Reversible change in embryonic diapause intensity by mild temperature in the Chinese rice grasshopper, Oxya chinensis

The effects of temperature on maintenance and termination of embryonic diapause were investigated in Jining (35.4°N, 116.6°E) and Sihong (33.5°N, 118.2°E) strains of the Chinese rice grasshopper, Oxya chinensis Thunberg (Orthoptera: Catantopidae). Eggs of both strains entered diapause when incubated at 30, 25, or 20 °C. Chilling at 8 °C had an evident effect on diapause termination and almost all eggs chilled for 60 days ended diapause development. Chilling of eggs at 8 °C for only 20 days failed to result in any hatching at 20 °C, suggesting that such level of chilling was not enough to induce diapause termination. However, the treatment combining incubation of eggs at 30 °C for varying lengths of time with subsequent incubation to 20 °C had a distinct effect on the completion of diapause of the eggs. The results indicate that there were two temperature optima, that is, low temperature (chilling) and high temperature, for diapause development in this grasshopper species. Incubation of chilled eggs at 20 °C for 5–15 days followed by further incubation at 25 °C reduced termination of diapause significantly compared with the eggs only chilled at 8 °C. Exposure of eggs chilled at 8 °C to a pulse of 25 °C from 1 to 7 days, separated by a 20-day interval at 8 °C, resulted in a decrease in the percentage of successfully hatched eggs as the length of the pulse of 25 °C increased. The results suggest that diapause intensity may be restored at moderately high temperatures. This reversible change in diapause intensity would play an important role in maintaining diapause before winter.     

Termination and reinduction of reproductive diapause by photo-period and temperature in males of the grasshopper, Oedipoda miniata

ABSTRACT. Previous publications have demonstrated that O.miniata adults exhibit an aestival reproductive diapause during the dry summer in Israel; females do not lay and males show only little mating behaviour. It was also reported previously that photoperiod and temperatures corresponding to autumn in Jerusalem terminate diapause and induce intense male mating behaviour, whereas continuous illumination and high temperatures maintain diapause. In the present study we used mating behaviour as an indicator for investigating the effects of several photoperiod—temperature combinations on reproductive diapause in O.miniata males. Mating behaviour was induced and diapause terminated under the following conditions: LD 10.5:13.5, T = 38-27°C (the temperatures during the photophase and scotophase, respectively); LD 12:12, T=38–27°C; LD 12:12, T=26–13°C; LD 14:10, T = 26–13°C. In contrast, diapause was maintained under LD 14:10, T = 38–27°C and under LL with high but variable temperatures. In further experiments males that were already sexually active, and which had been kept under LD 12:12, T = 38–27°C, were transferred to LL with high variable temperatures. Similarly, postdiapause males which had been kept under LD 14:10 and T=26–13°C were transferred to T = 38–27°C without changing the photoperiod. In both instances mating behaviour declined, then disappeared, thus reinduction of the diapause occurred under such conditions which previously were found to maintain diapause. O.miniata is therefore a 'short day' and/or 'low temperature' insect. This is the first report on complete control of photoperiod—temperature over aestival reproductive diapause and its reversibility in a male insect.     

Effects of temperature of adults and eggs on the induction of embryonic diapause in the band-legged ground cricket, Dianemobius nigrofasciatus

