Synaptonemal complex analysis in spermatocytes of tilapia, Oreochromis niloticus (Pisces, Cichlidae).

Some adaptations of the synaptonemal complex (SC) whole-mounting technique first used in plants permitted its application to meiotic studies in tilapia, Oreochromis niloticus. Direct observation of the chromosome pairing process and bivalent structure during the meiotic prophase of this fish species by light and electron microscopy permitted the analysis of SCs in autosomes and the possible identification of sex chromosomes. The analysis of SCs in spermatocytes of O. niloticus revealed that all 22 bivalent chromosomes completely paired, except for the occurrence of a size heteromorphism in the terminal region of the largest bivalent associated with the presence of an incompletely paired segment during the synapsis process, which may be the cytological visualization of an XX/XY sex chromosome system in this species.     

Bone development in the jaw of Nile tilapia Oreochromis niloticus (Pisces: Cichlidae)

East African cichlids have evolved feeding apparatus morphologies adapted to their diverse feeding behaviors. The evolution of the oral jaw morphologies is accomplished by the diversity of bone formation during development. To further understand this evolutionary process, we examined the skeletal elements of the jaw and their temporal and sequential emergence, categorized by developmental stages, using the Nile tilapia Oreochromis niloticus as a model cichlid. We found that chondrogenesis started in Stage 17. The deposition of osteoid for the dermal bones commenced in Stage 18. The uptake of calcium dramatically shifted from the surface of larvae to the gills in Stage 20. The bone mineralization of the skeleton began in Stage 25. These data provide important information regarding the sequential events of craniofacial development in East African cichlids and lay the groundwork for studying the molecular mechanisms underlying adaptation of jaw structure to feeding behavior.     

Development of the embryo, larva and early juvenile of Nile tilapia Oreochromis niloticus (Pisces: Cichlidae). Developmental staging system

We described the developmental stages for the embryonic, larval and early juvenile periods of Nile tilapia Oreochromis niloticus to elucidate sequential events of craniofacial development. Craniofacial development of cichlids, especially differentiation and morphogenesis of the pharyngeal skeleton, progresses until about 30 days postfertilization (dpf). Because there is no comprehensive report describing the sequential processes of craniofacial development up to 30 dpf, we newly defined 32 stages using a numbered staging system. For embryonic development, we defined 18 stages (stages 1-18), which were grouped into seven periods named the zygote, cleavage, blastula, gastrula, segmentation, pharyngula and hatching periods. For larval development, we defined seven stages (stages 19-25), which were grouped into two periods, early larval and late larval. For juvenile development until 30 dpf, we defined seven stages (stages 26-32) in the early juvenile period. This developmental staging system for Nile tilapia O. niloticus will benefit researchers investigating skeletogenesis throughout tilapia ontogeny and will also facilitate comparative evolutionary developmental biology studies of haplochromine cichlids, which comprise the species flocks of Lakes Malawi and Victoria.     

Kin cohesiveness and possible inbreeding in the mouthbrooding tilapia Sarotherodon melanotheron (Pisces Cichlidae)

Four microsatellite markers were used to study genetic variation among individuals of the mouthbrooding tilapia Sarotherodon melanotheron (Rüppel 1852) caught in separate but adjacent shoals. A comparison was also made with fish from six other localities. Populations originating from riverine environments appear to be panmictic, while samples from open waters such as lagoons showed highly significant heterozygote deficiencies. For instance, at the 33-allele locus SMEL4, 32 homozygous individuals were observed among the 82 individuals from the same lagoon location instead of only five homozygotes expected if random mating occurred. A further assessment of the genetic similarity of individuals within each shoal, validated by robust permutation techniques requiring no precise knowledge of gene frequencies, showed that related individuals tend to aggregate, and suggested that mating occurs preferentially within small groups of kin. Cichlids are often presented as a group of fish where microallopatric speciation takes place. The possible link between kin aggregation, inbreeding and shoaling behaviour we propose here may have important consequences for our understanding of the mechanisms involved in this fast speciation process.     

The inheritance of aggressiveness in the convict cichlid fish, Cichlasoma nigrofasciatum (Pisces: Cichlidae).

