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|
| | The
damselfishes are some of the most common and conspicuous coral reef fishes in
the region and are often selected as the subjects for ecological and behavioral
studies. Their taxonomy is straightforward; there are only four genera in the
Caribbean with relatively few species. Two genera include several species each:
the ubiquitous Stegastes spp.
with six species (and one fresh-water species) and Chromis
spp. with two shallow and four deep-water species. There are two Abudefduf
spp. and the single Microspathodon
chrysurus. The Caribbean Stegastes
spp. have been moved around from genus to genus over the recent past,
spending some time as Eupomacentrus spp. and Pomacentrus spp. |
| |
Pomacentrid larvae closely resemble juvenile damselfish in form and are easy to
recognize. Larval damselfishes are characterized by a rounded body with a wide
caudal peduncle, continuous spinous and soft dorsal fins, the absence of stout
spines on the head, large round eyes, and a relatively small terminal mouth. Distinguishing
the damselfish species in the larval and newly-settled stages can be a challenge
in this family where meristics can broadly overlap, especially within Stegastes
spp. The marking patterns that separate species of Stegastes
spp. typically diverge only at the late juvenile stage and new recruits
can share many of the features that will later distinguish the species (i.e. ontogenetic
homologies). The two regional Abudefduf
spp. are also very similar as larvae and new recruits. As a result, some
descriptions of damselfish early life history and many illustrations of small
juvenile damselfishes in guidebooks and on the web are incorrect. | |
| |
| | |
|
| | The
damselfishes of this genus are known for marked coloration and pattern changes
as they grow on the reef, mostly converging on a uniform dusky appearance as adults.
Pre-transitional larval Stegastes spp. are mostly colorless, have few distinguishing
markings, and overlap extensively in meristics and thus DNA sequencing is necessary
to assign some larval types to species. Damselfish larvae rapidly acquire juvenile
markings during the night of settlement and transitional larvae are common in
collections, even before they have the chance to transform in a trap. |
| | New
recruits of some Stegastes spp. share the markings that are used as diagnostic
characters for distinguishing later juvenile stages. These shared markings include
the pattern of spots and stripes on the head, the size, shape, and position of
the dorsal fin ocellus, the spot at the top of the pectoral fin base, and the
upper caudal peduncle saddle spot. These characters may not be consistent for
identifying juveniles below 20 mm SL and many species identifications of photographs
of small juveniles on the web and in reef fish guides are mistaken. Furthermore,
the published descriptions of juveniles of this genus are almost always incongruent
and depend on highly variable characters, such as shades of yellow or the degree
of striping. The most egregious example of this is the commonplace assignment
of any juvenile with blue head stripes to S.
diencaeus. | |
| Ontogenetic homologies in
pomacentrid early life history: This interesting phenomenon of short-lived,
presumably vestigial, markings that appear on early juveniles is somewhat analogous
to the homologies in ontogeny shown in mammalian embryology. In this case, however,
perhaps ontogeny does recapitulate phylogeny as ancestral markings may persist
in the earliest juveniles of an entire clade. The similarity may extend to the
mechanism for the persistence: just as obsolete embryological structures persist
because of reduced selection pressure within the womb (for example the hindlimbs
in dolphin embryos), these early markings may persist because of reduced selection
pressure for species recognition among the smallest juvenile stages on the reef.
Alternatively, a more intriguing explanation may be that this phenomenon is an
adaptation to disguise species identity at the time of extreme vulnerability to
competitive interactions from adults. | |
| Species Identification:
Among the larval Stegastes spp., S.
partitus diverges first during the transition phase by not developing
the dorsal fin ocellus shared by all of the other reef species. The dorsal fin
ocellus is the first transitional marking appearing on larvae and, slightly later,
the outline of iridescent spots and stripes appear on the head and upper body
and the spotted species S. adustus
and S. planifrons can
be distinguished from the striped species S.
diencaeus, S. leucostictus,
and S. variabilis. |
| |
New recruits of S. partitus
share the blue spots and stripes on the head and iris characteristic of the other
species, although these are usually not conspicuous and disappear with growth.
Once the dorsal fin ocellus is present on transitional recruits, S.
adustus and S. planifrons
separate out by having only spots, not stripes, on the top of the head (as well
as distinctive color patterns). The three remaining reef species, all with a dorsal
fin ocellus and stripes on the head, share most basic markings as new recruits
(S. diencaeus, S.
leucostictus, and S. variabilis).
