195
7 Aggregation Spawning: Biological Aspects of the Early Life History
have distinctive spindle-shaped eggs up to about 2.5 mm long and 0.5 mm wide.
Most free-spawned eggs are slightly buoyant, nearly transparent, with yolk and oil
globule(s), as well as a hard outer chorion. The yolk provides food for the embryo
in the egg and yolk sac larva stages. The oil globule may contain materials, such as
triacylglycerol (TAG) lipids, important for larval nutrition. Most pelagic reef fi sh
eggs hatch within 24 h to yolk-sac larvae. Species with negatively buoyant, adhesive,
small eggs (triggerfi shes-Balistidae, rabbitfi shes-Siganidae) also hatch quickly.
Eggs attached to the bottom or other objects (damselfi shes-Pomacentridae) are
generally larger, not spherical, take longer to hatch and are ready to feed within
hours of hatching.
Yolk sac larvae – Pelagic eggs at hatching are essentially embryos attached to the
large yolk sac, often with slight positive buoyancy, and have very limited swimming
ability. Over a few days they develop functional eyes, a complete gut with mouth
and anus, rudimentary fi ns and nervous system. The benthic eggs of triggerfi shes
and rabbitfi shes quickly hatch into larvae similar to those with pelagic eggs. Others
with demersal eggs (e.g. damselfi shes) go through the yolk sac stage in the egg
capsule for some days and at hatching are capable of weak swimming and ready to
feed quickly. Most fi shes with demersal eggs do not aggregate for spawning. Otoliths
(ear bones) begin to develop in the yolk sac stage.
Early stage larvae (prefl exion) – Pelagic larvae initiate feeding on external foods once
the systems necessary (complete gut, pigmented eyes, ability to orientate) have been
developed during the yolk sac stage. Larvae possess a notochord without any caudal
fi n supporting structures. The nearly transparent bodies of early larvae, an adaptation to pelagic life, have the body musculature divided into myomeres, which have
a near one-to-one correspondence with the eventual number of vertebrae. Pigment is
often limited to the eyes, over the gut area and as scattered chromatophores or
melanophores on the body. There is a medial fringe of thin tissue, the fi n fold,
around the posterior end which becomes differentiated into the medial fi ns.
Rudimentary pectoral fi ns are usually present. A swim bladder may form and usually
infl ates some days after the larvae start to feed, reducing negative buoyancy. The
otoliths develop with rings being laid down which allow for age determination, if
the rings are formed daily or at another known interval, since otolith formation.
Late stage larvae (post-fl exion) – As larvae grow, the posterior end of the notochord
fl exes upward in the process called “fl exion” establishing the location where the
supporting elements of the caudal fi n will develop. Once fl exion is complete, the
larva is in the “post-fl exion” stage. Formation of caudal fi n supporting elements
occurs over time and the vertebrae develop. Larvae often become more pigmented,
and develop specialized structures in some families (some may start prior to fl exion).
Swimming ability is greatly increased once the caudal fi n is developed. The larval
stage is considered to end when all external meristic characters (fi n spines and rays,
etc.) develop and temporary special features for pelagic life are lost.
The pelagic juvenile stage – Still living in open water, they have nearly the full
morphology of the benthic stages, but usually are partially transparent and lack the
7 Aggregation Spawning: Biological Aspects of the Early Life History
have distinctive spindle-shaped eggs up to about 2.5 mm long and 0.5 mm wide.
Most free-spawned eggs are slightly buoyant, nearly transparent, with yolk and oil
globule(s), as well as a hard outer chorion. The yolk provides food for the embryo
in the egg and yolk sac larva stages. The oil globule may contain materials, such as
triacylglycerol (TAG) lipids, important for larval nutrition. Most pelagic reef fi sh
eggs hatch within 24 h to yolk-sac larvae. Species with negatively buoyant, adhesive,
small eggs (triggerfi shes-Balistidae, rabbitfi shes-Siganidae) also hatch quickly.
Eggs attached to the bottom or other objects (damselfi shes-Pomacentridae) are
generally larger, not spherical, take longer to hatch and are ready to feed within
hours of hatching.
Yolk sac larvae – Pelagic eggs at hatching are essentially embryos attached to the
large yolk sac, often with slight positive buoyancy, and have very limited swimming
ability. Over a few days they develop functional eyes, a complete gut with mouth
and anus, rudimentary fi ns and nervous system. The benthic eggs of triggerfi shes
and rabbitfi shes quickly hatch into larvae similar to those with pelagic eggs. Others
with demersal eggs (e.g. damselfi shes) go through the yolk sac stage in the egg
capsule for some days and at hatching are capable of weak swimming and ready to
feed quickly. Most fi shes with demersal eggs do not aggregate for spawning. Otoliths
(ear bones) begin to develop in the yolk sac stage.
Early stage larvae (prefl exion) – Pelagic larvae initiate feeding on external foods once
the systems necessary (complete gut, pigmented eyes, ability to orientate) have been
developed during the yolk sac stage. Larvae possess a notochord without any caudal
fi n supporting structures. The nearly transparent bodies of early larvae, an adaptation to pelagic life, have the body musculature divided into myomeres, which have
a near one-to-one correspondence with the eventual number of vertebrae. Pigment is
often limited to the eyes, over the gut area and as scattered chromatophores or
melanophores on the body. There is a medial fringe of thin tissue, the fi n fold,
around the posterior end which becomes differentiated into the medial fi ns.
Rudimentary pectoral fi ns are usually present. A swim bladder may form and usually
infl ates some days after the larvae start to feed, reducing negative buoyancy. The
otoliths develop with rings being laid down which allow for age determination, if
the rings are formed daily or at another known interval, since otolith formation.
Late stage larvae (post-fl exion) – As larvae grow, the posterior end of the notochord
fl exes upward in the process called “fl exion” establishing the location where the
supporting elements of the caudal fi n will develop. Once fl exion is complete, the
larva is in the “post-fl exion” stage. Formation of caudal fi n supporting elements
occurs over time and the vertebrae develop. Larvae often become more pigmented,
and develop specialized structures in some families (some may start prior to fl exion).
Swimming ability is greatly increased once the caudal fi n is developed. The larval
stage is considered to end when all external meristic characters (fi n spines and rays,
etc.) develop and temporary special features for pelagic life are lost.
The pelagic juvenile stage – Still living in open water, they have nearly the full
morphology of the benthic stages, but usually are partially transparent and lack the