Probable evolution of the coelacanth's reproductive style: lecithotrophy and orally feeding embryos in cichlid fishes and in Latimeria chalumnae

Synopsis The living coelacanth is a livebearer. Yolk seems to be the main source of nutrients and of oxygen to the embryo (fetus). Long before birth, young may also possibly feed orally on histotrophe secretion and egg debris. This type of reproduction evolved, as in most other fishes, from oviparity. The Carboniferous coelacanth Rhabdoderma exiguum had eggs of much lesser yolk volume and may represent an earlier form of oviparity with hiding, guarding or brooding type of parental care. The Jurassic coelacanth Holophagus (Undina) and the Cretaceous Axelrodichthys appear to have already evolved the internal-bearing style. Much of this evolutionary sequence is similar to that in cichlids. Ancestral cichlids are substrate tenders and nesters, with small eggs, little yolk and a feeding larva with indirect development. Mouthbrooding cichlids evolved a few, large eggs with denser yolk, direct development and, ultimately, orally feeding embryos while yolk is still in ample supply. Mixed feeding from yolk and orally ingested food in cichlids and in coelacanths is shown to be an enhanced mode of food delivery to the embryos over that from each source separately, in order to produce directly a better developed or larger young at the time of release, i.e. independence. Increase in egg size is regarded as an environmentally induced, altered pattern of yolk synthesis and an initial component of the epigenetic mechanism leading towards greater specialization. Carotenoids are incorporated within the yolk to assist the oxidative metabolism of the developing embryo.     

Autonomous yolk protein synthesis in ovaries ofDrosophila cultured in vivo

Summary Immature ovaries ofDrosophila mercatorum were injected into young larvae and into adult males ofD. mercatorum, D. melanogaster, D. hydei, D. virilis, andZaprionius vittiger. These homo- and heteroplastic transplantations allow normal vitellogenesis to occur in the donor ovary. By SDS gel electrophoresis, we identified the major species-specific yolk proteins of mature eggs (stage 14) which were exclusively of donor-specific origin. Other experiments withD. hydei andZ. vittiger showed that, when females were used as hosts, the host-specific yolk proteins became incorporated into the donor eggs. When two immature ovaries, one ofD. mercatorum and one ofD. hydei, were co-cultured in males, again only the donor-specific yolk proteins were found in the mature eggs implying that these yolk proteins were not released into the host hemolymph.A parthenogenetic strain ofD. mercatorum was used to demonstrate the ability of transplanted immature ovaries to produce viable eggs which can give rise to fertile adults.The role of the species-specific yolk proteins is discussed with respect to the dual origin of these proteins during normal vitellogenesis, i.e., an autonomous synthesis within the ovary itself in addition to the well-known production by the fat body. Further experiments with pupae as hosts indicate that even in the absence of juvenile hormone and in the presence of high doses of ecdysone, vitellogenesis can proceed within the donor ovary.Based on these experiments, a new hyopthesis on the hormonal control of vitellogenesis inDrosophila is presented. We propose that yolk proteins derived from the fat body are controlled by juvenile hormone, whereas the independent and autonomous vitellogenesis within the ovary itself is controlled by endogenously synthesized ecdysone.     

The occurrence of Eubothrium fragile (Cestoda: Pseudophyllidae) in twaite shad, Alosa fallax (Lacépède) in the River Severn

The occurrence, size and maturity changes of Eubothrium fragile have been studied in postlarvae, juveniles and adult twaìte shad, Alosa fallax , from several locations in the River Severn. Parasites were only found in adult shad and not in post-larvae or juveniles. No juvenile or recently acquired cestodes were identified as such, but adults were present in shad throughout the whole period of their spawning migration. A large proportion of the parasites were gravid upon arrival in the river and, although eggs were subsequently released into fresh water, there was no loss of cestodes from the fish. It was concluded that E. fragile is a marine species, that the parasites found in adult shad in fresh water were the residue of a marine life cycle and that the eggs released in fresh water were part of the parasite's natural reproductive wastage. The distribution and biology of E. fragile were discussed and it is considered that it is typical of the marine species of the genus.     