Nevertheless, with the characters described below, juveniles of these species
should be categorized correctly. It should be noted, however, that intermediate
individuals do occur. This may be a result of variability within species, although
the possibility of occasional hybridization should not be excluded. DNA sequencing
analyses, underway at present on this group, will confirm the identification of
larvae and new recruits of these species and explore the hybridization question
further. | |
| The most troublesome feature of
juvenile markings in these damselfishes is the rapid change in the size, shape,
and position of the spots, stripes, and the dorsal fin ocellus. Overlying these
ontogenetic changes is a high degree of variability within species. My DNA sequencing
of these damselfishes reveals that many features are inconsistent for separating
species, especially the size and position of the dorsal fin ocellus and the intensity
of spots on the head and dorsal fin. | |
| | |
| | |
|
| | Diagnosis:
Damselfishes with 12 dorsal spines and a mode of 15-16 dorsal soft rays comprise
the genera Stegastes spp. and Microspathodon
chrysurus. Fin ray counts broadly overlap among Stegastes
spp. with most species having 13-14 soft anal fin rays and 18-20 pectoral
fin rays (S. adustus and
M. chrysurus have
a mode of 21 pectoral fin rays). Given this overlap, it is likely that pre-transitional
larvae will require DNA sequencing to reliably distinguish the species. |
| |
Notes on juvenile markings: New recruits (10-15 mm SL) of S. planifrons
have only a few discrete small and round blue spots on the head and upper body
with only a single spot on the upper iris (no stripe). There is a large black
ocellus centered on the last three dorsal spines and first three dorsal soft rays
(about 40% on the fin, 60% on the body) and a caudal peduncle saddle made up of
a pale area surrounded by dusky shading followed by a distinct black spot. Although
it is a diagnostic feature for later juveniles, new recruits do not have a spot
at the top of the pectoral fin base. Ontogenetic homologies include a short-lived
blue ring around the dorsal fin ocellus and blue edging around the caudal peduncle
saddle spot. | |
| New recruits of S. planifrons
are separated from S. diencaeus,
S. leucostictus, and
S. variabilis by having
no stripes on the iris or the top of the head. In addition, they are uniformly
yellow or dusky yellow (S. diencaeus
is the only other species that can be uniformly yellow, although often new recruits
are not). New recruits of S. planifrons are separated from S.
adustus by color as well as having much smaller blue spots (and only one
on the iris) and from S. partitus
by having the dorsal fin ocellus and caudal peduncle saddle. | |
| Later juveniles (over 15 mm
SL) are characterized by a uniform yellow body, a black dorsal fin ocellus, a
large black upper caudal peduncle spot, and a prominent black spot at the pectoral
fin base. | |
| Description: | |
|
|
| Stegastes planifrons new recruit |
| 10.9 mm SL | | Belize,
BZ98-704 | |  | | |  |
| | |
| | |
|
| | Diagnosis:
Damselfishes with 12 dorsal spines and a mode of 15-16 soft rays comprise the
genera Stegastes spp. and Microspathodon
chrysurus. Although fin ray counts broadly overlap among the other Stegastes
spp. with most species having 13-14 soft anal fin rays and 18-20 pectoral
fin rays, S. adustus and
M. chrysurus have
a mode of 21 pectoral fin rays. The two latter species have a slight divergence
in anal fin soft rays, with S. adustus
having 13-15, while M. chrysurus
has 12-13. | |
| Notes on juvenile markings:
New recruits (10-15 mm SL) of S. adustus have | |
| New recruits of S. adustus
are separated from S. diencaeus,
S. leucostictus, and
S. variabilis by having
no stripes | |
| Later juveniles (over 20 mm
SL) are characterized by | |
| Description: | |
|
|
| Stegastes adustus new recruit |
| 10.9 mm SL | | Belize,
BZ98-704 | | | | | |
| | |
| | |
|
| | Diagnosis:
Damselfishes with 12 dorsal spines and a mode of 15-16 soft rays comprise the
genera Stegastes spp. and Microspathodon
chrysurus. Fin ray counts broadly overlap among Stegastes
spp. with most species having 13-14 soft anal fin rays and 18-20 pectoral
fin rays (S. adustus and
M. chrysurus have
a mode of 21 pectoral fin rays). Given this overlap, larvae and even early juveniles
can require DNA sequencing to reliably distinguish the species. | |
| Notes on juvenile markings:
New recruits (10-15 mm SL) of S. leucostictus have a pattern of blue stripes
and spots on the head and upper body with two spots and a stripe on the iris.
The upper part of the head and anterior body have a dusky blue background color.
