Sri Lanka J. Aquat. Sci. 26(1) (2021): 25 - 36 http://doi.org/10.4038/sljas.v26i1.7582
This work licensed under a Creative Commons Attribution-NonCommercial
4.0 International License
Embryonic and larval development in the freshwater angelfish
K. G. D. D. Thilakarathne1, G. N. Hirimuthugoda1, P. H. T. Lakkana2 and S. Kumburegama1*
1Department of Zoology, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka.
2Faculty of Applied Science and Technology, Uva Wellassa University, Badulla, Sri Lanka.
Received: 25.08.2020 Revised: 25.02.2021 Accepted: 27.01.2021 Published online: 15.03.2021
Abstract The freshwater angelfish, Pterophyllum scalare like many other cichlids show parental care of embryos and larvae. This
study was carried out to investigate the embryonic development of P. scalare, which shows biparental care and substrate brooding.
During the study adult reproductive behavior and parental care was observed. Once the eggs were fertilized upon spawning, the
early and later embryonic stages were observed, documented and various embryo length measurements were analyzed to
characterize the developmental pattern in this species. Whole mounts and bone and cartilage measurements of acid free double
stained larvae were analyzed to further understand the developmental rates in the hatched larvae. The developmental events were
compared with those of other documented cichlid species as well as with the zebrafish, Danio rerio (Family Cyprinidae), which
does not show parental care. After fertilization, cleavage division of the P. scalare embryo starts 1.30 hours post fertilization (hpf).
On average, cleavage, blastula, gastrula, segmentation and pharyngula periods of embryogenesis are observed for approximately 3
½, 11 ½, 9 ½, 36 and 12 hours respectively. Ultimately, P. scalare embryos hatched around 72 (hpf). Head Length, dorsal, caudal
and anal fins show positive allometric growth while body depth and digestive tract show almost isometric growth. The study
highlights that similar to a few other studied cichlids, P. scalare embryogenesis and larval development occur at a slower rate of
development compared to D. rerio. In cichlids including P. scalare, parental care may allow these embryos the luxury of developing
at a slower rate whereas the lack thereof for D. rerio embryos may necessitate faster development.
Keywords: Pterophyllum scalare, egg, embryo, development, fish larvae
Even though, for many cultured fish, sufficient
information on their reproductive behavior is
available their embryonic development at often
times is ignored. However, an understanding of
embryonic development is required not only for
captive breeding in aquaculture but also for various
experimental studies in areas such as developmental
biology, neurobiology, toxicology and pathology
(Rahman et al. 2009). For example, the zebrafish,
Danio rerio is a model organism widely used in
vertebrate developmental studies. This species in
addition to a few other species such as the rainbow
trout (Oncorhynchus mykiss) and medaka (Oryzias
latipes) are among the few species of fish where
embryonic development has been studied in detail.
Family Cichlidae is one of the largest and one of the
most successful groups of bony fish consisting of
about 1,300 species (Nelson 2006). Many members
of this family such as freshwater angelfish
(Pterophyllum scalare), Oscar (Astronotus
ocellatus), Discus (Symphysodon sp.), Blood red
parrot fish (Hybrid), Ram cichlid (Mikrogeophagus
ramirezi), Cichlasoma dimerus, Golden eye cichlid
(Nannacara anomala) and Humped-head cichlid
(Hybrid) are widely used in ornamental fish industry
and they are among the most popular ornamental
fish species cultured in many countries. These fish
are relatively cheap and can tolerate a range of
physiochemical conditions. Almost all freshwater
cichlids provide parental care for their offspring.
They display uniparental or biparental care by either
mouth brooding or by guarding nests or egg clutches
that are attached to external substrates, referred to as
substrate brooding (Goodwin et al. 1998).
Among the cichlids, the Freshwater angelfish, a
native of tropical South American waters is
considered a relatively cheap, ideal community tank
fish that has become a popular aquarium fish in
26 K. G. D. D. Thilakarathne et al
many countries around the world. This fish comes in
several colors and forms such as Cobra angelfish,
black zebra angelfish, half black angelfish and Koi
angelfish (Padilla and Williams, 2004). The
reproductive behavior of this species is relatively
well documented. They show biparental care where
both parents are involved in substrate brooding.
