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Hatchlings of clown loach (3.8 mm in total length, TL) are pelagic. When
reared at 26−28 °C, they become benthic 3 days after hatching (dah), when their swim
bladder is inflated. The fin development sequence (pectorals < caudal <
dorsal = anal < pelvic) is typical of the clade Ostariophysi. All fins and
finrays are fully elongated at 20 mm TL, but the finfold persists until
25 − 26 mm TL (start of juvenile stage). Melanophores appear at 3 dah,
they form a 5-bar pattern at 5 dah, then two bars (III and V) vanish progressively,
producing at 26 mm TL the 3-bar pattern that is typical of adults.
Throughout the ontogeny, the pigment pattern exhibits a structural regularity (bars spaced
at regular intervals), which is interpreted in a functional perspective by reference to
the maintaining of crypsis and signalling throughout. Exogenous feeding commences at 4 dah
(5.5 mm TL). Food intake (FI) increases rapidly, from 6%
wet body mass (WM) at 5.5 mm TL to >20%
WM in fish > 7 mm TL. Gut evacuation rate
(Rg) increases with increasing meal size
and fish size, as a result of gut coiling (from 8 to 15 mm TL), and is
highest at 11 mm TL (about 10% WM h-1 in fish
feeding maximally). The allometric increase of FI and
Rg during the early larval stages is
accompanied by increasing capacities for growth, so early sizes differences amplify
rapidly during the ontogeny. Nevertheless, growth remains slow (mean of 0.4 mm TL
day-1 from 4 to 29 dah; 0.9 mm TL day-1
for top growers). By contrast, unfed fish display long resistance to starvation (until
14−15 dah). The combination of slow growth and long resistance to starvation is discussed
in respect to the reproductive phenology of the species, as the capacity of making
metabolic economies prevails over fast growth for seasonal strategists spawning mainly at
the start of the rainy season.
The knowledge of how fish survive and grow at different temperatures, and how these traits vary between life stages, is essential to evaluate the effects of climate change on wild fish and implement effective strategies in aquaculture. These issues are addressed in this study through a series of experiments that evaluate the effect of temperature (23–34 °C) on the embryos and larvae of clown loach, Chromobotia macracanthus. This species is endemic to the rivers of Sumatra and Borneo, highly praised on the ornamental fish market, and has been reproduced in captivity recently. No embryo survived a 24-h exposure to 34 °C until the age of 3 days after hatching (dah); mortality was high at 32 °C at 2 and 3 dah, whereas it was low and similar from 1 to 4 dah at 23–29 °C (<10%). Yolk absorption was proportional to water temperature (Q10°C
of 1.69 in the 23–32 °C range), but fish reared at cold temperatures were larger than others at the start of exogenous feeding (5.7 vs. 5.5 mm TL, at 23 and 32 °C, respectively). The survival of larvae fed Artemia nauplii ad libitum was high at 23–32 °C (80–100%), but almost null at 34 °C. Growth models at different temperatures were produced from weekly measurements in two experiments, and tested by comparing their predictions with the results of a third experiment. Throughout the larval stage, the optimal temperature for growth (T°opt)
was close to 29 °C, and departures from T°opt
resulted in substantial growth penalties (–30% SGR for –5.1 °C and + 3.1 °C). High survival, fast growth (0.7 mm day-1) and limited size dispersal at T°opt
are encouraging perspectives for the aquaculture of clown loach. From an ecological perspective, the species has an atypical thermal biology, as it is less thermophilic than other tropical fishes, but more stenothermal than temperate fishes exhibiting similar values of T°opt
, both traits being of particular concern in the context of global warming.