Reproductive biology and larval morphology of the marine plotosid Cnidoglanis macrocephalus (Teleostei) in a seasonally closed Australian estuary

Monthly trends shown by gonadosomatic indices, the prevalence of the different gonadal stages, and the size distribution of the oocytes, indicate that the large marine and commercially important plotosid Cnidoglanis macrocephalus spawns in Wilson Inlet between October and January. The conclusion that spawning occurs within this seasonally closed estuary was confirmed by the presence of males in large nests and by the capture of newly-hatched, yolk sac larvae from one of those nests. The fact that C. macrocephalus, which is also widely distributed in coastal marine waters throughout much of southern Australia, can spawn within Wilson Inlet would be of particular value to this species in those periods when closure of the estuary would preclude a seawards spawning migration. Sexual maturity is size dependent, with spawning rarely occurring before fish have reached a total length of 425 mm. Sexual maturity was attained by a few fish at the end of their second year, by several at the end of their third year and by most, if not all fish, at the end of their fourth year. Comparisons with data for the more northern and permanently open Swan Estuary indicate that C. macrocephalus also spawns within that system and that the spawning time of this species is related to water temperature. The adult male guards the larvae under its pelvic fins in burrows. The larvae increased in total length from 29 mm just after hatching to 43 mm in the 17–18 days after capture, during which time their yolk sac was resorbed. Details are given of the morphology, morphometrics, meristics and pigmentation of larval C. macrocephalus. In comparison with the larvae of three other plotosid genera, the larva of C. macrocephalus is far larger in size and more developed at hatching and takes a shorter time to transform into a juvenile.     

Size‐dependent reproductive success of wild zebrafish Danio rerio in the laboratory

Size‐dependent reproductive success of wild zebrafish Danio rerio was studied under controlled conditions in the laboratory to further understand the influence of spawner body size on reproductive output and egg and larval traits. Three different spawner size categories attained by size‐selective harvesting of the F₁‐offspring of wild D. rerio were established and their reproductive performance compared during a 5 day period. As to be expected, large females spawned more frequently and had significantly greater clutch sizes than small females. Contrary to expectations, small females produced larger eggs when measured as egg diameter with similar amounts of yolk compared to eggs spawned by large spawners. Eggs from small fish, however, suffered from higher egg mortality than the eggs of large individuals. Embryos from small‐sized spawners also hatched later than offspring from eggs laid by large females. Larval standard length (L_S)‐at‐hatch did not differ between the size categories, but the offspring of the large fish had significantly larger area‐at‐hatch and greater yolk‐sac volume indicating better condition. Offspring growth rates were generally similar between offspring from all size categories, but they were significantly higher for offspring spawned by small females in terms of L_S between days 60 and 90 post‐fertilization. Despite temporarily higher growth rates among the small fish offspring, the smaller energy reserves at hatching translated into lower condition later in ontogeny. It appeared that the influence of spawner body size on egg and larval traits was relatively pronounced early in development and seemed to remain in terms of condition, but not in growth, after the onset of exogenous feeding. Further studies are needed to explore the mechanisms behind the differences in offspring quality between large‐ and small‐sized spawners by disentangling size‐dependent maternal and paternal effects on reproductive variables in D. rerio.     

Egg size and emergence timing affect morphology and behavior in juvenile Chinook Salmon,Oncorhynchus tshawytscha

Variation in early life history traits often leads to differentially expressed morphological and behavioral phenotypes. We investigated whether variation in egg size and emergence timing influence subsequent morphology associated with migration timing in juvenile spring Chinook Salmon, Oncorhynchus tshawytscha. Based on evidence for a positive relationship between growth rate and migration timing, we predicted that fish from small eggs and fish that emerged earlier would have similar morphology to fall migrants, while fish from large eggs and individuals that emerged later would be more similar to older spring yearling migrants. We sorted eyed embryos within females into two size categories: small and large. We collected early and late‐emerging juveniles from each egg size category. We used landmark‐based geometric morphometrics and found that egg size appears to drive morphological differences. Egg size shows evidence for an absolute rather than relative effect on body morphology. Fish from small eggs were morphologically more similar to fall migrants, while fish from large eggs were morphologically more similar to older spring yearling migrants. Previous research has shown that the body morphology of fish that prefer the surface or bottom location in a tank soon after emergence also correlates with the morphological variations between wild fall and spring migrants, respectively. We found that late‐emerging fish spent more time near the surface. Our study shows that subtle differences in early life history characteristics may correlate with a diversity of future phenotypes.     