However, similar to many other species their
embryonic development is often overlooked (Cacho
et al. 2007). Hence, the present study focused on
understanding the embryonic and later development
of freshwater angelfish as an example of a cichlid
that demonstrate both substrate guarding and
MATERIALS AND METHODS
Reproductive behavior and embryonic development
of freshwater angelfish
After introducing several freshwater angelfish into a
1 x 2 m tank, they were allowed to pair. Once the
fish paired, three pairs were used to study their
reproductive behavior. Each pair was put into a
separate 100 liter tank with gravel-filtered water.
The fish were fed twice daily. A clean,
longitudinally split PVC tube (about 20 cm long)
was placed at an angle of 45° to the floor of the tank
as a spawning substrate. Once egg laying was over,
a plastic pipette was used to suck several embryos
from the PVC surface to study different stages of
embryonic development. The embryos were
observed under a Leica dissecting microscope and
photographed using a Nikon digital camera mounted
on the microscope. In addition, embryo
measurements were taken using a Zeiss Primo star
inverted microscope with the aid of Zen 2012
software. The length and width of the embryo,
length and width of the yolk and the height of the
blastoderm were measured for all early stage
embryos before hatching. Once the embryos hatched
the total length, length and width of head and the
yolk sac, lengths of tail, trunk, anal fin, caudal fin
and dorsal fin were measured. This data was
analyzed and compared using MS-Excel 2010 and
Minitab 16. The mean, standard deviation, standard
errors of the measurements were calculated for each
All embryonic stages were fixed in 4%
paraformaldehyde for whole mount preparation and
bone and cartilage staining. Later developmental
stages from seven to 21 days post fertilized were
first anesthetized using MS222 before fixing in 4%
paraformaldehyde overnight at 4°C. All fixed
embryos were stored at 4°C until further analysis.
a. Acid free double staining
An acid free double staining procedure based on
Walker and Kimmel (2007) was utilized to study
cartilage and bone development in later stages.
Photographs and measurements of bones and
cartilage were taken using a Zeiss Primo star
inverted microscope or a Leica Dissecting
microscope attached with a Nikon digital camera.
b. Whole mount staining
Whole mount staining with Borax Carmine of later
developmental stages was performed to study the
morphology. Embryos fixed in 4%
paraformaldehyde were transferred into distilled
water and 50% alcohol for 15 minutes each to
remove excess fixative. Next, embryos were stained
in Borax Carmine for 15 minutes and transferred to
50% alcohol for 2 minutes. Embryos were washed
in 70% alcohol for 2 minutes and distained in acid
alcohol for 2 minutes. Incubation in Ammonia
alcohol for 2 minutes was used to stop the reaction.
Next, the embryos were dehydrated in 90% alcohol
followed by several washes in Absolute alcohol.
Finally, the embryos were cleared in clove oil and
observed under a light microscope.
Larval body measurements were used to determine
the pattern of development in relation to body length
of P. scalare. The allometric patterns of
development of P. scalare were determined by the
growth coefficient using the equation Y = aXb,
where Y is the dependent variable/measured
character and X is the independent variable/total
length (TL), a is the intercept and b is the growth
coefficient. When isometric growth occurred, b = 1,
whereas either positive or negative allometric
growth is indicated if b > 1 or b < 1.
K. G. D. D. Thilakarathne et al
Reproductive behavior and parental care in P.
Before the onset of egg laying, the male and female
that were allowed to pair beforehand followed
ritualistic premating as well as mating behaviors.
Reproductive tubes were clearly visible in both male
and female a few days before spawning. Usually,
three or four hours before mating the male and
female fish cleaned the artificial substrate provided
(a PVC tube) using their mouth for nearly 1-2 hours.
They spawned approximately between 12.00 noon
and 3.00 pm and they were able to spawn regularly
at 10-day intervals as long as the previous egg clutch
was removed immediately after they end spawning.
When the female started egg laying the male
followed the female and fertilized the eggs. This was
observed for nearly 45 to 60 minutes. Next, the male
and female fanned the eggs and removed possibly
unfertilized and damaged eggs. Both parents
showed parental care. After the embryos hatched, 3-
4 day old fries were taken into the mouth of the
parents and deposited on nearby aquatic plants.