Abstract.  Eggs laid by adult female Dianemobius nigrofasciatus , reared under long-day (LD 16 : 8 h, 25 °C) or short-day (LD 12 : 12 h, 25 °C) conditions from the nymphal stage, are kept at several constant temperatures. At 22.5–30.0 °C, eggs laid by long-day adults show lower incidences of diapause than those laid by short-day adults. In both eggs laid by adults under long-day conditions and those under short-day conditions, the higher the temperature at which the eggs are kept, the lower the incidence of diapause. When eggs of long-day adults are exposed to a low-temperature pulse (10 °C, 24 h) on the day of deposition (day 0), the incidence of diapause increases. The low-temperature pulse on day 1 does not increase the incidence of diapause. By contrast, when the eggs of short-day adults are exposed to a high-temperature pulse (35 °C, 24 h) on day 0 or day 1, the incidence of diapause decreases. The temperature pulses on day 0 are more effective at diapause prevention. Staining of diapause eggs by the Feulgen–Rossenbeck method shows that the eggs enter diapause at the blastoderm stage, which is on day 1 or day 2 at 25 °C. The exposure of adults to long days and higher temperatures prevents the eggs from entering diapause. In D. nigrofasciatus , embryonic diapause is controlled by maternal effects, adult photoperiod and temperature, and egg temperature before or at diapause.     

Laboratory studies of egg development and diapause in Isoperla obscura (Plecoptera) from a mountain stream in Norway

SUMMARY 1. A laboratory study of egg development of the stonefly Isoperla obscura (Zetterstedt) collected from the stream Flybekken (southern Norway, 61°25'N, 8°48'E, 1373 m a.s.l.) showed a short diapause followed by a prolonged period of postdiapause quiescence.
2. Diapause occurred over a wide range of temperatures (−20°C to +8°C), but 0–1°C was the most favourable for fast diapause development and successful hatching. Diapause development required temperatures below 12°C, but sub-zero diapause temperatures increased mortality during the postdiapause phase, as well as reducing hatching success.
3. The threshold for initiation of postdiapausal development was about 1–2°C. Development rate increased and variation in development time decreased at higher water temperatures. These may be important characteristics to ensure seasonal and geographical synchrony of development in individual populations. Low postdiapausal temperatures were unfavourable, despite a subsequent increase in water temperature. No response to postdiapausal photoperiod was detected.
4. The results agreed well with conditions noted in the natural environment, and with the abundance of this species at high altitudes and latitudes in Fennoscandia. Nevertheless, Isoperla obscura also occurs in streams in the maritime parts of western Norway. The results of the present study suggest that egg development in these populations will prove to be different.     

Environmental and genetic control of the larval diapause of the Southwestern corn borer, Diatraea grandiosella

ABSTRACT. The interaction of photoperiod and temperature in the regulation of the induction and termination of the larval diapause of the Southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera), was examined. A population originating from south-eastern Missouri had critical daylengths for diapause induction of about 15h 5min (ecological threshold) and llh (physiological threshold). The ecological threshold was more stable than was the physiological threshold at temperatures lower than 25°C. Above 25°C the diapause response was suppressed. The insect appears to measure photoperiods in a stationary manner since a stepwise increase or decrease in photoperiod did not affect the incidence of diapause. In the critical region of the photoperiodic response curve, a higher incidence of diapause was found among females than among males. Females entered diapause later than did males, but resumed active development earlier than males. The rate of diapause development was more temperature dependent than was the rate of diapause induction, yet it was also clearly under photoperiodic control. The temperature coefficient (Q₁₀) for this process was about 4. Several other factors including sex-linkage, age, and geographic adaptations are involved in controlling the rate of diapause development, even more so than they are in controlling diapause induction. In the laboratory, the intensity of diapause declined gradually without larvae being exposed to non-diapause inducing conditions. Incubation of field-collected larvae revealed that their sensitivity to diapause maintaining photoperiods had ended by January. Three generations of selection of a Mississippi population of D. grandiosella at 30°C and LD 12:12 led to the production of an essentially diapause-free strain and a diapause strain.     