Intraspecific comparisons in aggression were made between X-irradiated (2 times 500 R) ancestors and untreated control animals. Bites delivered to sub-adult conspecifics were counted to measure aggressiveness. When compared with corresponding controls, a significant reduction in male aggressiveness was found in the post-irradiation F1 generation. A greater variation in bites delivered occurred among the F2 post-irradiation generations than among the control equivalents. Three distinct back-cross generations were obtained as follows. a single F1 female from the irradiated line was mated to (a) her high aggressive (I-A) and (b) her low aggressive (I-NA) F2 son; a F1 control female was mated to her low aggressive F2 son. In the back-cross generations, both I-A and C-NA males were more aggressive than I-NA males. No differences were found between I-A and C-NA back-cross males. The findings are discussed in terms of a two-factor model based on Mendelian laws.     

Sound production by Simochromis diagramma (Günther) (Pisces, Cichlidae).

Two types of sounds produced by the cichlid fish Simochromis diagramma, are recorded and analysed. A br-r-r sound, with a main frequency of 200 Hz and a chewing sound with a main frequency of 6,000-10,000 Hz are produced during threatening; the former sound can also be heard during quivering. No sound was recorded with fighting or anxious fishes.     

Male-female Parental Roles in Sarotherodon galilaeus (Pisces: Cichlidae)

Sarotherodon galilaeus is a predominantly biparental, sexually monomorphic mouthbrooder. 15 spawnings of pairs in the weight range 150–450 g were observed in aquaria. Only clutches of females larger than 320 g were reared jointly; with those of smaller females, the first fish to pick up eggs brooded alone, while the comparatively few eggs left for the second fish were invariably eaten. The latency after which eggs were picked up increased with the duration of spawning and indirectly with fish size. This increase was larger in males than in females, so large males started to collect eggs later than their mates. In small pairs (female weight < 200 g), by contrast, the male would pick up eggs first and carry them alone. This is the case in a smaller congeneric species, S. melanotheron, indicating that the apportioning of broodcare is based on a unitary, size-dependent mechanism among Sarotherodon. Yet picking-up order was not entirely determined by these equations. Some fish did not show any reaction towards the eggs and occasionally males became care-givers despite expected longer latency, seemingly because of aggressive dominance.     

Evolution of centromeric satellite DNA and its use in phylogenetic studies of the Sparidae family (Pisces, Perciformes).

In this paper, we use the EcoRI centromeric satellite DNA family conserved in Sparidae as a taxonomic and a phylogenetic marker. The analyses of 56 monomeric units (187 bp in size) obtained by means of cloning and PCR from 10 sparid species indicate that this repetitive DNA evolves by concerted evolution. Different phylogenetic inference methods, such as neighbor-joining and UPGMA, group the 56 repeats by taxonomic affinity and support the existence of at least two monophyletic groups within the Sparidae family. These results reinforce the recent taxonomic revision of the genera Sparus and Pagrus and contradict previous classifications of the Sparidae family.     

Local adaptations in populations of rock-dwelling haplochromines (Pisces:Cichlidae) from southern Lake Victoria

The radiation of cichlid fishes in the African great lakes is often described as adaptive, because, at a superficial level, cichlid fishes seem adapted to the ecological niches they occupy. However, adaptedness has rarely been studied. We''ve investigated to what extent island populations of three species of the rock-dwelling genus Neochromis, endemic to Lake Victoria, are adapted anatomically to exploit locally abundant resources. Specifically, we asked whether different resource environments were reflected in differences in the feeding apparatus, both within species and between species. In populations of two specialized biters, the algae scrapers N. rufocaudalis and N. omnicaeruleus, the biting force of the lower jaw increased with increasing amount of items that require biting in the diet. N. greenwoodi is a less specialized biter; we found differences between two populations in the hyoid position and in the premaxilla that enhance suction feeding. These adaptations were related to the amount of items requiring suction. Comparing across three sympatric pairs of species, in each case different diets were reflected in differences in anatomy.     

Evolutionary relationships within three Tilapiine genera (Pisces: Cichlidae)

Species of the cichlid genera Tilapia, Sarotherodon and Oreochromis were studied by electrophoresis at 25 enzyme loci. The levels of similarities within and between genera, and the distribution of genetic identities at individual loci are similar to those found in other fish species. The evolutionary relationships between the species remains equivocal, and it is still not possible to decide between the hypotheses of Trewavas (1980) or that of Peters & Berns (1978, 1982). Further work needs to be undertaken on additional Tilapia and Sarotherodon species. The unexpected relationship of O. jipe to the other Oreochromis species is probably the strongest argument against Trewavas' hypothesis. But the close similarity of the maternal mouth brooders to each other, and the closer relationship of S. galilaeus to the Oreochromis species than to the Tilapia does not favour the hypothesis of Peters & Berns (1982). The difficulties in obtaining pure species from the wild due to widespread introduction of non-endemic species are likely to hinder progress in evolutionary and aquacultural studies.     