Reproductive biology of four large riverine fishes (Cyprinidae) in a man-made lake,Orange River,South Africa

The reproductive biology ofBarbus holubi, B. kimberleyensis, Labeo capensis andL. umbratus was examined in a large reservoir on the Orange River, South Africa. The findings are integrated into the existing knowledge on largeBarbus andLabeo species, which coexist in most river systems in Africa and Asia.LargeBarbus spawn on gravel beds within the river channel during floods in spring or summer. In continuously flowing regulated rivers, time of spawning is governed by water temperatures. They have moderate fecundity; large eggs, incubation time of several days and the larvae are initially immobile with large yolk sacs. In the impoundment, they spawn in the inflowing regulated river withB. kimberleyensis spawning four to six weeks later than the more cold-tolerantB. holubi, the dominant largeBarbus. Survival is generally good and the juveniles disperse throughout the lake, but unseasonal release of cold water from an upstream impoundment may cause poor reproductive success.In contrast,Labeo species generally spawn on newly flooded ground, usually leaving the main river channel. Spawning may or may not be preceded by a longitudinal migration. Labeos are relatively fecund with small (30%Barbus size) eggs which hatch quickly and the larvae swim in bursts up into the water column before sinking down again.L. capensis does not require a longitudinal spawning migration and breeds throughout the lake, depending on local conditions. Large temporal variation in gonadal development within the population can result in more than one spawning. Dispersal within the lake is poor.L. umbratus uses larger inflowing tributaries for spawning thanL. capensis and its juveniles have a much greater power of dispersal. Early dependence on external feeding and undependable occurrence of conditions for spawning and juvenile feeding makes for variable reproductive success.     

The effects of food and temperature regimes on life-history responses to fish kairomones in Daphnia hyalina × galeata*

Life-history responses to two concentrations of fish released info-chemicals at two temperature and food regimes were investigated for one clone of Daphnia hyalina × galeata. The presence of fish kairomones had a negative impact on size at maturity, carbon allocation to individual eggs and size of neonates in all treatments. Food concentration and temperature had positive effects on size of adult stages, independent of kairomone treatment. However, kairomone treatment were not found to interact with food or temperature. Age at maturity was positively influenced by increased temperature and food concentrations, whereas no direct kairomone effects were detected for this trait. Clutch size was not directly influenced by kairomone treatment, whereas both food concentration and temperature had strong, positive effects.     

Embryonic, larval and juvenile development of the roughskin sculpin,Trachidermus fasciatus (Scorpaeniformes: Cottidae)

Embryonic, larval and juvenile development of the catadromous roughskin sculpin,Trachidermus fasciatus, were described using eggs spawned in an aquarium. The eggs, measuring 1.98–2.21 mm in diameter, were light reddish-yellow and had many oil globules, 0.05–0.18 mm in diameter. Hatching occurred 30 days after spawning at 2.3–11.3°C. The newly-hatched larvae, measuring 6.9–7.3 mm BL, had a single oil globule, 9–10+25–26=34–36 myomeres and 6 or 7 large stellate melanophores dorsally along the gut. The yolk was almost resorbed, number of pectoral-fin rays attained 16–17, and two parietal, one nuchal and four preopercular spines were formed, 5 days after hatching, at 8.2–8.4 mm BL. The oil globule disappeared, and one supracleithral spine was formed, 11 days after hatching, at 8.9–9.5 mm BL. Notochord flexion began 15 days after hatching, at 9.7–10.3 mm BL. A posttemporal spine was formed 20 days after hatching, at 10.7–10.9 mm BL. The first dorsal fin spines (VII–VIII), second dorsal fin and anal fin rays (18–19, 16–18, respectively) appeared 23 days after hatching, at 12.0–13.7 mm BL. The pelvic fin spine and rays (I, 4) were formed and black bands on the head and sides of the body began to develop 27 days after hatching, at 13.8–15.8 mm BL. Newly-hatched larvae swam just below the surface in the aquaria. Preflexion larvae (8.9–9.5 mm BL), in which the oil globule had disappeared, swam in the middle layer, while juveniles (13.8–15.8 mm BL) began swimming on the bottom of the aquaria. Swimming behavior observed in the aquaria suggested that the fish started to change to a demersal existence at the juvenile stage.     