Effects of development, temperature and salinity on metabolism in eggs and yolk-sac larvae of milkfish, Chanos chanos (Forsskål)

Oxygen uptake rates and yolk-inclusive dry weiGhts were measured during the egg and yolk-sac larval stages of milkfish, Chanos chanos (Forsskal). Oxygen uptake by eggs and yolk-sac larvae was measured to assess the effects of four salinities (20,25,30,35 ppt) at 28°C. The effects of three temperatures (23,28,33°C) on oxygen uptake by yolk-sac larvae were determined at a salinity of 35 ppt. Dry weights were measured throughout embryonic development at 28°C and the yolk-sac stage at 23.28 and 33°C.
Oxygen uptake rates of eggs increased more than fivefold during embryogenesis (0.07±0.03 to 0.40 ± 03 μl O₂ egg ⁻¹ h ⁻¹;blastula to prehatch stage). Larval oxygen uptake did not change with age but was affected by rearing temperature (0.33 ± 0.08, 0.44 ± 0.07 and 0.63 ± 0.13 μl O₂ larva ⁻¹ h⁻¹ at 23, 28 and 33°C, respectively; Q₁₀= 1.93). Acute temperature changes from 28 to 33°C caused significant increases in oxygen uptake by embryos (Q ₁₀= 1.69–3.58) and yolk-sac larvae (Q ₁₀=2.55). Salinity did not affect metabolic rates.
Dry weight of eggs incubated at 28°C decreased 13% from fertilization to hatching. Incubation temperatures from 23–33°C did not affect dry weights at hatching. Rearing temperatures significantly affected the rate of larval yolk absorption (Q ₁₀= 2.25).     

Physiology of diapause and cold hardiness in overwintering pupae of the apple leaf miner Phyllonorycter ringoniella in Japan

Abstract The apple leaf miner Phyllonorycter ringoniella (Matsumura) (Lepidoptera: Gracillariidae) overwinters as a diapausing pupa. The diapause rate reaches 100% in early October. Diapause intensity decreases gradually from early October and diapause terminates in early February. The fresh body weight of diapausing pupae is 1.6 times that of non-diapausing pupae. The main cryoprotectant in P. ringoniella pupae is trehalose. Three stages are distinguishable as indicated by the correlations between diapause intensity, levels of cold hardiness and the trehalose content: diapause induction occurred in October, diapause development from November to December, and post-diapause quiescence from January to April. During diapause induction, the pupae accumulate low levels of trehalose and do not survive exposure to −15 °C. During diapause development, the pupae gradually accumulate more trehalose and show some ability to survive exposure to −15 °C, but not to −20 °C. During post-diapause quiescence, the pupae accumulate relatively more trehalose and cold hardiness fully develops, but decreases quickly in April. The trehalose content in pupae sampled in December is unaffected by acclimation temperatures in the range 0–30 °C, but decreases in pupae sampled in March after acclimation at temperatures from 5 to 15 °C. These results suggest that overwintering pupae of P. ringoniella have the ability to accumulate trehalose and develop a high level of cold hardiness during diapause development.     

Effect of temperature and sex on growth patterns in nymphs of the mayfly Hexagenia hilineata in the laboratory

SUMMARY. Eggs collected from Hexagenia bilineata females were successfully reared in the laboratory at temperatures of 15, 20, 25 and 30°C. Eggs did not hatch at 10°C and although hatching was successful at 35°C, all nymphs at this temperature died while in early instars.
Survival of nymphs between the approximate size interval of 4–14 mm showed a significant decrease with increased temperatures. Nymphs at 15°C, however, generally did not survive transformation to the subaduit stage.
The growth pattern of individual nymphs was well described by a logistic curve at most temperatures. Furthermore, growth pattern was significantly affected by both temperature and sex.
Rate of development from oviposition to first emergence increased with increasing temperatures in a linear fashion between 15 and 30°C. The relationship was equally well described by a hyperbolic equation and a power-law equation. By extrapolation from the hyperbolic equation, the lower threshold temperature for development was estimated to be 10.1°C3.1°C. The degree (°C)-days required for development from oviposition to first emergence was calculated to be 2337 days with 95% confidence limits of 2045–2727 days under laboratory conditions.     