The blue stripes and spots are heavier than in other conspecifics and usually
there is a third line of blue spots between the major stripe on the head and the
stripe directly behind the upper eye. Furthermore, the stripes typically break
up into a pattern of numerous bright spots on the anterior dorsal fin. There is
a large black ocellus ringed in blue centered on the last two dorsal fin spines
and first three dorsal fin soft rays. The ocellus is typically mostly on the fin
(usually 70% or more) and often higher than wide. There is a space of about one
scale between the lateral line and the blue ring. In addition, the scales with
blue below the ocellus on the body are outlined rather than merged into a blue
line. The spot migrates rapidly onto the fin, and by the time the juveniles reach
20 mm SL almost all of the black spot is over the dorsal fin. There is a dark
spot at the top edge of the pectoral fin base. There is no caudal peduncle saddle
spot. | | | New
recruits of S. leucostictus are separated from S.
planifrons and S. adustus
by having stripes along with the spots on the head and iris and from S.
partitus by having a dorsal fin ocellus. The more difficult separation
concerns new recruits (up to 15 mm SL) of S. leucostictus, which are separated
from S. variabilis and
S. diencaeus only by subtle
characters, i.e. having the dorsal fin ocellus centered off the body, usually
more than 70% over the dorsal fin. In S. leucostictus, there is a space
about one scale wide between the blue ring and the lateral line, while in the
other species the blue ring is less than a scale above the lateral line. In addition,
the blue ring below the dorsal fin ocellus is outlined on each scale distinctly
in S. leucostictus and merged into a line on the other two species. On
the head, the blue spots usually form a third row between the major head stripe
(from the upper jaw towards the dorsal fin) and the minor stripe (from the upper
eye towards the upper body), whereas in the other two species this additional
row is reduced and appears later, after 15 mm SL. S. leucostictus recruits
and juveniles do not have the caudal peduncle saddle spot and this is a useful
character to exclude S. leucostictus when it is present, but the absence
of the saddle means little in new recruits, when S.
variabilis may not yet have developed a saddle and when S.
diencaeus variably shows none or a transient saddle mark. Also S. leucostictus
recruits and juveniles always show the dark spot at the top of the pectoral fin,
so the absence of the spot indicates it is not S. leucostictus, however
the presence does not exclude the other species. | |
Later juveniles (over 20
mm SL) are yellow with a blue (often bright blue) upper head and anterior body
and dorsal fin with numerous iridescent blue stripes and spots, a dorsal fin ocellus
mostly off of the body (often completely over the fin rays and subsequently the
edges becoming indistinct and disappearing), a spot at the top of the pectoral
fin base and no caudal peduncle saddle. Some juvenile damselfishes are identical
in all respects to these S. leucostictus, but do have a caudal peduncle
saddle, and these are automatically assigned to S.
variabilis (DNA sequencing should confirm this conclusion). |
| | Description:
| | |
|
| Stegastes leucostictus juvenile |
| 21.1 mm SL | |
Species ID: | | - three blue stripes on each
side | | -numerous blue spots on spinous dorsal
fin | | -ocellus all on the fin |
| -pectoral fin base spot | | -no
caudal peduncle saddle spot | |
| St. Thomas, USVI , ST107 | |  | | |  | | |  | | |  |
| | |
| | |
| | |
|
| | Diagnosis:
Damselfishes with 12 dorsal spines and a mode of 15-16 dorsal soft rays comprise
the genera Stegastes spp. and Microspathodon
chrysurus. Fin ray counts broadly overlap among Stegastes
spp. with most species having 13-14 soft anal fin rays and 18-20 pectoral
fin rays (S. adustus and
M. chrysurus have
a mode of 21 pectoral fin rays). Given this overlap, larvae and even early juveniles
can require DNA sequencing to reliably distinguish the species. | |
| Notes on juvenile markings:
New recruits (10-15 mm SL) of S. variabilis have a pattern of blue stripes
and spots on the head and upper body with two spots and a stripe on the iris.
The upper part of the head and anterior body have a dusky blue background color.
There is a large black ocellus ringed in blue centered on the last two dorsal
fin spines and first three dorsal fin soft rays. The ocellus is typically 50%
on the fin, and 50% on the body. The blue edging below the ocellus on the body
is usually an intact blue line and it comes close to the lateral line (usually
less than one scale away). There is a characteristic blue-edged black spot on
the caudal peduncle just behind the last dorsal fin ray. | |
| New recruits of S. variabilis
are separated from S. planifrons
and S. adustus by having
stripes along with the spots on the head and iris and from S.
partitus by having a dorsal fin ocellus. New recruits of S. variabilis
develop a prominent caudal peduncle saddle mark, which serves to distinguish them
from the otherwise very similar S.
leucostictus and S. diencaeus.
