506
12 Species Case Studies
12.17.3 Other Small Species of Scarids
In the Indo-west Pacifi c small scarids known to aggregate include Bleeker’s
parrotfi sh – Chlorurus bleekeri , and greenthroat parrotfi sh – Scarus prasignathos ,
and others reported need further confi rmation (surf parrotfi sh – S. rivulatus , Myers
1999 ) . While longnose parrotfi sh, Hipposcarus longiceps (formerly known as “ H.
harid ”), is at the upper limit of “small” scarids, and its Indian Ocean replacement
has been previously reported to group-spawn (Gladstone 1996 ) , nearly all observations have been of pair-spawning (Chap. 12.16 ). The behaviour of this species closely
resembles its western Atlantic counterpart the redfi n parrotfi sh, S. rubripinne , even
regarding similarities in colour patterns of initial and terminal phase fi sh between
the two species. Some other IWP genera, Calotomus and Leptoscarus are reported
to spawn in groups of individuals (Robertson et al. 1982 ) , but whether these represent aggregations migrating to a spawning area is unknown. The seagrass parrotfi sh,
Leptoscarus vaigiensis , is a gonochorist (no sex change), the fi rst parrotfi sh reported
to be so (Robertson et al. 1982 )
The occurrence of group- and pair-spawning in the same area is common in
many species, with terminal phase males often courting females that are also
involved in group-spawning activities. It appears there is a “critical mass” factor in
determining whether group spawning will occur based on the numbers of initial
phase fi sh present: Bleeker’s parrotfi sh pair-spawn hundreds of times on a study reef
in Palau, but never group-spawned. About 100 m down the reef a group of about 50
initial phase Bleeker’s parrotfi sh group-spawn. The example is good evidence of the
caution needed when making generalities about spawning in a particular species,
particularly based on experience at one small site. Colin and Bell ( 1991 ) examined
spawning by scarids and labrids within an area of reef channel at Enewetak Atoll
and although they found 13 species of scarids spawning in this area, none would be
considered to aggregate. The small bullethead parrotfi sh, C. sordidus , which aggregates in other areas of Enewetak, was not common in their study area, hence no
aggregations were seen there.
Since small scarids in the IWP seem to spawn with a tidal timing, the situation
with bullethead parrotfi sh is interesting. It appears in several locations that spawning occurs only when tides are in the morning; at other tidal phases they either do
not spawn or spawn at some time that has not been noted. In Palau, the aggregation
of bullethead parrotfi sh is replaced by surgeonfi shes (striped bristletooth –
Ctenochaetus striatus and brown surgeonfi sh – Acanthurus nigrofuscus ) when high
tides are in the afternoon. Most scarids have unusual and distinctive spindle-shaped
pelagic eggs about 1.2–2.2 mm in length and only 0.4–0.6 mm wide. The eggs of
some have bent pointed ends (Colin and Bell 1991 ) . Only a small number of species
have more typical spherical eggs (e.g. bicolor parrotfi sh – Cetoscarus bicolor and
bumphead parrotfi sh – Bolbometopon muricatum ). One possible explanation for
this egg type is that the shape may have less resistance to vertical movement in the
water column due to buoyancy and this shape might either help to keep them closer
to the surface or increase their ascent rate away from benthic predators.
12 Species Case Studies
12.17.3 Other Small Species of Scarids
In the Indo-west Pacifi c small scarids known to aggregate include Bleeker’s
parrotfi sh – Chlorurus bleekeri , and greenthroat parrotfi sh – Scarus prasignathos ,
and others reported need further confi rmation (surf parrotfi sh – S. rivulatus , Myers
1999 ) . While longnose parrotfi sh, Hipposcarus longiceps (formerly known as “ H.
harid ”), is at the upper limit of “small” scarids, and its Indian Ocean replacement
has been previously reported to group-spawn (Gladstone 1996 ) , nearly all observations have been of pair-spawning (Chap. 12.16 ). The behaviour of this species closely
resembles its western Atlantic counterpart the redfi n parrotfi sh, S. rubripinne , even
regarding similarities in colour patterns of initial and terminal phase fi sh between
the two species. Some other IWP genera, Calotomus and Leptoscarus are reported
to spawn in groups of individuals (Robertson et al. 1982 ) , but whether these represent aggregations migrating to a spawning area is unknown. The seagrass parrotfi sh,
Leptoscarus vaigiensis , is a gonochorist (no sex change), the fi rst parrotfi sh reported
to be so (Robertson et al. 1982 )
The occurrence of group- and pair-spawning in the same area is common in
many species, with terminal phase males often courting females that are also
involved in group-spawning activities. It appears there is a “critical mass” factor in
determining whether group spawning will occur based on the numbers of initial
phase fi sh present: Bleeker’s parrotfi sh pair-spawn hundreds of times on a study reef
in Palau, but never group-spawned. About 100 m down the reef a group of about 50
initial phase Bleeker’s parrotfi sh group-spawn. The example is good evidence of the
caution needed when making generalities about spawning in a particular species,
particularly based on experience at one small site. Colin and Bell ( 1991 ) examined
spawning by scarids and labrids within an area of reef channel at Enewetak Atoll
and although they found 13 species of scarids spawning in this area, none would be
considered to aggregate. The small bullethead parrotfi sh, C. sordidus , which aggregates in other areas of Enewetak, was not common in their study area, hence no
aggregations were seen there.
Since small scarids in the IWP seem to spawn with a tidal timing, the situation
with bullethead parrotfi sh is interesting. It appears in several locations that spawning occurs only when tides are in the morning; at other tidal phases they either do
not spawn or spawn at some time that has not been noted. In Palau, the aggregation
of bullethead parrotfi sh is replaced by surgeonfi shes (striped bristletooth –
Ctenochaetus striatus and brown surgeonfi sh – Acanthurus nigrofuscus ) when high
tides are in the afternoon. Most scarids have unusual and distinctive spindle-shaped
pelagic eggs about 1.2–2.2 mm in length and only 0.4–0.6 mm wide. The eggs of
some have bent pointed ends (Colin and Bell 1991 ) . Only a small number of species
have more typical spherical eggs (e.g. bicolor parrotfi sh – Cetoscarus bicolor and
bumphead parrotfi sh – Bolbometopon muricatum ). One possible explanation for
this egg type is that the shape may have less resistance to vertical movement in the
water column due to buoyancy and this shape might either help to keep them closer
to the surface or increase their ascent rate away from benthic predators.