Freiwasserbeobachtungen an Tilapien (Pisces,Cichlidae) in Ostafrika

T. grahami ♂♂ haben in Kolonien dicht beieinander liegende Reviere; sie beziehen sie frühmorgens bei völliger Dunkelheit und verlassen sie am späten Nachmittag wieder. Einige daraufhin beobachtete ♂♂ waren immer in der gleichen Grube zu finden. Es wird angenommen, daß dies generell gilt. Kämpfe sind äußerst selten. Revier-♂♂ kämpfen durchschnittlich nur etwa 2mal pro Woche. Revierlose ♂♂ rufen öfter Kämpfe hervor, und da Kämpfe auch nachts zu bemerken sind, ist eine teilweise Unabhängigkeit des Kämpfens vom Revierverhalten zu folgern. Ersatzhandlungen fürs Kämpfen sind das Graben und das Vertreiben von Jungfischen. Es wird angenommen, daß diese Handlungen wegen der beengten Lebensbedingungen einen positiven Selektionswert haben. Aus den gleichen Gründen sind wohl Balzhandlungen reduziert, die Laichdauer auf eine Sekunde zusammengeschrumpft und Jungfische früher als in jeder anderen Cichlidenart geschlechtsreif. Diese Frühreife hat bei T. alcalica zu einem Kampfgleichgewicht zwischen sehr unterschiedlich großen ♂♂ geführt. Sonst zeigt die Art keine Unterschiede im Verhalten zu T. grahami. Bei den Geißeltilapien T. variabilis und T. rukwaensis kann die Reviergröße in sehr weiten Grenzen schwanken. In großen Revieren schwimmen die ♂♂ viel an ihren Reviergrenzen entlang, ihr Gebiet auf diese Weise gegen Reviernachbarn markierend und auf ♀♀ hin kontrollierend. In kleinen Revieren bietet sich mehr ein Bild wie bei den Sodatilapien T. grahami und T. alcalica. Aus dem Vergleich der Gruben, der Balz und des Laichens von Soda- und Geißeltilapien lassen sich verwandtschaftliche Beziehungen zwischen beiden Gruppen erschließen. Es werden also vor allem Revierverhaltensweisen in ihrer Anpassung an wechselnde Umweltbedingungen beschrieben und Balzhandlungen zur Klärung verwandtschaftlicher Beziehungen verwandt.     

Development of the islets, exocrine pancreas, and related ducts in the Nile tilapia, Oreochromis niloticus (Pisces: Cichlidae)

Pancreatic development and the relationship of the islets with the pancreatic, hepatic, and bile ducts were studied in the Nile tilapia, Oreochromis niloticus, from hatching to the onset of maturity at 7 months. The number of islets formed during development was counted, using either serial sections or dithizone staining of isolated islets. There was a general increase in islet number with both age and size. Tilapia housed in individual tanks grew more quickly and had more islets than siblings of the same age left in crowded conditions. The pancreas is a compact organ in early development, and at 1 day posthatch (dph) a single principal islet, positive for all hormones tested (insulin, SST-14, SST-28, glucagon, and PYY), is partially surrounded by exocrine pancreas. However, the exocrine pancreas becomes more disseminated in older fish, following blood vessels along the mesenteries and entering the liver to form a hepatopancreas. The epithelium of the pancreatic duct system from the intercalated ducts to the main duct entering the duodenum was positive for glucagon and SST-14 in 8 and 16 dph tilapia. Individual insulin-immunopositive cells were found in one specimen. At this early stage in development, therefore, the pancreatic duct epithelial cells appear to be pluripotent and may give rise to the small islets found near the pancreatic ducts in 16-37 dph tilapia. Glucagon, SST-14, and some PPY-positive enteroendocrine cells were present in the intestine of the 8 dph larva and in the first part of the intestine of the 16 dph juvenile. Glucagon and SST-14-positive inclusions were found in the apical cytoplasm of the mid-gut epithelium of the 16 dph tilapia. These hormones may have been absorbed from the gut lumen, since they are produced in both the pancreatic ducts and the enteroendocrine cells. At least three hepatic ducts join the cystic duct to form the bile duct, which runs alongside the pancreatic duct to the duodenum.     

