A nematode (Captivonema cretacea gen. et sp. n.) preserved in a clitellate cocoon wall from the Early Cretaceous

A fossil worm-like organism, ca. 0.5 mm long, showing cuticle, gut and setae. is contained in the solid part of the wall of a clitellate cocoon from the Early Cretaceous (Barremian). It is interpreted as having been entrapped and embedded in the intitially viscous cocoon secretion which solidified and thus prevented decay. Size, shape, posture, and an unsegmented but annulated cuticle with irregularly distributed setae have led to the identification of the fossil as a nematode. described as Captivonema cretacea gen. et sp. n., of uncertain family and order affinity. Fossil association end mode of preservation indicate a free-living life habit in damp plant litter.     

POTENTIAL MECHANISMS FOR SEX RATIO ADJUSTMENT IN MAMMALS AND BIRDS

Sex ratio skews in relation to a variety of environmental or parental conditions have frequently been reported among mammals and, though less commonly, among birds. However, the adaptive significance of such sex ratio variation remains unclear. This has, in part, been attributed to the absence of a low-cost physiological mechanism for sex ratio manipulation by the parent. It is shown here that several recent findings in reproductive biology are suggestive of many potential pathways by which gonadotropins and steroid hormones could interfere with the sex ratio at birth. And these hormone levels are well-known to be influenced by many parameters which have been invoked in correlating with offspring sex ratios. Hence, it is argued that the significant, but inconsistent sex ratio biases reported in mammalian and avian populations are coherent with current knowledge on reproductive physiology in those species. However, whether such variations can be viewed at as a consequence of physiological constraint or as adaptive sex ratio adjustment, has still to be determined.     

A new Aequidens species from the Rio Trombetas, Brasil, and redescription of Aequidens pallidus (Teleostei, Cichlidae)

Aequidens pallidus (Heckel) reaches c. 140 mm standard length and occurs in the drainages of the blackwater rivers Negro, Puraquequara, Preto da Eva and Uatumã in Brasil. Aequidens duopunctata Haseman is a junior synonym. Aequidens tubicen sp.n., was collected in the Trombelas and Mapuera rivers a little upstream of Cachoeira Porteira. The largest specimen is 116 mm standard length. It differs from all other Aequidens species in the possession of a dark spot at the inner angle of the preopercle and adjacent cheek. Aequidens pallidus and A. tubicen form a group diagnosed by a high number of vertebrae (14 + 13 = 27) and three colour pattern synapomorphies: lateral band positioned more dorsally and the midlateral spot positioned more posteriorly than in other Aequidens species, and caudal spot preceded by a large light spot at the dorsal margin of the caudal peduncle.     

Cichlid Fishes from the La Plata Basin. Part V. Description of Aequidens plagiozonatus sp.n. (Teleostei, Cichlidae) from the Paraguay River System

Aequidens plagiozonatus sp.n. is described from 49 specimens collected in the northern Paraguay river system. It is the only known species of Aequidens Eigenmann & Bray in the La Plata basin. Distinguishing characteristics include the slanting vertical bars on the sides, discontinuous horizontal lateral band, immaculate or very sparsely dotted caudal fin and short pectoral fin.     

Selection of nest sites by a hole-nesting duck, the Goldeneye Bucephala clangula

Nest site preferences were examined for a population of Goldeneye Ducks breeding in nest boxes in Värmland, central Sweden. Some nest boxes were occupied more often than others even if females returning to the same nest box were excluded from the analysis. Nest boxes located higher up trees were occupied more often than those close to the ground and some spatial 'cluster groups' of boxes were occupied more often than others. Otherwise nest site prefernces were not related to any measured physical attributes of the boxes. Prefernces for nest boxes seemed to be based mainly on a tendency for females to select those that had been occupied by other females in the preceding year, especially if they had bred successfully. As a result of this, the occupancy of nest boxes was not random over years but rather progressed in a series of runs; a period of consecutive years in which a box was occupied was followed by a period of years in which it was empty.
There were reproductive consequences for these prefernces in that females occupying preferred boxes were less likely to lose their clutch to a predator. These females also bred earlier in the year and produced larger clutches and broods than females breeding in other boxes.     

Intraspecific VariabiIity in Acrobeloides nanus (de Man) Anderson (Nematoda, Cephalobidae) and a Note on External Morphology

Uniparental free-living nematodes, Acrobeloides nanus (de Man, 1880) Anderson, 1968, were isolated from the raw humus layer at two coniferous forest sites in central Sweden. Intraspecific variability in progeny from single females cultured both on agar and in soil was studied. Females from field populations were much shorter than those from laboratory cultures and significant differences in length between cultured specimens from the two sites were found. No significant difference in length of animals from comparable cultures on agar and in soil was obtained. The substrate had some effect on the tail shape, but not on the labial probolae shape. No significant differences in taxonomic characters between populations from the two localities were observed. Enzyme analyses by horizontal starch gel electrophoresis showed qualitative differences in isozyme patterns of malate dehydrogenase (MDH) and superoxide dismutase (SOD) in populations from the two sites, while the esterase patterns were unstable with time and less suitable for population analysis.