Figure 3. Localized patterns and morphology of pigmentation in Mexican tetras (A. mexicanus). A. Surface fish larva after two days of development showing melanic chromatophores (yellow arrows); dorsal view with anterior to the top. B. Dendritic melanin granules along the circular margin of a juvenile surface fish eye, and within a field of melanophores anterior to the eye in both surface (yellow arrow) and subsurface cells (white arrow). C. Melanophores along the lateral line of a juvenile surface fish showing typical dendritic morphologies within (yellow arrow) and below (white arrow) the epidermis. D. Terminal edge along the posterior field of an adult surface fish scale, pigmented with melanophores (yellow arrows) and xanthophores (white arrowheads). E. Pigmentation in the caudal fin of a juvenile surface fish showing a highly concentrated area of melanophores extending from the lateral line into the tail (dashed yellow arrow), and linear patterns melanophores and xanthophores aligned along the branched fin rays. F. Magnified view of dendritic melanophores in the concentrated area shown in ‘E’. G. Magnified view of the branched caudal fin rays of an adult cavefish. Blood vessels are visible (red), and both melanophores (yellow arrows) and xanthophores (white arrowheads) are detectable along lateral margins of the fin rays. H. Molino cavefish juvenile in left-lateral view. Black melanophores are not visible in any regions of the body. Xanthophores are discernable as yellow-orange spots on margins of the olfactory pit (dashed yellow arrow), on the optic tectum (dashed red arrow) and along dorsal sides (white arrows) and flanks of the body. I. Magnified, lateral view of the head of a Molino cavefish juvenile. A remnant of the embryonic eye is centered within a transparent eye orbit (dashed black arrow), and xanthophore pigmentation is visible within tissues of the olfactory pit (dashed yellow arrow) and optic tectum (dashed red arrow). J–L. Pigmentation in different body regions of a Molino cavefish juvenile exposed to daily treatments of high-intensity light for 15 days. Overall body coloration is noticeably orange due to increased xanthophore pigment along body and fin rays (J,K, dashed red arrows) and regions of the head and mouth (L, dashed yellow arrows). There is also an increase in the presence of iridescent cells (iridophores) along the lateral stripe and sides of the head (J,L, dashed white arrows) and other areas (not shown). af, adipose fin. Stereomicrographs by Michael J. Boyle. Figure 4. Comparative pigmentation of an A. mexicanus cavefish after exposure to high light. A. Adult male cavefish in right-lateral view. This fish was maintained in an aquarium under ambient light for several months. Regions of pigmentation along the dorsal side of the body (white arrows), within surface tissues of the head (dashed yellow arrows), and posterior flank (yellow arrows) point to areas for direct comparison during and after experimental treatments. B. The same fish as shown in ‘A’ after 45 days of daily exposure to high light treatments. All areas pointed to in ‘A’ have increased in the level and distribution of melanin production. Other areas of melanin increase include the pigmented band dorsal to the lateral line, cells along the base of the adipose fin (red arrow), and around the mouth, and olfactory pits (dashed red arrow). C. The same fish as shown in ‘A’ (and ‘B’) after 72 days of daily exposure to high light treatments. Macrophotographic images by Scott Arledge and Michael J. Boyle. pigmentation (melanophores, xanthophores, iridophores). Cavefish were not adversely affected by low pH or lowered O2 levels; surface fish showed signs of stress under those treatments. The primary interpretation of our test results indicate that Astyanax cavefish exhibit the capacity to adapt rapidly to significant abiotic changes in aquatic environments. As they show relatively rapid responses to light exposure, they may have shown similar response times when adapting to cave environments in the past. II. MATERIALS and METHODS A. Animal model and husbandry The research model tested for all experiments is the freshwater teleost fish, Astyanax mexicanus (Mexican tetra). This species consists of one surface-dwelling (SF, surface fish) morphotype, and multiple cave-dwelling (CF, cavefish) morphotypes across stream systems. All SF stocks were collected from the Guadalupe River, Texas. BOYLE, ARLEDGE, THOMAS, TOMKINS, AND GULIUZZA Testing the cavefish model 2023 ICC 125
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