Early embryonic development
Embryonic stages from the zygote up to the
gastrula stage were considered under early
embryonic development. As soon as a sperm
fertilizes the egg, the 1-cell stage zygote forms and
embryonic development starts at the animal pole
(AP) of the embryo. The zygote of freshwater
angelfish was observed for about 90 minutes after
fertilization (Table 1). These eggs are oval-shaped,
slightly pale in colour and full of yolk-granules
(Figure 1A). The zygote diameter along the
horizontal axis is about 1.5±0.05 mm.
Typically, 2-cell stage to 64-cell stage is
referred to as the cleavage period. Similar to other
fish, P. scalare cleavage planes were incomplete,
restricted to the animal pole cytoplasm where there
is no yolk. The cleavage period in freshwater
angelfish embryos was observed for nearly three
hours with 30-40 minute intervals between cleavage
events (Table 1, Figure 1B-G, Figure 2).
The blastula period of fish generally includes
stages from 128-cell to dome stage. During the
blastula stage, the number of blastomeres increased
but the size of the blastomeres decreased. Resulting
blastomeres over laid the yolk at the AP. About 7
hpf, the shape of the blastomere layer changed from
“high” to “sphere” to “dome” (Figure 1H-K). The
blastula period in P. scalare embryos was observed
for about 11 ½ hours (Table 1, Figure 1H-K and
Following the blastula stage the embryo
undergoes gastrulation. Gastrulation is the period
where there is extensive cell movement. In P.
scalare, the rate of development decreased soon
after the “dome” stage, at the beginning of epiboly
(Figure 1L, Figure 2) indicating this change.
Epiboly is where overlying cells of the blastoderm
gradually moves over the yolk towards the vegetal
pole (VP) of the embryo. Depending on the
percentage of yolk covered, 30%, 50%, 75% and
90% epiboly stages were recorded at 16, 20, 22 and
23 ¾ hpf respectively (Figure1L-O). After 50%
epiboly, the progressing vegetal margin thickened
(germ ring formation) suggesting that internal cell
movements like involution started around this time.
Soon after completion of epiboly, the head and tail
buds of the developing embryo were observed
(Figure 1P). It takes nearly 9 ½ hours for P. scalare
to complete gastrulation.
28 K. G. D. D. Thilakarathne et al
Table 1 Embryogenesis of P. scalare at an average temperature of 29° C in comparison to two other cichlids, O. niloticus and C. dimerus (Korzelecka-
Orkisz et al. 2012; Fujimura and Okada 2007; Meijide and Guerrero 2000; Kupren et al. 2014) and the cyprinid model organism, D. rerio (Kimmel
et al. 1995).
Hours post fertilization
Distinguishing features of P. scalare
Cytoplasmic accumulation to form the blastodisc at the animal pole and the
appearance of a distinct perivitelline space
1st vertical Cleavage
2nd vertical Cleavage at right angle to 1st.Two rows of 2 cells
Two rows of 4 cells
Four rows of 4 cells
Four rows of 8 cells
Horizontal plane, 2 tires
Five irregular tires
Nine irregular tires
11 tires, Yolk syncytial layer forms
>11 tires, Blastoderm has mound of cells
Egg became spherical
Marked beginning of epiboly
K. G. D. D. Thilakarathne et al
Hours post fertilization
Distinguishing features of P. scalare
Blastoderm cover 30% of yolk
Blastoderm cover 50% of yolk
Local thickening of the blastoderm
Blastoderm covered 75% of yolk
Blastoderm covered 90% of yolk, embryo became thickened at the anterior end, brain
started to form in the anterior end of the embryo, yolk plug can be seen at the vegetal
Tail started to form, 1st somite appears
Optic vesicle and otic placode start to form
Head become more visible
Melanophores on the yolk surface of each side of the embryo gradually differentiated
Pharyngula stage. Tail is completely separated from yolk, Heart started pumping and
blood circulation is clearly visible around the yolk and the head region
P. scalare has 26 somites when they hatched
30 K. G. D. D. Thilakarathne et al
Fig 1 Embryonic stages of P. scalare. A) 1-cell stage, B) 2-cell stage, C) 4-cell stage, D) 8-cell stage, E)
16-cell stage, F) 32-cell stage, G) 64-cell stage, H) 128-cell stage, I) 256-cell stage, J) High stage, K) Dome
stage, L) 30% epiboly stage, M) 50% epiboly stage, N) 75% epiboly stage, O) 90% epiboly stage, P) Bud
stage, Q) 3-Somite stage, R) 12 somite stage, S) 17-somite stage and T) 19-somite stage.