Effects of photoperiod and temperature on the development and diapause of the bark beetle Ips typographus

Abstract:  Diapause was induced in a Central European population of Ips typographus grown at 20°C when the day length decreased below 16 h [50% diapause incidence occurred in the 14.7:9.3 h L:D (light:dark) regime]. The non-diapausing adults fed on days 2–6 and 10–14 after the ecdysis and swarmed after the second feeding bout with chorionated eggs in the ovaries and sperm in the spermiducts. Neither gonads nor the flight muscles matured and no swarming occurred in the diapausing adults. The development from egg to adult took about 34 days in both 18:6 h (no diapause) and 12:12 h L:D (diapause) regimes, but it was extended by up to 30% without diapause induction when only larvae or pupae were exposed to L:D 12:12 h. Diapause was induced in insects reared at L:D 12:12 h through the last larval and the pupal instars and/or in the adult stage. Temperature ≥ 23°C prevented diapause induction at L:D 12:12 h but diapause occurred at L:D 14:10 h associated with 26:6°C thermoperiod. The effect of thermoperiods on the developmental rate requires further research. Exposure of the non-diapausing adults to 5°C for several days blocked feeding and evoked a diapause-like state, whereas diapausing adults fed and their gonads slowly developed at this temperature. Diapausing adults exposed in forest to low night temperatures and transferred in October to 20°C readily reproduced at 18:6, but not 12:12 h L:D photoperiods. After 2-months at 5°C and darkness, they became insensitive to the photoperiod, matured and most of them also swarmed at 20°C in the 12:12 h L:D regime. In a Scandinavian population, diapause occurred at 18:6 h L:D and was terminated either by exposure to 5°C or by very long photoperiod (L:D 20:4 h) combined with high temperature (23°C).     

Temperature-induced changes of survival, development and yolk partitioning in Chondrostoma nasus

Fertilized Chondrostoma nasus eggs were incubated at 10, 13, 16 and 19° C until full resorption of the yolk sac. High survival was observed at 10–16° C (89–92% at the onset of external feeding), whereas at 19) C survival was depressed (76%). The time at which 5, 50 and 95% of individuals had hatched, filled the swim bladder, ingested the first food and fully resorbed the yolk sac was determined. An increase in temperature accelerated development and made it more synchronous. Within the period from fertilization to hatching embryonic development was theoretically arrested (t_{0 dev}) at 8·8° C, and growth was arrested (t_0gr) at 8·86° C. For the whole endogenous feeding period (from fertilization to full yolk resorption) the amount of matter transformed into tissue was temperature independent between 10° and 19° C. Respiration increased exponentially with age; the respiration increase was faster at higher temperatures, but, in general, metabolic expenditures of C. nasus were low. As a consequence, the efficiency of utilizing yolk energy for growth was high as compared with other fish species (57% during the whole endogenous feeding period); it was temperature independent. However, time was used less efficiently at low temperatures, increasing a risk of predation. Within the endogenous feeding period a shift from lower to higher temperatures for optimal yolk utilization efficiency was observed. The temperatures optimal for survival and energetic performance seem to be 13–16° C for egg incubation and 15–18° C for rearing of yolk-feeding larvae. Chondrostoma nasus is a potential candidate for aquaculture for restocking purposes.     

The roles of temperature and diapause in the life history of a temperate-zone dragonfly: Argia vivida (Odonata: Coenagrionidae)

Abstract. 1. The life cycle of Argia vivida Hagen generally took longer to complete in the field than was predicted on the basis of the thermal sum accumulated in laboratory rearing.
2. The prediction of a bivoltine life-cycle from geothermal sites with either a constant annual temperature of 26°C or thermal range of 11–31°C was not borne out because the intervention of short-day induced developmental delays in later larval instars extended the life cycle to 1 year.
3. This diapause, which synchronizes adult emergence with favourable summer temperatures, was also present in larvae from sites with annual temperature ranges of 0–33°C and 5–20°C.
4. At these colder sites completion of the life cycle takes 2 and 3 years respectively and dragonflies must be in cold-resistant stages during the winter. A long-day diapause, principally affecting late-instar larvae below a certain size during the summer, achieves this.
5. Large diurnal temperature fluctuations at the 0–33°C site markedly increase the useful thermal energy available to larvae for growth over that predicted by the thermal sum equation.
6. The interaction between the effects of temperatures favourable for growth and day-length-governed diapause, synchronize the emergence of the low-temperature sensitive adult stage of this tropical dragonfly with northern-latitude summers at a variety of habitats.     

