68
swim bladder or swim bladder horns to the ear is now known to enhance the hearing
sensitivity and frequency range during social communication in adult Chaetodon
species (see Sect. 4.2 ). In the tholichthys larvae of C. ocellatus the distance between
the swim bladder and the ear increases to about 1.2 mm as fi sh grow. This distance
does not increase after transformation to the juvenile stage (with continuing
increases in fi sh size) as the long swim bladder horns increase in length with no
signifi cant change in hearing sensitivity (Webb et al. 2012 ) suggesting that the distance between the air within the horns and the ear is of functional importance even
in early life history stages.
2.4 Evolution of the Laterophysic Connection in Chaetodon
Blum ( 1988 ) determined that the medial opening in the supracleithrum is a synapomorphy that defi nes the genus Chaetodon . Webb ( 1998 ) then defi ned the LC as an
association of swim bladder horns with the medial opening in the supracleithrum.
Webb et al. ( 2006 ) defi ned two Direct LC variants (Fig. 4a, b ) and four Indirect LC
variants (Fig. 4c–f ) among Chaetodon species. Smith et al. ( 2003 ) mapped LC characters on a new hypothesis of phylogenetic relationships based on morphological
characters and suggested that Chaetodon ancestor had a Direct LC. This was surprising given the correlation of swim bladder morphology with LC type among
Chaetodon species and because Chaetodon species with an Indirect LC have a
swim bladder that is quite similar to those non- Chaetodon genera in lack an LC
(e.g., Forcipiger , Hemitaurichthys ; Fig. 5 ).
More recent phylogenetic analyses (using molecular evidence) identifi ed four
Chaetodon clades (Fessler and Westneat 2007 ; Bellwood et al. 2010 ). A Direct LC
is found in Clades 3 and 4, but an Indirect LC is found in Clades 2, 3, and 4. Data
are not available for any species in Clade 1 (Table 1 ). In contrast to the mapping of
LC type in Smith et al. ( 2003 ), a mapping of LC variants on a molecular phylogeny
(Fessler and Westneat 2007 ), in which Clade 2 is considered to be the sister group
to Clades 3 + 4, suggests that an Indirect LC is the ancestral LC type in Chaetodon .
Unfortunately, the mapping of LC variants on a more recent molecular phylogeny
(Bellwood et al. 2010 ) sheds no light on the identity of the ancestral LC condition
because it shows Clades 2, 3, and 4 as an unresolved trichotomy. Regardless, the
occurrence of one LC variant in each of the Chaetodon subgenera, which are intact
within the four clades, substantiates the value of the subgenera as taxonomic units
that presumably evolved in response to different selective pressures associated with
LC structure, and presumably function. The distribution of LC variants among subgenera reveals that most of the variants evolved at least two times within the genus
Chaetodon (Table 1 ).
It was hypothesized that ecological correlates (e.g., feeding habit) might shed
light on the functional signifi cance of the evolution of LC morphology (Webb and
Smith 2000 ). However, of the 15 Chaetodon species now known to be obligate corallivores that are monogamous and territorial (Table 1 ), seven have a Direct LC and
T.C. Tricas and J.F. Webb
swim bladder or swim bladder horns to the ear is now known to enhance the hearing
sensitivity and frequency range during social communication in adult Chaetodon
species (see Sect. 4.2 ). In the tholichthys larvae of C. ocellatus the distance between
the swim bladder and the ear increases to about 1.2 mm as fi sh grow. This distance
does not increase after transformation to the juvenile stage (with continuing
increases in fi sh size) as the long swim bladder horns increase in length with no
signifi cant change in hearing sensitivity (Webb et al. 2012 ) suggesting that the distance between the air within the horns and the ear is of functional importance even
in early life history stages.
2.4 Evolution of the Laterophysic Connection in Chaetodon
Blum ( 1988 ) determined that the medial opening in the supracleithrum is a synapomorphy that defi nes the genus Chaetodon . Webb ( 1998 ) then defi ned the LC as an
association of swim bladder horns with the medial opening in the supracleithrum.
Webb et al. ( 2006 ) defi ned two Direct LC variants (Fig. 4a, b ) and four Indirect LC
variants (Fig. 4c–f ) among Chaetodon species. Smith et al. ( 2003 ) mapped LC characters on a new hypothesis of phylogenetic relationships based on morphological
characters and suggested that Chaetodon ancestor had a Direct LC. This was surprising given the correlation of swim bladder morphology with LC type among
Chaetodon species and because Chaetodon species with an Indirect LC have a
swim bladder that is quite similar to those non- Chaetodon genera in lack an LC
(e.g., Forcipiger , Hemitaurichthys ; Fig. 5 ).
More recent phylogenetic analyses (using molecular evidence) identifi ed four
Chaetodon clades (Fessler and Westneat 2007 ; Bellwood et al. 2010 ). A Direct LC
is found in Clades 3 and 4, but an Indirect LC is found in Clades 2, 3, and 4. Data
are not available for any species in Clade 1 (Table 1 ). In contrast to the mapping of
LC type in Smith et al. ( 2003 ), a mapping of LC variants on a molecular phylogeny
(Fessler and Westneat 2007 ), in which Clade 2 is considered to be the sister group
to Clades 3 + 4, suggests that an Indirect LC is the ancestral LC type in Chaetodon .
Unfortunately, the mapping of LC variants on a more recent molecular phylogeny
(Bellwood et al. 2010 ) sheds no light on the identity of the ancestral LC condition
because it shows Clades 2, 3, and 4 as an unresolved trichotomy. Regardless, the
occurrence of one LC variant in each of the Chaetodon subgenera, which are intact
within the four clades, substantiates the value of the subgenera as taxonomic units
that presumably evolved in response to different selective pressures associated with
LC structure, and presumably function. The distribution of LC variants among subgenera reveals that most of the variants evolved at least two times within the genus
Chaetodon (Table 1 ).
It was hypothesized that ecological correlates (e.g., feeding habit) might shed
light on the functional signifi cance of the evolution of LC morphology (Webb and
Smith 2000 ). However, of the 15 Chaetodon species now known to be obligate corallivores that are monogamous and territorial (Table 1 ), seven have a Direct LC and
T.C. Tricas and J.F. Webb