Behavioral repertoire of biparental care in Apistogramma trifasciata (Pisces: Cichlidae)

Cichlid fish exhibit a complex behavioral repertoire, mainly regarding social and reproductive behaviors. Nevertheless, although dwarf cichlids exhibit a remarkable diversity of behaviors, only a few studies have been conducted with these fish. Apistogramma trifasciata is a common dwarf cichlid in the Neotropics and widely used as ornamental fish. Both males and females are territorial and engage in biparental care. Here we observed the behavior of A. trifasciata males and females to construct an ethogram. We registered the parental care of a couple since the spawn until the moment when the offspring became independent. Striking behaviors were observed, such as male-male contests involving circling displays and mouth wrestling, and parental care, which involved mouth brooding and parental color change. Finally, we suggest that our results may be used to establish this species as a model for parental care and territoriality studies.     

Bi-parental mouth brooding in Tilapia galilaea (Pisces, Cichlidae)

New evidence shows that Tilapia galilaea (Artédi) is almost certainly a bi-parental mouth brooder throughout its range, and that this habit is not confined only to races found in Israel. Previous classification of this species as a maternal brooder in Africa appears to be based on lack of observations; it is the only member of the genus definitely known to be a bi-parental brooder. The size of the eggs and the development of their adhesive stalk system is intermediate between those of substrate spawners and mouth brooders. Among the latter the adhesive stalk system of the only paternal brooding species is rudimentary but it is entirely absent in the maternal brooding species. The conclusion is drawn that maternal and paternal mouth brooders did not evolve directly from substrate spawners but independently from bi-parental brooders.     

Astronotus ocellatus (Cichlidae:Pisces) and Macropodus opercularis (Anabatidae:Pisces) as predators of immature Aedes fluviatilis (Diptera:Culicidae) and Biomphalaria glabrata (Mollusca:Planorbidae).

Two fish species, Astronotus ocellatus (Cichlidae) and Macropodus opercularis (Anabatidae) were tested for predacious behavior toward immature mosquitoes (Aedes fluviatilis, Diptera:Culicidae) and schistosomiasis snail hosts (Biomphalaria glabrata, Mollusca:Planorbidae), in the presence or absence of non-living food and in laboratory conditions. A. ocellatus, a species indigenous to Brazil, was a very efficient predator of both organisms (alpha = 0.05); M. opercularis, an exotic species, preyed well on immature mosquitoes, but small snails and snail egg-masses were ingested only irregularly. Both fish species seemed to prefer live to non-living food.     

Naphthalene-induced changes in carbohydrate metabolism in Sarotherodon mossambicus Peters (Pisces: Cichlidae)

Sarotherodon mossambicus Peters was exposed to naphthalene concentrations ranging from 7.9 to 0.92 mg/l for periods ranging from 4 days to 10 weeks and changes in glycogen, glucose, pyruvic acid and lactic acid contents of liver and muscle were studied. While glucose levels remained unaffected, major effects could be seen in the other constituents following both lethal and sublethal naphthalene treatment. Whereas glycogen and pyruvic acid levels showed considerable reduction, lactic acid levels increased markedly. These changes are indicative of a metabolic stress leading to a shift to anaerobic metabolism. The effects do not appear to be related to the feeding and growth rates of the naphthalene-intoxicated fish.     

A preliminary investigation of sex change inPseudotropheus lombardoi (Pisces: Cichlidae)

Synopsis Peters (1975) suggested the possibility of adult sex change in certain cichlids of Lake Malawi. When adultPseudotropheus lombardoi in male coloration were found mouthbrooding eggs under natural conditions in Lake Malawi, one of the possible explanations for this female-type behaviour was that sex change had occurred, but with the retention of male coloration. Behavioural investigations based on current models of social systems in sex-changing species were conducted in an attempt to substantiate this hypothesis. These observations were supplemented by an histological examination of the gonads of individuals of both sexes. A pronounced advantage in the mating success of dominant males over non-dominant males was noted. Similarly, large females had a greater reproductive success than smaller females. Thus, the possibility that sex change occurred inP. lombardoi following the size advantage model (Warner 1975) was investigated. However, histological studies did not provide conclusive evidence of sex change; only undeveloped oocytes were found in the testes of all males examined. It is postulated that gonads of maleP. lombardoi pass through an intersexual juvenile period. Later, testicular elements dominate within a gonad still containing immature oocyte tissues. It is further suggested that femaleP. lombardoi are dimorphic, some having male coloration and others having female coloration.