A study on diapause of zooplankton in a tropical river - The Blue Nile

SUMMARY. The zooplankton of the Blue Nile overcomes the adverse conditions of the flood by entering into a state of diapause. Resting stages were found in the river bottom, flowing water and rain pools. They were blown to different localities by wind and can withstand long periods of desiccation. Some species of zooplankton go into diapause more than once a year, notably during their seasonal peaks of abundance. Release from diapause occurred in the temperature range 20–30°C and was facilitated by longer photoperiods and decreased concentrations of suspended silt. Various species of zooplankton were induced in facultative diapause when stored in complete darkness.     

THE EFFECT OF TEMPERATURE ON EGGS AND IMMATURE STAGES OF CULEX ANNULIROSTRIS SKUSE (DIPTERA: CULICIDAE)

Abstract
No immature stages of Culex annulirostris were found during field sampling in 1979–1980 when the average water temperature was < 17 °C; they reappeared when the average water temperature was 19 °C and reached the peak density (mean 107 immatures/cylinder) at 26.5 °C.
The effect of 6 temperatures (15–40°C) on egg hatching, development and survival of the immature stages of Cx annulirostris in the laboratory showed that at 15 and 40°C, eggs failed to hatch and larvae died in the first instars. The optimum temperatures for egg hatching and the survival of immature stages were 25 and 30°C. At these temperatures, 85 and 82% respectively of egg rafts hatched, the mean number of larvae per raft was 258 ± 9.8 and 260 ± 11.4 with immature survival of 83.5 and 79.0% respectively. Mean time to hatch at 20–35°C ranged from 1.2 d (35°C) to 2.9 d (20 °C). Developmental times from first instar to adult ranged from 7.1 d (35 °C) to 25.2 d (20 °C). The threshold for development of the immatures was 15.6 ± 2.5°C and the thermal constant was 142.9 ± 26.5 day—degrees (incubation temperatures 20–35°C). At less suitable temperatures of 20 and 35 °C, hatching (57.5 and 45%), number larvae per raft (mean 139.8 ± 9.8 and 102.6 ± 14.2) and survival were low.     

Effects of photoperiod and temperature on the development of Bonagota cranaodes

Abstract.  The Brazilian apple leafroller, Bonagota cranaodes (Meyrick) (Lepidoptera: Tortricidae) is reared in the laboratory under a long-day (LD 14 : 10 h) and a short-day (LD 7 : 17 h) photoperiod at 22 °C, and under two different temperatures (10–13 °C and 21–22 °C). The development time from larval to adult eclosion do not differ between the two photoperiods, but did between the two temperature regimes. However, the larvae do not enter diapause, even under short day conditions and low temperatures. The number of adults obtained does not differ with temperature and light conditions. Field captures with pheromone traps show that Brazilian apple leafroller occurs in apple orchards throughout the year and the population densities are lower in winter. Accordingly, control measures should be taken during the off-season.     