Embryonic and larval development of a Japanese spinous loach,Cobitis takatsuensis

The embryonic and larval development ofCobitis takatsuensis, a mountain stream spinous loach, was surveyed by incubating artificially inseminated eggs. The mean diameter of the inflated eggs and mean total length of newly-hatched larvae were 2.7 mm and 5.7 mm, respectively. The eggs were spherical, transparent and unpigmented, with a pale yellow yolk and no oil globule. The daily cumulative temperature to hatching was estimated to be 70–110°C. day. Hatched larvae were unpigmented with outer gill filaments on their cheeks, as in otherCobitis species, but the melanophores were comparatively less obvious at each developmental stage. The larvae started feeding eleven days after hatching yolk absorption being completed sixteen days after hatching. All the fin rays were fully developed and the juvenile stage reached at 16 mm TL, 38 days after hatching. Embryonic and larval developmental traits ofC. takatsuensis, such as egg size, clutch size and larval pigmentation, were similar to the Korean species,Niwaella multifasciata, that lives in the upper reaches of the Nak-tong river, andN. delicata, which inhabits Japanese mountain streams, rather than to its congeners. Among cobitine fishes, the spawning of a small number of larger eggs yielding larger larvae without pigmentation, characteristics shared byC. takatsuensis, N. multifasciata andN. delicata, is attributable to adaptation to cold mountain streams.     

Early ontogeny of Adinia xenica (Pisces,Cyprinodontiformes): 1. The development of embryos in hiding

Synopsis Examination of living and stained preserved embryos of the fundulid Adinia xenica from the intertidal salt marshes of the northern Gulf of Mexico, revealed a saltatory pattern of development. Embryonic structures indicate an ability to tolerate low oxygen conditions, primarily by extensive vascularization of the large yolksac and pericardial headfold, and capillaries covering the entire body. Adhesive fibres of the chorion hold the egg to the chosen substrate, away from anoxic sediments. Prior to hatching most embryos enter a resting interval of up to four weeks, with a mean duration of 8 days (in the laboratory). Growth ceases, development is limited to some skeletal calcification, and metabolic rate, as indicated by bradycardia and yolk resorption, is greatly reduced. This step distributes hatching and recruitment over time in a variety of environmental conditions. Hatching is considered to result from a variable summation of internal and external factors; ease of its induction increases during the resting interval. Upon hatching most embryos are well developed and ready to feed exogenously, there being essentially no free-embryo phase. The considerable individual variation in timing and relative order of some ontogenic events may provide opportunities for environmentally induced heterochronies and thus assist A. xenica to develop in a range of habitats and conditions. Reproductive style includes hiding the eggs in protected locations, and a fractional spawning pattern. Consideration of this plus morphological adaptations, egg size and adhesivity places A. xenica in the reproductive guild of brood-hiding cavity spawners.     

Egg size-dependent expression of growth hormone receptor accompanies compensatory growth in fish

Large egg size usually boosts offspring survival, but mothers have to trade off egg size against egg number. Therefore, females often produce smaller eggs when environmental conditions for offspring are favourable, which is subsequently compensated for by accelerated juvenile growth. How this rapid growth is modulated on a molecular level is still unclear. As the somatotropic axis is a key regulator of early growth in vertebrates, we investigated the effect of egg size on three key genes belonging to this axis, at different ontogenetic stages in a mouthbrooding cichlid (Simochromis pleurospilus). The expression levels of one of them, the growth hormone receptor (GHR), were significantly higher in large than in small eggs, but remarkably, this pattern was reversed after hatching: young originating from small eggs had significantly higher GHR expression levels as yolk sac larvae and as juveniles. GHR expression in yolk sac larvae was positively correlated with juvenile growth rate and correspondingly fish originating from small eggs grew faster. This enabled them to catch up fully in size within eight weeks with conspecifics from larger eggs. This is the first evidence for a potential link between egg size, an important maternal effect, and offspring gene expression, which mediates an adaptive adjustment in a relevant hormonal axis.     

