RESUMO
Gamma-ray bursts (GRBs) and their afterglows are the most brilliant transient events in the Universe. Both the bursts themselves and their afterglows have been predicted to be visible out to redshifts of z approximately 20, and therefore to be powerful probes of the early Universe. The burst GRB 000131, at z = 4.50, was hitherto the most distant such event identified. Here we report the discovery of the bright near-infrared afterglow of GRB 050904 (ref. 4). From our measurements of the near-infrared afterglow, and our failure to detect the optical afterglow, we determine the photometric redshift of the burst to be z = 6.39 - 0.12 + 0.11 (refs 5-7). Subsequently, it was measured spectroscopically to be z = 6.29 +/- 0.01, in agreement with our photometric estimate. These results demonstrate that GRBs can be used to trace the star formation, metallicity, and reionization histories of the early Universe.
RESUMO
The relationship between the morphology and functions of the feeding appendages of first-stage zoeae of the lithodid crab Placetron wosnessenskii Schalfeew during ingestion is explored in this study. The preoral chambers of these zoeae are bordered on all sides, with the labrum and mandibles forming the anterior borders, the paragnaths and sternal projection together creating the posterior boundaries, and the maxillules forming the sides. The maxillules are the sole pair of appendages responsible for prey manipulation immediately preceding ingestion. Maxillules are capable of remarkable plasticity of movement, enabling them to grasp, control, and redirect violently struggling prey (Artemia sp. metanauplii). The asymmetrical mandibles tear and grind the prey, working against each other with rotating motions. Two separate ratchet-like coordinations of the appendages were seen, each of which enabled the zoea to maintain a firm grasp on the prey while renewing points of leverage for ingestion. The mandibles hold prey in position while the maxillules regrab it to push it farther into the mouth. Similarly, the labrum holds the prey while the mandibles prepare for a new grinding rotation. Capture and ingestion of an algal cell by a rapid outward flinging and inward clasping of the mouthparts was seen in one videotaped sequence. Gut fluorescence after introduction of various algal species reveals an ability to ingest a range of particle sizes. This plasticity of feeding behaviors allows the zoeae to ingest a range of food items, and thus meet their nutritional needs.