In comparison to older life stages, the embryonic stages of fishes generally have narrow tolerance ranges for environmental conditions, as regards water quality, temperature and mechanical shocks. The knowledge of these factors is indispensable to appraise the threats brought about by climate or anthropogenic changes upon their resilience, and to define adequate ways of incubating their eggs for an efficient propagation of the species under controlled conditions. Clown loach eggs have a narrow thermal tolerance range in comparison to other tropical and temperate fishes. Hatching occurs at 22–30 °C, and non-deformed larvae can only be obtained at 23.8–30.2 °C. Furthermore, the thermal tolerance of any particular progeny was found dependent on the maintenance temperature of the female parent, thereby making the actual tolerance no broader than 4.5 °C. The (log-log) relationship between the duration of the incubation period and temperature was characterized by a shallow slope, which is more typical of coldwater fishes, as is a narrow thermal tolerance range. On the other hand, clown loach hatched more rapidly (20 h at 26 °C) than predicted by existing models on the basis of water temperature and egg diameter, a feature that is shared by other warmwater fishes producing eggs that undergo a strong swelling process (about three times the ova diameter in clown loach). Clown loach embryos are strongly sensitive to mechanical shocks, but their development is not viable either in protracted steady state conditions, in absence of water movement, as they develop various deformities (e.g. pericardial oedema). This is thought to originate from a hypoxic microenvironment around the embryo, as a consequence of an oxygen gradient developing inside and outside the egg, since the boundary diffusion layer is not refreshed by water movement. This issue is worsened by strong egg swelling and incubation at warm temperature.
The clown loach Chromobotia macracanthus, endemic to Indonesia, is a
major species on the international market of ornamental freshwater fish. In order to
satisfy an increasing demand with a sustainable alternative to the massive capture of wild
juveniles, research has been dedicated to the artificial propagation and domestication of
this species. The present study, the first of a series, focused on favourable maintenance
conditions for broodfish sexual maturation, criteria for identification of ripe fish,
efficiency of hormone-induced breeding treatments, predictability of their latency
response, and on the comparison of reproductive performances of fish from populations of
Sumatra and Borneo Islands (in total, 112 females of 46 to 404 g body weight). When reared
in fully controlled conditions in large water recirculation systems, broodfish originating
from Sumatra had reproductive performances similar to or slightly higher than those
maturing in the wild (ovulation rate of 93% vs. 82%, relative fecundity of 109 277 vs.
103 550 ova kg-1 and fertilization rate of 73% vs. 61%, respectively). In the
same rearing conditions, captive females from Borneo (n = 22) showed
lower ovulation rate (77%), relative fecundity (76 262 ova kg-1) and
fertilization rate (50%) than those originating from Sumatra (n = 28). By
contrast, the mean individual weight of ova (around 0.8 mg) was independent from the
origin or maintenance conditions of females. An initial modal follicle diameter ≥1.02 mm
generally led to high ovulation success (>80%) after hormonal treatment and is
recommended as the main criterion for selecting female broodfish. Two hormonal treatments
for inducing oocyte maturation and ovulation (T1: two successive injections of Ovaprim at
a 6 h-interval; T2: one injection of human chorionic gonadotropin (hCG)- and one of
Ovaprim 24 h later), produced similar results in terms of ovulation rate, quantity and
quality of ova collected. With both treatments, the latency decreased with increasing
water temperature, then increased again at temperatures >28–29 °C. To our
knowledge, such U-shaped relationship between the latency response and
temperature has never been documented in teleost fishes.
Large differences in reproductive traits were observed between populations of Oreochromis niloticus sampled in 9 manmade lakes of Ivory Coast. In order to assess whether these variations in reproductive characteristics resulted from short term adaptation or from a longer evolutionary process, living specimens were caught in the two most differentiated populations in term of life history traits, and placed in a common environment in culture conditions. Genetic analysis of fish from these two populations were performed using four microsatellite markers and revealed that both descended originally from the same strain (Bouaké station) which was constituted from broodfish initially caught in the Nile and Volta basins. Fish from the two populations were subjected to a common environment (pond and aquariums) for five months. Then, their reproductive characteristics were analysed and no significant differences were found in fecundity, egg size and spawning frequency. These results indicate that reproductive differences between the two populations, originally observed in the two reservoirs, mostly reflect the phenotypic plasticity of the species in facing different environmental conditions.
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