FITNESS CONSEQUENCES OF VARIATION IN EGG SIZE AND FOOD ABUNDANCE IN BROOK TROUT SALVELINUS FONTINALIS

Relationships between egg size and juvenile survival in brook trout, Salvelinus fontinalis, were determined experimentally at two levels of food abundance and then incorporated into a model that related maternal fitness to egg size and food supply. Egg volume was positively correlated with juvenile size at hatching and size at yolk sac resorption but had no significant effect on embryonic survival or development time. Juvenile survival was linearly related to egg size throughout the first 50 days of exogenous feeding at high and low food levels. The effects of egg size and food abundance on juvenile survival were not additive. Decreased food abundance significantly increased mortality among the smallest eggs but had a negligible effect on the largest eggs. Model simulations indicate that maternal fitness is a curvilinear function of egg size and that food supply influences both the height and the shape of the function. The fitness functions provide empirical support for the hypothesis that selection favors an increase in offspring size with reductions in resource abundance.     

Life cycle and mating behavior of <Emphasis Type="Italic">Helicotylenchus multicinctus</Emphasis> (Nematoda: Hoplolaimidae) on excised <Emphasis Type="Italic">Musa cavendishii</Emphasis> roots

The life cycle and mating behavior of Helicotylenchus multicinctus (Nematoda: Hoplolaimidae) were observed in vitro on excised roots of Musa cavendishii in gnotobiotic culture. Eggs hatched into juveniles whose appearance and structure were similar to those of the adults. Juveniles grew in size and each juvenile stage was terminated by a molt. H. multicinctus had four juvenile stages. The first molt occurred outside the egg shortly after hatching. After the final molt the juveniles differentiated into adult males and females. Mating was required for reproduction. After mating, fertilized females began to lay eggs. The life cycle from second stage juvenile to second stage juvenile was completed in 39 days.     

Some Consequences of Size Variability in Juvenile Prickly Sculpin, Cottus asper

Variation in size among fishes can have important ecological consequences. The origin and persistence of variation in size among a cohort of juvenile prickly sculpin, Cottus asper, was explored experimentally by manipulating the size variability in juvenile fish and the presence of conspecific adult fishes in a factorial design. I found that adult fish significantly suppressed the growth and survivorship of juveniles, while the variance treatment alone had no effect on growth or survivorship. The presence of adults was also associated with a decrease in size variability in the high variance treatment. An analysis of juvenile growth patterns revealed no evidence for size dependent growth or temporal correlations in growth (`growth autocorrelation'), and are consistent with a lack of amplification of size variance through time. Juvenile C. asper appear to have been feeding on a relatively homogeneous resource and utilizing those resources similarly. Thus, patterns of individual growth and the presence of adult conspecifics may both serve to dampen size differences in this coastal fish species.     

Age and early life history of juvenile scotia sea icefish, <Emphasis Type="Italic">Chaenocephalus aceratus</Emphasis>, from Elephant and the South Shetland Islands

The otolith microstructure of juvenile Scotia Sea icefish (Chaenocephalus aceratus) was analyzed from samples collected around Elephant and South Shetland Islands, with the aim to validate previous annual ageing and to give new insight into its early life history timings. Fish were caught by bottom trawl fishing conducted on the continental shelf between 100 and 500 m depth. To determine the timing and position of the first annulus on sagittal otoliths, microincrements were counted on juvenile otoliths previously aged 1+ year old by counting annuli in sectioned otolith. Assuming that microincrements were laid down daily, age ranged from 406 to 578 days in fish measuring 13–19 cm TL, thus corroborating previous results. The relationship between fish size and otolith size/weight was estimated using the least square linear regression method. The relationship between age and otolith size was also estimated to determine the otolith length in 1-year old fish, which was approximately 1.58 mm. In all samples the otolith core was characterized by an evident strong check, assumed to be laid down at the beginning of exogenous feeding of yolk sac larvae. The yolk sac duration estimated from hatch to the first feeding check was longer than other channichthyids, lasting 29–45 days. Hatching dates were backcalculated from the date of capture using the age estimates, indicating C. aceratus sampled off Elephant and South Shetland Islands hatched over a long period lasting from July to December, with a peak in November. As a result, the potential larval dispersion driven by local oceanographic features is discussed.     