Recruits that have not developed the saddle can be separated from S.
leucostictus by having the ocellus well onto the body (a less useful character
is usually no additional row of spots between the major stripe on the head and
the minor stripe from the top of the eye, and sparser spotting on the anterior
dorsal fin). Early recruits that have not developed the saddle or have an incipient
saddle are difficult to separate from S.
diencaeus when life colors are not apparent (S.
diencaeus recruits should not have the anterior dusky blue wash on the
head and upper body and the stripes are more magenta than blue). Some S. variabilis
up to 20 mm SL have indistinct caudal peduncle saddles and many S.
diencaeus of the same size have similar-appearing saddles. In that case,
the distinction becomes subtle: S. variabilis has more dusky shading over
the head and body, especially including the posterior dorsal fin and the anal
fin while this area is not dusky in S.
diencaeus. Unfortunately, degrees of duskiness are poor characters to
separate individuals. DNA sequence comparisons underway at present should resolve
the line of separation. | |
| Later juveniles (over 20 mm
SL) are are yellow with a blue (often bright blue) upper head and anterior body
and dorsal fin with numerous iridescent blue stripes and spots, a dorsal fin ocellus
over the body and fin (moving off the body when large juveniles over 30 mm SL),
and a prominent black caudal peduncle saddle. | |
| Description: | |
|
|
| Stegastes planifrons new recruit |
| 10.9 mm SL | | Belize,
BZ98-704 | | | | | |
| | |
| | |
|
| | Diagnosis:
Damselfishes with 12 dorsal spines and a mode of 15-16 dorsal soft rays comprise
the genera Stegastes spp. and Microspathodon
chrysurus. Fin ray counts broadly overlap among Stegastes
spp. with most species having 13-14 soft anal fin rays and 18-20 pectoral
fin rays (S. adustus and
M. chrysurus have
a mode of 21 pectoral fin rays). Given this overlap, larvae and even early juveniles
can require DNA sequencing to reliably distinguish the species. |
|
| Notes on juvenile markings: This
species is distinct in the field in having magenta stripes and spots on
the head, not strictly blue as in other juvenile damselfishes. This important
feature is rarely mentioned in books and guides (in Randall's 2nd edition the |
| stripes
are described as violet, in the 3rd edition as blue). There are several possible
reasons for this. One is that the difference is sometimes subtle and affected
by lighting. Magenta is made up of equal parts red and blue, and red is a color
that attenuates rapidly underwater and thus in deeper water (and in photographs)
the magenta would turn towards blue. Also, the red color disappears promptly in
preservative, leaving the blue spots and lines indistinguishable from those on
other juvenile damselfishes. Add to that the relatively low abundance of juveniles
on many reefs, certainly compared to the ubiquitous
S. leucostictus and S.
variabilis, and there is ample room for confusion. The possibility that
there is some variation in the occurrence of magenta in this species will be explored
by DNA sequence identifications of candidate juveniles. | 
| Photograph
by Gregory Taylor
saltyzoo.com | | |
| New recruits (10-15 mm SL)
of S. diencaeus have a pattern of magenta stripes and spots on the head
and upper body with two spots and a stripe on the iris. There is no additional
row of spots between the major stripe on the head and the minor stripe from the
top of the eye (this additional row emerges after about 15-20 mm SL, often just
three spots in a row). There is a large black ocellus ringed in blue centered
on the last three dorsal fin spines and first three dorsal fin soft rays (about
50% on the fin, 50% on the body). Although the top of the head and anterior body
may be duskier than the rest of the body in new recruits, it is not blue. Ontogenetic
homologies include a transient darkening of the scales at the saddle area of the
caudal peduncle, a dark spot at the top of the pectoral fin base, and some darkening
of the anterior upper body (but not clearly blue). | |
| New recruits of S. diencaeus
are separated from S. planifrons
and S. adustus by having
stripes along with the spots on the head and iris and from S.
partitus by having a dorsal fin ocellus. New recruits of S. diencaeus
are most easily separated from the other striped species when the S. diencaeus
have no blue wash over the anterior upper body and head, but some individuals
do show a dusky anterior coloration and these individuals can be problematic.
Other features separating the new recruits from
S. leucostictus are having the ocellus half onto the body, the blue
ring below the ocellus not broken up, no additional row of spots between the major
stripe on the head and the minor stripe from the top of the eye, and sparser spotting
on the anterior dorsal fin. New recruits of S. diencaeus can also be separated
from S. variabilis by
having only a faint caudal peduncle saddle (if any) but, when compared to recruits
of S. variabilis that
have not yet developed their saddle, the distinction is difficult without life
colors. Unfortunately, some S. diencaeus recruits up to 20 mm SL have some
form of faint caudal peduncle saddle and some S.
variabilis of the same size can have similar unformed saddles. In that
case, the distinction can become subtle: S.
variabilis has more dusky shading over the head and body, especially including
the posterior dorsal fin and the anal fin which are not dusky in S. diencaeus.