K. G. D. D. Thilakarathne et al
Fig 2 Embryonic developmental rate in P. scalare at different stages as a function of hpf. Stages 1-26 in Y-
axis denote 1-Cell (zygote) to Hatching. Stages 2 to 7 are cleavage period. From stage 8 to 14 are blastula
period. The gastrula period consists of stage 15 to 22 and rest of the stages belongs to segmentation period
where stage 31 is hatching.
Later embryonic development
Starting from somite formation up to hatching
was considered under later embryonic
development. In addition to somitogenesis,
head and eye developmental rates, ossification
events (development of the upper jaw, lower
jaw) and fin development (dorsal, anal, caudal
and pectoral fin) were analyzed.
Segmentation period or somitogenesis
generally follows at the completion of epiboly
and tail bud formation (Kimmel et al., 1995).
During this time somites, which will eventually
give rise to the dermis, muscle and skeletal
elements etc. are sequentially added from the
trunk to the tail. The first somite pair was
observed at the tail bud stage. Initial somites up
until the formation of the 10th somite took place
approximately at 30 minute intervals. However,
this interval gradually increased as the number
of somites increased (Figure 2). The optic
vesicle and the auditory placode were visible
around the 3rd somite stage (Figure 1Q). At
about the time of 5th somite formation, the head
became more defined. Cement glands on the
head appeared 50 hpf, approximately at 18-
somite stage (Figure 1T).
Pharyngula refers to the stage where the
embryo attains the classic vertebrate body
organization (Ballard 1981). This stage in P.
scalare was observed starting at 60 hpf. At this
time, the embryo had 25 somites and the tail was
completely separated from the yolk. Heart,
pigmented blood cells, digestive tract and eyes
were well developed. Heart started pumping
and blood circulation was clearly visible around
the yolk and the head region. P. scalare
embryos hatched at 72 hpf at which time they
had a total number of 26 somites and an average
total length of 1.60 ± 0.05 mm (Figure 3A). By
4 days post fertilization (dpf) the larva
developed melanocytes in the tail region and
yolk sac (Figure 3B). They did not have well
developed fins and as a result, the larvae were
not able to swim well. Instead, they attached to
the spawning substrate using their well-
developed dorsal and ventral cement glands
32 K. G. D. D. Thilakarathne et al
Fig 3 Different larval stages of P. scalare. A) Hatching larva 3 dpf (a-Chorion), B) 4 dpf larva (b-
Melanocytes on the tail and c-Yolk sac, C) 5 dpf larva (d-Optic cup and e-Heart), D) 6 dpf larva, E) Cement
glands on 4 dpf larva (f-Cement gland and g-Pectoral fin buds), F) Cement glands on 7 dpf larva (h-Cement
glands), G) 8 dpf larva (i-Slightly developed dorsal fin) and H) 18 dpf larva (j-Dorsal fin and k-Anal fin).
K. G. D. D. Thilakarathne et al
Morphology and developmental rate of the P.
P. scalare larval development was observed until 23
to document distinct features in the larval
development process. Around 4 dpf, the larvae
started to swim and the cement glands started to
disappear after 8 dpf. The pigmentation of the eye
started at 6 dpf, and continued up to 11 dpf (Figure
3D). The dorsal, anal and caudal fins were
continuous at this time (Figure 3G). After 11 dpf,
rays were observed on the fins (Figure 3H).