Mariculture of kurosoi,Sebastes schlegeli

Synopsis The technology of collecting developing larvae from female kurosoiSebastes schlegeli, and raising the larvae to juveniles (100 mm total length (TL)) to be released into the oopen sea, is presented. Gravid females 40–46 cm TL were captured in May–June 1977–1980 and held in the laboratory until parturition. Fecundity of fish in this size range was 100 000–184 000. Larvae were sequentially fed rotifers,Artemia nauplii, and young sand lance,Ammodytes personatus, until reaching 25 mm; this required 35 days and yielded a survival rate of 50%. Thereafter, the fish were reared in separate size groups to avoid cannibalism. Minced or chopped sand lance and commercial food were provided until the final size of 100 mm was attained. The growth of juvenile kurosoi from 25 to 100 mm required 85 days, with a survival rate of 90%. The effect of released cultured fish on the local stock is being determined from information on the recapture of tagged fish.     

Avoidance of Filial Cannibalism in the Amphipod Gammarus pulex

Adult animals that cannibalise juvenile conspecifics may gain energy but also risk filial cannibalism, that is, consumption of their own offspring. However, individuals vary in the magnitude of the costs and benefits of cannibalism depending on factors such as their current energy reserves or the probability that they have offspring in the vicinity. They may therefore also vary in the extent to which they participate in cannibalism. This study investigated whether the sex or brooding status of adult amphipods (Gammarus pulex) influenced whether they participated in cannibalism of juveniles. For females carrying embryos within their brood pouch, we also investigated two hypotheses to explain the presence or absence of cannibalistic behaviour by determining whether cannibalism was correlated with factors that might reflect energy demands (body length, brood size), or that might reflect a temporal change in cannibalistic behaviour (corresponding to stage of brood development). All reproductive classes of adults participated in some level of juvenile cannibalism, but females carrying offspring at an advanced stage of development (close to emergence from the brood pouch) consumed significantly fewer juveniles than other groups. Females thus appear to significantly reduce cannibalism of juveniles concurrent with the time when their own eggs are hatching within the brood pouch, prior to the release of their offspring. Because the experiment tested female responses to unfamiliar juveniles, this reflects a temporal change in behaviour rather than a response to phenotypic recognition cues, although additional direct recognition cannot be ruled out. Brooding females with large brood sizes or large body lengths, which might have disproportionately greater energetic demands, were not more likely to cannibalise juveniles. We also noted that juveniles that survived in trials where cannibalism occurred were significantly more likely to be found at the water surface, suggesting a possible adaptation to escape cannibalistic adults. Overall, our results provide evidence that amphipods use indirect temporal cues to avoid filial cannibalism.     

Development of eggs and larvae of Stolephorus commersonnii and taxonomic key to fish eggs of the Clupeidae and Engraulidae off China

Fish eggs of successive stages of embryonic development, which were identified as belonging to specimens of Commerson's anchovy Stolephorus commersonnii through comparative molecular techniques, were collected from Leqing Bay, Zhejiang, China. Some eggs were reared artificially to obtain samples of successive developmental stages of larvae. The fertilized eggs of S. commersonnii are ellipsoidal and non-adhesive. The surface of the egg membrane is smooth and the perivitelline space is narrow. There is a single oil globule in the irregularly segmented yolk. Newly hatched larvae are transparent and devoid of pigments. Development of the larvae occurs in the following sequence: 8?h after hatching, the anus unfolds; 12?h after hatching, the pectoral fins emerge; 24?h after hatching, the liver and branchial arches emerge, and the alimentary canal differentiates into the oesophagus and intestinal canal; 30?h after hatching, the eyes become pigmented; 36?h after hatching, the upper and lower jaws become distinct; 42?h after hatching, the stellate melanophores emerge; 72?h after hatching, the postlarval developmental stage begins with the emergence of the dorsal fin. Based on the morphology of eggs and larvae of S. commersonnii, a taxonomic key to fish eggs of the Clupeidae and Engraulidae off China is established to provide an efficient and convenient way to identify the egg specimens. The fish eggs of the Clupeidae and Engraulidae off China could be identified to some extent by the buoyancy, shape and diameter of eggs, the number of layers in the egg membrane, the size of the perivitelline space, the number and diameters of oil globules, and the distribution of pigments. Meanwhile, their prelarvae could be identified by the number and diameters of oil globules, the distribution of pigments, the location of the anus and the number of myomeres.