A winter skate (Leucoraja ocellata) egg capsule with a developing embryo in its early stages
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A winter skate (Leucoraja ocellata) egg capsule with a developing embryo in its early stages |
During the early
stages of development within the egg case, the ovum and associated yolk mass are
not structurally self-supportive, but are stabilized by a clear, gelatinous egg
jelly, often referred to as the albumen.
The albumen fills the interior of the egg capsule not occupied by the egg
itself, and serves to contain and limit the movement of the delicate ovum within
the egg capsule during the initial one third of the incubation period. It
also functions as a plug for the respiratory canals found on either side of the
egg case at the base of the anterior horns.
As the yolk develops its own self-supportive external membranes, the
albumen gradually decreases in viscosity and then dissolves completely, opening
the respiratory canals and allowing for ventilation and oxygenation of the
interior environment. The
time required for the canals to become fully unplugged and for seawater to enter
freely through one of both ends of the egg case is likely a function of
incubation temperature; in the clearnose skate (Raja eglanteria), for example, the process takes approximately 25-28
days. For the
remaining two thirds of incubation, the embryo develops in seawater.
During this time, water flow and gas
exchange is aided by the embryo, which constantly beats its elongate tail to
facilitate the movement of water into and out of the egg case through the open
respiratory canals. External gill
filaments aid in respiration prior to the formation of gills.
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A winter skate (Leucoraja ocellata) embryo removed from its egg capsule. Note the external gill filaments and the large external yolk sac. |
A winter skate (Leucoraja ocellata) embryo removed from its egg capsule |
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A thorny skate (Amblyraja radiata) egg capsule cut open to reveal a developing embryo |
A thorny skate (Amblyraja radiata) embryo in its egg capsule. Note the developing eye and the large extrenal yolk sac. |
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A thorny skate (Amblyraja radiata) embryo further along in the development stages. Note the increased size of the embryo relative to the external yolk sac. |
Lateral view of a developing thorny skate (Amblyraja radiata) embryo |
As development continues, the external yolk sac slowly diminishes in size and the embryo orients itself so that its rostrum faces the anterior end of the egg case (or the posterior part of the egg capsule in relation to the genital tract of the parent fish). The pectorals are folded dorsally over the body and the tail is rotated 180° so that the fanning motion continues to circulate seawater towards the gills of the embryo.
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An embryo of a thorny skate (Amblyraja radiata) in the later stages of its development. The external yolk sac is diminished, an internal yolk sac has formed, and the embryo is oriented with its rostrum facing anteriorly (or posteriorly in relation to the oviduct of the female). |
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An embryo of a thorny skate (Amblyraja radiata) in the later stages of its development, with its pectoral fins folded dorsally in preparation for emergence |
An embryo of a thorny skate (Amblyraja radiata) with its pectoral fins folded dorsally and a remaining external yolk sac |
After a developmental period lasting from a couple of months to over a year, the fully-formed skate emerges suddenly from the end of the capsule, pushing itself through a ruptured slit between the two anterior horns. A small internal yolk-sac measuring approximately 1-3mm in diameter is present at the time of hatching. During the first week post-hatching, this sac is resorbed completely, after which the young skate must feed for itself.
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Dorsal view of the embryo of a thorny skate (Amblyraja radiata) in the later stages of its development. The internal yolk sac is clearly visible. |
Ventral view of the embryo of a thorny skate (Amblyraja radiata) in the later stages of its development. The internal yolk sac is clearly visible, whereas the external yolk sac has been completely resorbed. |
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An embryo of a thorny skate (Amblyraja radiata) emerging from the anterior end of its egg capsule (or posterior end in relation to the oviduct of the mother) |
