Embryology and Early Life History of Rio Grande Silvery Minnow Hybognathus amarus (Teleostei: Leuciscidae) with Detailed Morphological Description of Its Larva
Understanding fundamental life history and ecological attributes of declining endemic fishes is essential for developing effective strategies for their conservation and recovery. In the Great Plains and desert rivers of North America, numerous imperiled leuciscids belong to a unique reproductive ecotype with drifting eggs and larvae (pelagophils). Herein, we synthesize three decades of research on the embryology and early life history of Rio Grande Silvery Minnow Hybognathus amarus, a federally endangered member of this ecotype, and explore how our findings can guide management and conservation of these sensitive taxa. We investigated three early developmental aspects of Hybognathus amarus through aquarium and laboratory studies: 1) egg morphology, development, and density, 2) larval development, growth, and behavior, and 3) morphologic and meristic analysis of larvae and early juveniles. Eggs nearly doubled in size at 10 min post-fertilization, were nonadhesive and nearly neutrally buoyant (specific gravity: 1.0011–1.0024), and hatched within 30 hrs in water about 23°C. Recently emerged protolarvae first transformed to mesolarvae after about one week, to metalarvae after about three weeks, and to juveniles after about six weeks at 20–24°C. Based on six candidate models, larval fish development (i.e., from protolarvae to early juveniles) was best explained by a cubic polynomial growth curve. While most protolarvae developed a gas bladder and began to feed within a week, the complete complement of fin rays (i.e., required for proficient swimming) had not fully formed until about one month post-hatching. Early developmental characteristics (e.g., egg specific gravity and larval fin ray formation), combined with river fragmentation, flow regulation, and habitat loss, can profoundly affect the upstream retention and recruitment of Hybognathus amarus and other native pelagophils. Long-term recovery of these highly imperiled species will depend on restoring sufficient seasonal flows, river and floodplain connectivity, and habitat complexity to promote their successful spawning, growth, and survival.
Photograph of a live egg and embryo of Hybognathus amarus, collected in the Middle Rio Grande (NM, Socorro County, Rio Grande at Sevilleta National Wildlife Refuge, La Joya, 7 June 2018; photo by Andrea D. Urioste), illustrating key characteristics (size [ca. 3.7 mm diameter] and clarity [diaphanous]) of nearly neutrally buoyant eggs and embryos emblematic of this reproductive guild.
Illustration of tank used to rear Hybognathus amarus from egg to early juvenile stages (189.3 L): (A) shows the 25 cm tubular air-stone used to maintain dissolved oxygen and provide continuous circular flow, (B) indicates flow direction, and (C) is the clear acrylic tray (37 × 15 × 8 cm) that was suspended 5 cm below the water surface to provide low-velocity habitat for larval fish (i.e., artificial backwater).
Measures for larval and early juvenile fishes. Yolk sac (Y) and pterygiophores are included in the width and depth measures but fins and finfolds are not. “B” in BPE and BPV means immediately behind. AMPM is anterior margin of most posterior myomere. PHP is measured to the end of notochord until the adult complement of principal caudal-fin rays is observed (postflexion mesolarval phase). Fin lengths (D, A, P1, and P2, encircled) are measured along the plane of fin from its origin to most distal margin. Images modified from Snyder et al. (2016).
Meristics (quantitative features) determined for larval and early juvenile cypriniforms. Meristic features comprise myomeres (noted by white diamonds), pterygiophores, fin rays (primary and rudimentary), and lateral line scales (or lateral series if the lateral line is incomplete; noted by white squares). Rudimentary rays are reported in lowercase Roman numerals, while principal median rays are denoted in Arabic numerals; rudimentary rays are not distinguished in paired fins. In the upper illustration, the most anterior and most posterior myomeres included in myomere counts (e.g., ODF, OD, PV) are transected by a vertical line and shaded dark gray. Images modified from Snyder et al. (2016).
Developmental phases of Hybognathus amarus. Embryonic stage names and anatomical features follow terminology of Mansueti and Hardy (1967). Embryonic stages identified by upper case letters and hours post-fertilization contained in brackets: (A) blastula [6 hrs], (B) gastrula [8 hrs], (C) early embryo stage [10 hrs], (D) tail-bud stage [12 hrs], (E) tail-free stage [14 hrs], (F) beginning of late embryo stage [18 hrs], (G) middle of late embryo stage (composite of two photographs) [22 hrs], and (H) end of late embryo stage [26 hrs], about 4 hrs prior to hatching. Structures that appear throughout embryonic development: 1: blastomeres; 2: yolk; 3: embryonic axis head; 4: embryonic axis tail; 5: cephalic region; 6: somites; 7: caudal region; 8: optic vesicles; 9: developing somites; 10: notochord; 11: auditory vesicle; 12: myomeres; 13: Kupffer’s vesicle; 14: chorion; 15: lens of the eye; 16: yolk sac; 17: perivitelline space.
Illustration of nearly neutrally buoyant egg of Hybognathus amarus with large, heavily yolked, almost fully developed embryo, large perivitelline space, and star-shaped micropyle. Bar is 1 mm.
Box plots representing daily lengths of larvae and early juveniles of Hybognathus amarus over 48 days. The box and its associated components represent all lengths, regardless of developmental phase, recorded during a single day. Boxes show the interquartile range (IQR; Q3–Q1), with the lower and upper bounds of the box respectively representing the first (Q1; 25th percentile) and third (Q3; 75th percentile) quartiles. Whisker caps represent the 10th (Q0) and 90th (Q4) percentile values, while the thin, black horizontal line (often obscured) represents the median (Q2; 50th percentile). Red horizontal lines represent the mean lengths; outliers are shown as individual blue dots. The day notable morphological features first appeared are indicated, as well as the range of days each larval phase occurred in the sample.
Body length growth of Hybognathus amarus across 48 days. The relationship between SL and age, in days, was best fit by a cubic polynomial model (see Table 2). Raw data points (SL per day, blue circles) are plotted, with model-predicted values (red diamonds).
Dorsal, lateral, and ventral views of Hybognathus amarus (MSB 49967): (A) recently hatched protolarva, 4.7 mm SL, 5.0 mm TL; (B) protolarva, 5.4 mm SL, 5.7 mm TL; (C) flexion mesolarva, 6.0 mm SL, 6.5 mm TL; (D) postflexion mesolarva, 7.2 mm SL, 7.9 mm TL. Rectangular icon below images is the artist’s stylized initials.
Dorsal, lateral, and ventral views of Hybognathus amarus (MSB 49967): (A) recently transformed metalarva, 10.1 mm SL, 12.0 mm TL; (B) metalarva, 12.1 mm SL, 14.8 mm TL; (C) recently transformed juvenile, 17.6 mm SL, 21.8 mm TL; (D) early juvenile, 28.0 mm SL, 35.6 mm TL. Rectangular icon below images is the artist’s stylized initials.
Stereoscopic image of live eggs and embryos of Platygobio gracilis (left, 2.4 mm) and Hybognathus amarus (right, 3.8 mm) from the Middle Rio Grande, NM collected in May 2018. Ocular micrometer increments are 0.1 mm.
Collection tray holding over 10,000 live eggs of Hybognathus amarus, which were taken in 30 min during a large spawning event in the Middle Rio Grande (NM, Socorro County, Rio Grande at Sevilleta National Wildlife Refuge, La Joya, 5 June 2018).
Contributor Notes
Associate Editor: T. Grande