Science Center Objects

By Kimberly Chojnacki, and Aaron DeLonay
July 14, 2016

With all the discussion of free embryos this summer, some readers may be left wondering what, exactly, is a ‘free embryo’.  A free embryo is a developing fish no longer within a protective chorion (egg envelope), from the time of hatch to the initiation of active feeding.  This life stage is marked by rapid development, growth, and change.  Newly hatched pallid sturgeon free embryos are generally 7-9 millimeters long with a large yolk-sac to fuel the rapid development and growth of the tiny fish.  The free embryos hatch without a well-developed mouth, eyes, gills, fins, or the fleshy, whiskerlike barbels near the mouth of sturgeon (figure 1).  Without fins, a newly hatched pallid sturgeon free embryo has limited ability to control its movement in the river current for several days after hatch.  Water temperature is considered to be the most important environmental factor influencing the development of this early life stage.

Pallid sturgeon free embryos on the day of hatch, approximately 8-9 mm (about 0.33 of an inch) in length.

Figure 1. Pallid sturgeon free embryos on the day of hatch, approximately 8-9 mm (about 0.33 of an inch) in length.

(Credit: Kimberly Chojnacki, U.S. Geological Survey. Public domain.)

At approximately 20 °C and 2 days after hatch, eyes of the free embryos have become pigmented, and pectoral fins buds, and barbel buds are visible (figure 2).

Pallid sturgeon free embryo at approximately 2 days after hatch, approximately 11-12 mm (about 0.45 of an inch) in length.

Figure 2. Pallid sturgeon free embryo at approximately 2 days after hatch, approximately 11-12 mm (about 0.45 of an inch) in length.

(Credit: Kimberly Chojnacki, U.S. Geological Survey. Public domain.)

By approximately the fifth day after hatch, the free embryos are developing gill filaments, a dorsal fin, and the barbels continue to grow (figure 3).

Pallid sturgeon free embryo at approximately 5 days after hatch, approximately 14-15 mm (about 0.57 of an inch) in length.

Figure 3. Pallid sturgeon free embryo at approximately 5 days after hatch, approximately 14-15 mm (about 0.57 of an inch) in length.

(Credit: Kimberly Chojnacki, U.S. Geological Survey. Public domain.)

At approximately 20 °C and 10 days after hatch, the pallid sturgeon free embryos look much more like miniature versions of their parents with a broad, flat rostrum (snout or nose), fleshy barbels bordering the protrusible mouth, and large pectoral fins (figure 4).  The free embryos are now approximately 17-19 millimeters long and are ready to begin active feeding and transition to the larval stage.

Pallid sturgeon free embryo at approximately 10 days after hatch, approximately 19-20 mm (about 0.77 of an inch) in length.

Figure 4. Pallid sturgeon free embryo at approximately 10 days after hatch, approximately 19-20 mm (about 0.77 of an inch) in length.

(Credit: Kimberly Chojnacki, U.S. Geological Survey. Public domain.)

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