P. scalare larvae exhibited its highest rate of
development from 5 dpf to 7 dpf where it increased
its body length from 1.7 mm to 2.4 mm. Typically,
a relatively constant rate of development in length
of the larva is seen from day 12 to 17 dpf. By 23 dpf
the average body length of P. scalare larvae was
15.04±0.03 mm. The growth or increase in body
length in relation to dpf of P. scalare larvae
followed an exponential curve (Figure 4). The
development of the upper jaw in P. scalare occurred
at a relatively slower rate until 12 dpf and then
rapidly increases from 13th to 21st dpf. On the other
hand, the rate of development of lower jaw was
initially high during 5th to 10th dpf where it increased
from 1.1 mm to 2.39 mm and thereafter it gradually
slowed. The fins: dorsal fin (a = 0.07, b = 1.42, R2 =
0.99), caudal fin (a = 0.34, b = 1.26, R2 = 0.93) and
anal fin (a = 0.11, b = 1.33, R2 = 0.99), head length
(a = 0.29, b = 1.26, R2 = 0.96) and eye diameter (a =
0.15, b = 1.39, R2 = 0.96) showed positive allometric
growth during larval development indicating that all
these structures grow at a faster rate in relation to the
body length increment (Figure 5). Meanwhile body
depth (a = 0.29, b = 0.96, R2 = 0.92) and linear length
of the digestive tract (a = 0.45, b = 0.98, R2 = 0.87)
showed nearly isometric growth indicating a
somewhat slower rate of development in contrast to
the body length.
Fig 4 Rate of change in total length of the P. scalare larva.
y = 4228.8e0.07x
R² = 0.97
34 K. G. D. D. Thilakarathne et al
Fig 5 Allometric growth of P. scalarae larva. Rate of development in eye diameter, ED (R2 = 0.96, n = 30),
body depth, BD (R2 = 0.99, n = 30), head length, HL (R2 = 0.96, n = 30), trunk length, TRL (R2 = 0.87, n =
30), anal fin height, AH (R2 = 0.99, n = 30), dorsal fin height, DH (R2 = 0.98, n = 30), caudal fin length, CL
(R2 = 0.98, n = 30) and upper jaw length, UL (R2 = 0.99, n = 30) of P. scalare after hatching.
In the animal kingdom, fish show the highest variety
of parental care. Parental care ranges from simple
burial of eggs to internal gestation and live bearing
(Goodwin et al. 1998). It may involve only the male,
female, or both parents. Some type of parental care
exists in 21% of the families of bony fishes. Among
the different types of parental care, guarding
behaviour is the predominant type observed among
fish. This guarding behaviour includes active
chasing of egg or fry predators (Gross and Sargent
1985). When considering male and female
involvement in parental care, it is apparent that
uniparental care is the most common type of
parental care among fishes and male parental care is
more common (49%) than female parental care
(7%). However, biparental care is found in 13% of
the families and it is considered to be the ancestral
form of care to uniparental care (Goodwin et al.
1998; Platania and Altenbach 1998; Nova and
Biparental care is commonly seen in cichlids
that show substrate guarding. A good example is
provided by P. scalare. Once a male and a female
pair, they remain as a monogamous pair for one to
three breeding cycles (Cacho et al. 2007). Next, as
was observed during the study, the male and female
fan the eggs to supply oxygen for the rapidly
developing embryos (Reebs 2001). Because of this
parental care, the survivorship of these embryos is
high (Korzelecka-Orkisz et al. 2012). However, due
to the costs involved in biparental care, the female
lays fewer eggs to compensate for this cost (Abadian
et al. 2012; Chellappa 1999).
During this study, the embryogenesis and larval
development of P. scalare was compared with the
data available for two other cichlids, Cichlasoma
Diamet er of eye/μm
Dept h of Body/μm
Length of Head/μm
Length of trunk/μm
Height of Anal fin/μm
Height of Dorsalfin/μm
Length of caudal fin/μm
Length of Upper Jaw/μm
UL = 0.14 TL
CL= -1301 + 0.34 TL
HL = -959 + 0.29 TL
BD = 0.29 TL
ED = -468.29 + 0.15 TL
TRL = 1232 + 0.42 TL
AH = 10.60 + 0.11 TL
DH = 23.00 + 0.07 TL
K. G. D. D. Thilakarathne et al
dimerus that shows substrate brooding similar to P.
scalare and Oriochromis niloticus that shows mouth
brooding (Table 1). Furthermore, these events were
compared to that of the cyprinid, D. rerio that does
not show any parental care. P. scalare eggs are
comparable to the eggs of C. dimerus, which are
1.65±0.05 mm in diameter along the horizontal axis.