Diapause in a Glasgow strain of the flour moth, Ephestia kuehniella

ABSTRACT. The incidence of diapause in Ephestia kuehniella Zeller from an unhealed granary in Scotland was influenced by both photoperiod and temperature. At 25°C, nearly 50% of larvae entered diapause when reared in continuous darkness (DD) and up to 30% did so in short photoperiods. Little diapause was detected around LD 14:10 but a second, smaller peak of about 20% occurred at LD 16:8 and LD 18:6, falling away again to nearly zero in continuous light. More larvae entered diapause when reared continuously at 15 or 20°C than at 25 and 30°C. However, when larvae reared from hatch at 25°C in LD 16:8 were transferred after 1 week to 15°C in LD 9:15, almost twice as many entered diapause as did those reared at 15°C throughout. The sensitive phase for diapause induction occurred near the start of the final instar. The mean duration of diapause was between 2 and 3 months in most photoperiods at 20 and 25°C, and was shorter at 15°C. However, in DD at 25°C, it lasted about 7 months. Termination of diapause was hastened in larvae reared at 25°C in DD by transferring them to LD 14:10, and also by chilling them at 7.5°C for 6 weeks before returning to 25°C in DD. In an unhealed store in southern England, viable adults emerged from May to July and originated from larvae which terminated diapause relatively late. It would appear from the results of transferring larvae back to the laboratory at various times during the winter that some phases of diapause development were completed quite early after exposure to low temperatures, although no further development took place in the store until temperatures rose again in April.     

Timing of life cycles in a seasonal environment: the temperature-dependence of embryogenesis and diapause in a grasshopper (Chorthippus brunneus Thunberg)

Summary The durations of the stages in the embryonic development of Chorthippus brunneus (Thunberg) have been estimated at constant temperatures of 20° C and 30° C. There was stage-dependent variation in the temperaturedependence of morphogenesis-a pattern apparently not previously described. The earliest and latest stages of morphogenesis proceeded relatively rapidly at higher temperatures. But intermediate stages, which would tend to occur around the diapause stage in a temperate winter, proceeded relatively rapidly at lower temperatures, as did diapause development. At a given temperature, there was individual variation between eggs such that some developed consistently faster than others. However, these intrinsic differences could be altered by exposure to diapause-breaking conditions at different morphological stages. The results are discussed in terms of their adaptive significance, their effect on population dynamics in a seasonal environment, and their implications for the nature of diapause in insects generally.     

Geographical variation in Japan in egg development of the mayfly, Ephoron shigae (Ephemeroptera: Polymitarcyidae)

1. The effect of temperature on embryonic development was compared in four populations, two bisexual and two unisexual, of Ephoron shigae , including one each near the northern and southern periphery of the species range in Japan.
2. Eggs from every population were chilled at 4, 8 or 12 °C for diapause development after 50 days at 20 °C for pre-diapause development (experiment I). Some eggs hatched during chilling at 8 °C or 12 °C, whereas no eggs hatched at 4 °C. The rate of hatching in a given condition of chilling was higher for the eggs from warmer winter environments.
3. Chilling at 4 or 8 °C effectively facilitated diapause development. Chilling at 12 °C was, in general, not so effective, but relatively effective for the eggs from warmer winter environments.
4. Eggs were incubated at 8, 12, 15 or 20 °C after chilling at 4 °C to examine the effect of temperature on post-diapause development (experiment II). The eggs incubated at higher temperature after chilling hatched quicker and more synchronously and had higher hatching success.
5. The relationship between temperature and the days required for hatching after chilling was well described by the power function. There was no significant difference in the slope of the regression lines (i.e. temperature dependency) among local populations. However, a longer time was required for hatching at a given temperature for the population from the colder winter environment.
6. There was no detectable difference in the observed intraspecific variations between unisexual and bisexual populations.     

