198
P.L. Colin
The point at which the bladder becomes functional (gas-fi lled) is considered “infl ation”
and can represent another “critical period”. For some larvae it is believed they must
rise to the surface to ingest air (Czesny et al. 2005 ) . In some living larvae the swim
bladder is visible as a silvery structure over the gut due to their generally transparent
nature; however the swim bladder is often masked by melanophores and not always
visible externally. Once infl ation has occurred, the larvae may change from negatively to neutrally buoyant, this change apparent in the angle at which the body is
held in the water. Data on time of infl ation are sparse, but, for example, in the red
grouper, Epinephelus morio, it was observed at 12 days post-hatching (Colin et al.
1996 ) and Drass et al. ( 2000 ) observed it 4–6 days post-hatching in 2.4 mm notochord length (NL) reared larvae of the red snapper, Lutjanus campechanus .
Flexion – Swimming of suffi cient speed and duration to match ambient currents is
nearly impossible for early larvae. Prior to fl exion, swimming is adequate for food
capture and perhaps to change depths to a limited degree, but not to migrate any
distance. During their early stages larvae might be concentrated by mechanisms,
such as by maintaining a given depth along a descending front, but may not actively
seek such mechanisms. Their swimming ability might allow them to remain, once
encountered, with high concentrations of food. While fl exion is easily noted in larval
specimens, the age of its occurrence is often not known for fi eld-caught specimens.
Rearing studies provide some indication that it may occur roughly half way (or more)
through the larval life. For example Clarke et al. ( 1997 ) reported notochord fl exion
in several species of snappers (Lutjanidae) to start at 11–12 days post-hatching and
to be complete at 16–18 days, while metamorphosis occurred at ages of 22–33 days.
For red grouper Colin et al. ( 1996 ) found fl exion to occur at about 16 days, with a
total pelagic life of 35 days.
7.4.2 Are There Differences in ELH Between Aggregation
and Non-aggregation Spawners?
Fish families with aggregation spawning also have non-aggregating species. Within
these families the larvae of the two spawning types do not appear different and the
requirements to promote survival to recruitment are probably similar. Are there,
however, possible differences in egg type or size for fi shes with different strategies?
Are all propagules equivalent? For egg types, it seems unlikely that differences
exist. If a family has two egg morphologies (e.g. spherical versus spindle in parrotfi shes), they are found for both types of spawning. Where dual modes of spawning
(pair- and group-spawning) exist in one species (e.g. parrotfi shes and wrasses), the
eggs produced are also identical, or nearly so, and individual females may alternate
between pair and group spawning. Robertson ( 1996 ) did report a 5% higher egg
volume in group-spawned versus pair-spawned eggs of T. bifasciatum . This represents a difference of less than 2% in diameter and may represent a slightly different
degree of hydration.
P.L. Colin
The point at which the bladder becomes functional (gas-fi lled) is considered “infl ation”
and can represent another “critical period”. For some larvae it is believed they must
rise to the surface to ingest air (Czesny et al. 2005 ) . In some living larvae the swim
bladder is visible as a silvery structure over the gut due to their generally transparent
nature; however the swim bladder is often masked by melanophores and not always
visible externally. Once infl ation has occurred, the larvae may change from negatively to neutrally buoyant, this change apparent in the angle at which the body is
held in the water. Data on time of infl ation are sparse, but, for example, in the red
grouper, Epinephelus morio, it was observed at 12 days post-hatching (Colin et al.
1996 ) and Drass et al. ( 2000 ) observed it 4–6 days post-hatching in 2.4 mm notochord length (NL) reared larvae of the red snapper, Lutjanus campechanus .
Flexion – Swimming of suffi cient speed and duration to match ambient currents is
nearly impossible for early larvae. Prior to fl exion, swimming is adequate for food
capture and perhaps to change depths to a limited degree, but not to migrate any
distance. During their early stages larvae might be concentrated by mechanisms,
such as by maintaining a given depth along a descending front, but may not actively
seek such mechanisms. Their swimming ability might allow them to remain, once
encountered, with high concentrations of food. While fl exion is easily noted in larval
specimens, the age of its occurrence is often not known for fi eld-caught specimens.
Rearing studies provide some indication that it may occur roughly half way (or more)
through the larval life. For example Clarke et al. ( 1997 ) reported notochord fl exion
in several species of snappers (Lutjanidae) to start at 11–12 days post-hatching and
to be complete at 16–18 days, while metamorphosis occurred at ages of 22–33 days.
For red grouper Colin et al. ( 1996 ) found fl exion to occur at about 16 days, with a
total pelagic life of 35 days.
7.4.2 Are There Differences in ELH Between Aggregation
and Non-aggregation Spawners?
Fish families with aggregation spawning also have non-aggregating species. Within
these families the larvae of the two spawning types do not appear different and the
requirements to promote survival to recruitment are probably similar. Are there,
however, possible differences in egg type or size for fi shes with different strategies?
Are all propagules equivalent? For egg types, it seems unlikely that differences
exist. If a family has two egg morphologies (e.g. spherical versus spindle in parrotfi shes), they are found for both types of spawning. Where dual modes of spawning
(pair- and group-spawning) exist in one species (e.g. parrotfi shes and wrasses), the
eggs produced are also identical, or nearly so, and individual females may alternate
between pair and group spawning. Robertson ( 1996 ) did report a 5% higher egg
volume in group-spawned versus pair-spawned eggs of T. bifasciatum . This represents a difference of less than 2% in diameter and may represent a slightly different
degree of hydration.