The eggs are sticky, which enables them to stick to
the substrate and to one another at the time of
deposition. This is possibly due to the mucous layer
surrounding the smooth translucent chorion
analogous to that observed in C. dimerus (Meijide
and Guerrero 2000).
Most teleost eggs are telolecithal (contains large
amount of yolk) and as a result most of the egg
cytoplasm is contained in a small area at the AP.
Hence, as indicated in P. scalare embryos, the
ensuing cleavage divisions occur only in this clear
cytoplasmic area called the blastodisc (Kimmel et al.
1995; Gilbert 2006). In D. rerio, the cleavage stages
from 2-cell to 64-cell stage are usually complete
within 2 hpf with only 15 minute intervals in
between two cleavage events (Kimmel et al. 1995).
However, the cleavage period in freshwater
angelfish embryos lasts for three hours with 30-40
minute intervals between cleavage events (Table 1
and Figure 1A-G). Similarly, in C. dimerus cleavage
is observed for 3 ½ hours with nearly 30 minute
intervals between consecutive stages (Meijide and
Guerrero, 2000) whereas in O. niloticus this period
is longer and extends for approximately 10 hours
(Fujimura and Okada 2007).
The blastula and the gastrula periods of P.
scalare exhibits a similar developmental pattern to
D. rerio (Kimmel et al. 1995) albeit at a slower rate
similar to O. niloticus and C. dimerus (Table 1). P.
scalare, embryos take approximately six hours to
complete the blastula stage in contrast to 2 ¼ hours
taken for the same stage in Zebrafish embryos
(Kimmel et al. 1995). The onset of gastrulation is
discernable by the slowing down of the rate of
development at the end of the dome stage.
Gastrulation is a crucial period during development
where there is extensive and coordinate set of cell
movements. The overall result of these cell
movement events is the production of the three
primary germ layers, which are the precursors that
will give rise to the different tissues in the body, and
the establishment of embryonic axes (Kimmel et al.
1995). Even though it takes approximately five
hours to complete gastrulation in D. rerio, it takes
about 9 ½ hours in P. scalare, similar to C. dimerus
and O. niloticus. Segmentation period or
somitogenesis generally follows at the completion
of epiboly and tail bud formation (Kimmel et al.
1995). During this time, several body structures like
the brain, eyes and the auditory placode become
more defined. Though Zebrafish embryos hatch
about 48 hpf (Kimmel et al. 1995), the P. scalare
embryos hatch 72 hpf. They attach to the spawning
substrate using well developed dorsal and ventral
cement glands which appear at pre-hatching at
approximately 18-somite stage (Groppelli et al.
2003). The whole organ is comprised of three pairs
of ductless glands, which are conspicuous elevations
on the larval head (Groppelli et al. 2003).
P. scalar larvae are referred to as altricial larvae
in which, many of the organs develop after the
larvae hatch or during or after metamorphosis (Falk-
Peterson and Hansen 2001). After these larvae
hatched, the yolk sac gradually decreased in size and
they undergo drastic metamorphosis where the eyes,
jaws, and fins develop rapidly. The rate of
lengthening abruptly decreases at the end of tail
lengthening at 17.5 dpf similar to what is observed
in Zebrafish (Kimmel et al. 1995). According to
Çelik et al. (2014), the growth coefficient in P.
scalare larva change from yolk sac larva to pre-larva
to post-larva. However, overall these changes are
similar to what is observed in other teleost fishes.
The benefits of parental care on the developmental
rate of fish embryos, larvae and juveniles have
received hardly any attention (Klug et al. 2014). It
may be that in cichlids parental care increase the
proportion of time spent in embryonic and larval
stages while reducing the time spent in juvenile
stages. Ultimately, parental care will lead to the
increase fitness of both the parents as well as their
P. scalare early and later embryonic development as
well as its larval development occur at a slower rate
similar to those of other studied cichlids but in
contrast to that in the cyprinid, D. rerio, which
develop at a much faster rate. Parental care is likely
to influence embryonic and larval development. In
cichlids including P. scalare, parental care may
allow these embryos to develop at a slower rate
whereas the lack thereof for D. rerio embryos may
necessitate faster development.
36 K. G. D. D. Thilakarathne et al
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