Biology

The University of Oregon zebrafish page gives an excellent short video of a developing embryo. See: http://zfish.uoregon.edu/

The following staging table was developed by A. Jordaan for quantifying development during the egg stage. Each position of development (POD) is described in column 2. Interesting facts and links to relevant pages in the website are in column 3. All pictures taken by A. Jordaan, except where noted.

1l.jpg (22967 bytes) POD 1: Fertilization, the sperm and egg fuse and the first cell results. Eggs used at UMaine are dissinfected to reduce the potential for diseases: Information on dissinfection. N. Schatz completed a dissinfection trial on haddock eggs (see poster with results).
2h.jpg (20813 bytes) POD 2: The 2 cell stage. A female cod can hold millions of eggs. Cod, haddock and halibut are batch spawners, which mean they spawn a number of times in one season. Egg quality is best during the middle batches of eggs.
3h.jpg (17794 bytes) POD 3: The 4 cell stage. Cod undergo a spawning ritual during courtship, presumably to allow for a coordinated release of gametes.
4b.jpg (20297 bytes) POD 4: The 8 cell stage. Cod, haddock and halibut are all broadcast spawners, releasing eggs into the ocean to drift with currents (for more information go to ECOLOGY).
5h.jpg (25821 bytes) POD 5: The 16 cell stage.  
6a.jpg (26007 bytes) POD 6: The 32 cell stage.  
7h.jpg (28931 bytes) POD 7: Early BLASTULA stage (~ 64 cells)  
8b.jpg (22164 bytes) POD 8: Mid BLASTULA (Cells are still distinct) The morula period is highly sensitive to mechanical stress. Eggs should not be transported during this period.
9Be.jpg (22141 bytes) Morula stage begins, can no longer easily see individual cells when viewing full egg.

Egg looks similar to single-cell stage at fertilization.

 Cells are massed at the animal pole and begin to receive signals specifying their role in the segregation of the cell types.
9bside.jpg (18380 bytes) Eggs must be rotated on side as in picture.

POD 9: As in picture, cell-mass elongated

POD 10: Cell-mass ball-like with ring

 At this point all cell division is complete. All changes in the egg from here until hatch involve differentiation of cells.
M15.jpg (8238 bytes) POD 11: As seen in picture, but inner circle is symetrical with outer edge.

POD 12: As seen in picture.

  
M16.jpg (7978 bytes) POD 13: The "lemon wedge" stage, as in picture.

POD 14: The "kidney bean" stage,

POD 15: Now cresent shaped.

  
last of wedge and first axis.JPG (26840 bytes) POD 13-17: Note how cell mass inside eggs has formed a cup. This is called gastrulation.

POD 16: First indication of fold appearing where body axis will lie.

  
20c.jpg (33288 bytes) POD 17: Body axis forming. All cell types converge here. This is where the notochord begins to form.
21b.jpg (23548 bytes) POD 18: Body axis formed. This is my favorite point during egg development.  
22Aa.jpg (27661 bytes) POD 19: Hole closing, but not quite. Development of optic vessicles.  
23c.jpg (29497 bytes) POD 20: Closure complete. Tail-bud stage. This is another period where eggs are sensitive to mechanical stress and transport should be avoided.
23Ac.jpg (26353 bytes) POD 21: Head still in form of POD 20, but tail resembles POD 22 (adjacent picture).

POD 22: Body length is size of 1/2 of egg. Tail has diamond-like shape.

 Note how large the yolk-sac is. This is the only investment that the mother makes in her offspring.
26b.jpg (25857 bytes) POD 23: First sign of "fins".

POD 24: Can see a second set of small fins developing just below head. First set look more angular (as next picture). Also should be able to locate a small knot, heart.

 Development of organ systems.
31Ad.jpg (23403 bytes) POD 25: Heart still a small knot, but has developed some shape. Note shape of head. Also first melanophores appear (pigmentation).

POD 26: Heart elongates and has first beats. Exciting moments!

 Will beat very infrequently and irregularily at first, but quickly begins to assume a normal rythym.
31Ch.jpg (25875 bytes) POD 27: Tail less than 3/4 of distance around egg.

POD 28: Tail between 3/4 and touching head.

  
32Bc.jpg (32099 bytes) POD 29: Tail reaches head, eyes have no pigmentation.

POD 30: Eyes begin to have some pigmentation.

  
32Cc.jpg (20688 bytes) POD 31: Eyes fully pigmented, appear completely black. Larvae will start twiching in egg, ready to hatch. Once the eyes are fully pigmented the larvae should begin to hatch.
deadeggs.jpg (24748 bytes) 4 dead eggs Dead eggs generally sink to the bottom of tanks. The aeration and water flow is temporarily shut off, allowing live eggs to float to the surface. Dead eggs are siphoned off the bottom.
wpe1.jpg (19159 bytes) POD 28-29: These larvae hatch at a very premature stage. Note the lack of eye pigmentation, small size and large yolk-sac. Some eggs may hatch at this point but larvae are poorly developed.
veryprematurenextday.jpg (23408 bytes) POD 31, Larval POD 1: These are larvae from the same tank as the premature larvae above, but the following day. Note the increased development state, especially the increased pigmentation of the eyes. The etched grid has 1mm2 squares. These are used to calibrate the image software and get measurements such as standard length, body depth and yolk-sac area.
12 degree sample at hatch.JPG (56913 bytes) Hatch in warmer water: Larval POD 1-2. The larvae are curled because they have just hatched. The larvae uncurl in a couple of hours. Research by A. Jordaan has shown that larvae hatch at different sizes at different temperatures because they hacth at different points in development (see PDF file).
2 degree sample at hatch.JPG (56959 bytes) Hatch in colder water: Larval POD 2-3. J. Guebtner completed research on the bioenergetics of larval cod.
y-s larvae whole.jpg (13414 bytes) YOLK-SAC period The yolk-sac period is thought to experience extremely high mortality in the field.
rotifer.JPG (17238 bytes) TRANSITION period. While still using the yolk-sac for energy, the larvae begin feeding. For a short period of time they are using both sources of nutrition. The picture is a rotifer, the food of choice for cod and haddock. Check out CULTURE information. The greatest focus of UMaine has been the development of nutritional adequate dry diets for the early weaning of cod. B. Baskerville-Bridges, C. Callen and J. Muscato have all contributed in the creation of diets and early weaning protocols used at UMaine.
Cod_after_SB_inflation.jpg (85390 bytes) Note the digesting rotifers in this larval gut, where the greenish mass is. Also the inflated swim bladder. There is a small amount of yolk remaining just below the gut. For abstracts of research see COD or HADDOCK research pages.
Starving vs not.jpg (8150 bytes) LARVAL Period: ~ 4mm- ~ 15mm: The larval period continues until the finfold is replaced by fins. Morrison (1993) gives a full desciption The picture shows 2 cod larvae of the same age (15 days, 8ºC). The top larvae is feeding and at a length of 7 mm. The bottom larvae is not feeding and is only 4 mm. More information see EARLY LIFE HISTORY.
cod.gif (10486 bytes) JUVENILE and ADULT period. (Photo credit: ) Research at UMaine on juveniles has focussed on temperature effects and bioenergetics. S. Hansen has conducted work on the bioenergetics of juvenile cod.