Unfortunately, degrees of duskiness are poor characters to separate individuals.
DNA sequence comparisons underway at present should resolve the line of separation. |
| | Later
juveniles (over 20 mm SL) are characterized by magenta stripes on the head and
upper body, a relatively uniform yellow body (without obviously blue shading on
the anterior upper body and head), a dorsal fin ocellus distinctly over both the
body and fin, no caudal peduncle saddle spot, and no black spot at the top of
the pectoral fin base. | |
| Occasional problematic individuals
do have some blue shading over the top of the head and more than the usual blue
spotting and these may be confused with (or hybrids of) S.
leucostictus (both species missing the caudal peduncle saddle). DNA sequence
comparisons underway at present should resolve the line of separation. |
| | Description:
| | |
|
| Stegastes planifrons new recruit |
| 10.9 mm SL | | Belize,
BZ98-704 | | | | | |
| | |
| | |
|
| | Diagnosis:
Damselfishes with 12 dorsal spines and a mode of 15-16 dorsal soft rays comprise
the genera Stegastes spp. and Microspathodon
chrysurus. Fin ray counts broadly overlap among Stegastes
spp. with most species having 13-14 soft anal fin rays and 18-20 pectoral
fin rays (S. adustus and
M. chrysurus have
a mode of 21 pectoral fin rays). Given this overlap, it is likely that pre-transitional
larvae will require DNA sequencing to reliably distinguish the species. |
| |
Notes on juvenile markings: New recruits (10-15 mm SL) of S. partitus
have | | | New
recruits of S. partitus are separated | |
| Later juveniles (over 20 mm
SL) are characterized by | |
| Description: | |
|
|
| Stegastes planifrons new recruit |
| 10.9 mm SL | | Belize,
BZ98-704 | | | | | |
| | |
| | |
|
| | Diagnosis:
Damselfishes with 12 dorsal spines and a mode of 15-16 dorsal soft rays comprise
the genera Stegastes spp.
and Microspathodon chrysurus. Although fin ray counts broadly overlap among
the other Stegastes spp.
with most species having 13-14 soft anal fin rays and 18-20 pectoral fin rays,
S. adustus and M.
chrysurus have a mode of 21 pectoral fin rays. The two latter species
have a slight divergence in anal fin soft rays, with S.
adustus having 13-15, while M. chrysurus has 12-13. Late larvae
and transitional recruits of M. chrysurus have the distinctive blackening
of the pectoral fin rays that confirms the identification. (ML) | |
| Description: |
| | Microspathodon
chrysurus new recruit, 10.9 mm SL | | |
|
| | |
|
| | Diagnosis:
Damselfishes with 13 dorsal spines comprise the Abudefduf
spp. along with the deep-water Chromis enchrysura,
C. insolata, and C. scotti. Abudefduf saxatilis has a mode of 13 dorsal
soft rays and 12 anal soft rays (D-XIII,13 A-II,11-12 Pect-18-19). A. taurus has
fewer median fin soft rays, with a mode of 12 dorsal soft rays and 10 anal soft
rays (D-XIII,12 A-II,10 Pect-18-19). The Chromis
spp. have modes of 12 and 11 or 12 and 12. (ML) | |
| Description: | |
|
|
| Abudefduf saxatilis transitional recruit, |
| 10.6 mm SL | | (San
Blas, Panama, SB84-523) | | | | | | | | | | | | | | |
| | |
| | |
|
| | Diagnosis:
Damselfishes with 13 dorsal spines comprise the Abudefduf
spp. and the deep-water Chromis enchrysura,
C. insolata, and C. scotti. Abudefduf taurus has a mode of 12 dorsal soft
rays and 10 anal soft rays (D-XIII,12 A-II,10 Pect-18-19). A.
saxatilis has a mode of 13 dorsal soft rays and 12 anal soft rays (D-XIII,13
A-II,11-12 Pect-18-19). The Chromis spp. have
modes of 12 and 11 or 12 and 12 respectively. (ML) | | |
| Description: | |
|
|
| Abudefduf taurus
transitional recruit, | | 11.1 mm SL |
| (San Blas, Panama, SB83-137) | | | | | | | | | | | | | | |
| | |
| | |
|
| | Diagnosis:
Damselfishes (ML) | |
| Description: |
| | Stegastes
planifrons new recruit, 10.9 mm SL 10.9 mm SL | |
| | |
|
