using vibration attraction behavior (VAB), a behavior that is mediated through the mechanosensory function of their neuromasts. Since vibrations should be equal on both food extract and control sides of our experimental setup, differences in VAB do not likely contribute to the attraction of CF-like individuals to the food extract that is absent in SF-like fish. Our findings suggest that modified chemosensory capabilities in cavefish do indeed provide a sensory compensation for food-searching in dark cave environments. The fact that heightened chemosensory capabilities has been observed in both laboratory-raised Pachón and wild Subterráneo fish, which come from two independently derived stocks, also suggests a possible convergence of this capacity.
Furthermore, the fact that all food extract–responsive fish possessed enlarged nares and strongly reduced eyes shows a correlation between eye loss and modifications in olfactory-related morphology in this population. Although several mechanisms might underlie the observed heightened chemosensory capabilities in CF-like fish, modulations of naris size could be one contributing factor. Incidentally, our results also show that Astyanax can respond to a food extract to which they have never been exposed before (in this case, commercial granular fish food). The fact that only the CF-like individuals responded to this novel food might reflect a more generalist food detection ability of troglomorphic fish. If confirmed, such opportunism for multiple food sources might reflect a troglomorphic adaptation to an environment where seasonal water flows create sudden changes in nutrient provision.
To the best of our knowledge, this report is the first report to describe a behavioral experiment performed on wild Astyanax in their natural environment. The results obtained raise the intriguing possibility that olfactory capabilities might have evolved in cave-dwelling Astyanax, and it is tempting to propose that this change improves food-finding in dark environments, where vision is useless. Additional studies are certainly necessary to better characterize the observed differences in chemosensory response and to test whether they correspond to bona fide sensory adaptations in Astyanax mexicanus (and potentially other cave-dwelling animals as well).
Sampling and photography
For assessment of the phenotypes encountered in the Subterráneo cave, fish were caught with a seine net and transferred to inflatable plastic pools. They were individually photographed with a Canon EOS 650D camera (Canon U.S.A., Melville, NY, USA) in a narrow glass box and returned to the pool. Fieldwork was performed under the auspices of Mexican permit 02241/13 (to SR) delivered by Secretaria de Medio Ambiente y Recursos Naturales.
To collect fish for behavioral testing, a meat-baited minnow trap was set up overnight. The collected fish were placed in a 61-cm × 61-cm × 15-cm inflatable pool and were left for an acclimatization period of 48 h. Two 50-ml syringes were attached to a stationary tripod and connected to medical solution administration tubing containing a Luer stopper to control solution flow (Baxter, Thetford, UK). The ends of the two administration sets were attached to opposite corners of the plastic pool (Figure 2B). Food extract was prepared by adding 5 g of granular fish food (TetraDiskus; Tetra, Blacksburg, VA, USA) to 50 ml of local water, mixing, and filtering using glass microfiber filters (Whatman plc, Kent, UK) to remove any food particles. This solution was added to one syringe while a water control was added to the other. Solution flow from the food extract and control samples was initiated simultaneously, and the experiment was filmed from the top using a Sony DCR- SR200 Handycam camcorder equipped with NIGHTSHOT mode (Sony Electronics, San Diego, CA, USA). Food extract and control areas were defined arbitrarily as 20-cm × 20-cm zones adjacent to the two respective tube ends. To measure the time each fish spent in the food extract area, individual fish were manually tracked through frame-by-frame analysis of the movie using VirtualDub and ImageJ software (National Institutes of Health, Bethesda, MD, USA). A Mann–Whitney U test was utilized to determine the statistical significance of the results. After the test, the fish were collected, photographed individually and returned to the pool.
Naris size measurements
Naris circumference and standard body length were measured using ImageJ software and reported as a ratio to control for size variation between individuals.
We thank Ernesto Maldonado for help in obtaining the Mexican field work permit and Didier Casane, Julien Fumey and Karen Pottin for their help and enthusiasm during cave experimentation. This work was supported by ANR grants ASTYCO and BLINDTEST to SR. JB and AA receive ANR postdoctoral financial support.
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