Survival, development and food energy partitioning of nase larvae and early juveniles at different temperatures

Survival was generally high, 94–100%, for newly hatched larvae of the nase Chondrostoma nasus held at 10, 13, 16, 19, 22, 25 and 28° C up to day 66 post-fertilization. The developmental rate decreased with age and increased with temperature. Specific growth rates increased with temperature; within one temperature range growth rate decreased with ontogenetic development. Food consumption and respiration increased with temperature and body size. A temperature increase from 25 to 28° C resulted in slightly reduced survival, minor acceleration of developmental growth and respiration rates, and impeded skeleton formation. Growth efficiency of consumed energy decreased throughout the larval period from 55 to 67% at the first larval stage (L1) to 36–48% at the first juvenile stage (J₁). A similar trend for assimilation efficiency and its utilization for growth was observed. The constant temperatures required by larval nase ranged from a minimum 8–10° C to a maximum 25–28° C. A shift of optimum temperatures, 8–12, 13–16, 15–18, 19 and 22° C for nase spawning, embryonic development, yolk feeding larvae, early externally feeding larvae and, late larvae and juveniles, respectively, paralleled the spring rise in the river water temperature. Larval and juvenile nase show high survival, growth and energy conversion efficiencies compared with other fish species. On the other hand, low survival rates and growth can be attributed to external perturbations; thus, young nase may be considered a good indicator of the environmental and ecological integrity of river systems.     

Influence of photoperiod on low temperature acclimation for cold-hardiness in Drosophila auraria

ABSTRACT. Imagines of Drosophila auraria Peng, a reproductive diapause species, developed cold-hardiness at low temperatures to a greater extent when exposed to a diapause-inducing photoperiod (LD10:14 h) than when exposed to a diapause-preventing photoperiod (LD 16:8h). Imagines kept at 18°C, which was the temperature at which they were reared to eclosion, did not survive a test exposure to -5°C for 8 days regardless of age or photoperiod. When transferred to 10 or 5°C, either from eclosion or from 8 days after eclosion, the survival rate, on testing, rose with time since transfer and rose faster and higher with a photoperiod of LD 10:14h than with LD16:8h. Flies transferred to 15°C only showed improved ability to survive the test if they were kept in LD 10:14h. When cultured at 18°C to the age of 8 days after eclosion, diapause was terminated in about 30% of females even at LD 10:14h. In these post-diapause females the ability to develop cold-hardiness at lower temperatures was somewhat less than in the diapausing females, but apparently greater than in the non-diapause females. These results suggest that the physiological mechanism which promotes cold-hardiness under a diapause-inducing photoperiod is not directly linked to the process causing reproductive diapause.
In Sapporo, flies from a natural population became tolerant to cold in October when they entered diapause and daily mean temperature fell below 15°C and the light/dark cycle fell below LD 12:12h.     

Temperature- and salinity-dependent development of a Nordic strain of Ichthyophthirius multifiliis from rainbow trout

During the twentieth century evidence was presented which suggested the presence of various strains and races of the parasite Ichthyophthirius multifiliis Fouquet. However, ecological profiles of various parasite isolates from different climatic zones are sparse. Such stringent characterizations of parasite development at defined abiotic conditions could provide valuable criteria for the different races; profile comparison from various localities is one way to differentiate these strains. Baseline investigations were therefore performed on the associations between abiotic factors (temperature/salinity) and the development of theronts in tomocysts of I. multifiliis isolated from rainbow trout in a Danish trout farm. It was shown that tomocyst formation and theront development took place between 5 and 30°C. Development rates and sizes of theronts were clearly affected by temperature: theronts escaped tomocysts already after 16–27 h at 25°C and 30°C, whereas this process took 8–9 days at 5°C. Likewise, theront size decreased steadily from a maximum of 57.4 × 28.6 μm at 5°C to 28.6 × 20.0 μm at 30°C. This size variation was only partly associated with the number of theronts that appeared at different temperatures. The lowest number of theronts escaping from one tomocyst was indeed found at 5–7°C (mean 329–413). At 11.6, 17.0 and 21°C, the highest number of theronts appeared (mean 546–642). However, at 25 and 30°C, the number decreased (458 and 424, respectively). Additional studies on the salinity dependent development of the parasite (at 11.6°C) showed that salinities above 5 p.p.t. totally inhibited development. Even at 5 p.p.t. the developmental time significantly increased and the number of theronts produced from one tomocyst decreased.