Patterns of Evolution in the Feeding Mechanism of Actinopterygian Fishes

SYNOPSIS. Structural and functional patterns in the evolutionof the actinopterygian feeding mechanism are discussed in thecontext of the major monophyletic lineages of ray-finned fishes.A tripartite adductor mandibulae contained in a maxillary-palatoquadratechamber and a single mechanism of mandibular depression mediatedby the obliquus inferioris, sternohyoideus, and hyoid apparatusare primitive features of the Actinopterygii. Halecostome fishesare characterized by having an additional mechanism of mandibulardepression, the levator operculi—opercular series coupling,and a maxilla which swings anteriorly during prey capture. Theseinnovations provide the basis for feeding by inertial suctionwhich is the dominant mode of prey capture throughout the halecostomeradiation. A remarkably consistent kinematic profile occursin all suction-feeding halecostomes. Teleost fishes possessa number of specializations in the front jaws including a geniohyoideusmuscle, loss of the primitive suborbital adductor component,and a mobile premaxilla. Structural innovations in teleost pharyngealjaws include fusion of the dermal tooth plates with endoskeletalgill arch elements, the occurrence of a pharyngeal retractormuscle, and a shift in the origin of the pharyngohyoideus. Thesespecializations relate to increased functional versatility ofthe pharyngeal jaw apparatus as demonstrated by an electromyographicstudy of pharyngeal muscle activity in Esox and Ambloplites.The major feature of the evolution of the actinopterygian feedingmechanism is the increase in structural complexity in both thepharyngeal and front jaws. Structural diversification is a functionof the number of independent biomechanical pathways governingmovement.     

In vitro Regeneration of Mustard Plants (Brassica juncea var. RAI-5) on Cotyledon Explants from Non-irradiated, Irradiated and Mutagen-treated seed

Shoot formation in cotyledon explants of mustard (Brassica junceavar. Rai-5) was observed on Murashige and Skoog's medium supplementedwith NAA^* (1 mg l–1) and BA (1 mg l–1). Hypocotylsegments failed to differentiate shoots. Complete plants wereobtained when shoots were rooted in MS medium with NAA (1 mgl–1). EMS, a chemical mutagen, had an inhibitory effecton shoot regeneration. Gamma rays in doses above 2 kR suppressedshoot regeneration but stimulated callus growth. Brassica juncea, mustard, regeneration, tissue culture     

Long-term response and recovery to nutrient addition of a partitioned arctic lake

1. To study the bottom‐up linkages in arctic lakes, we treated one side of a partitioned lake with inorganic nitrogen and phosphorus for a 6‐week period each summer for 6 years starting in the summer of 1985. We took a variety of weekly measurements to determine the impact of the nutrient loading on the lake and continued weekly measurements for 2–6 years after the cessation of nutrient loading to observe the recovery of the treated side. The loading rates (2.91 mmol N m^?2 day^?1 and 0.23 mmol P m^?2 day^?1) were five times the calculated loading rates for Toolik Lake, located nearby. 2. In all 6 years of nutrient addition, phytoplankton biomass and productivity were greater in the treated sector than the reference sector. In the first 4 years of nutrient addition there was no flux of phosphorus from the mineral‐rich sediments. This changed in the last 2 years of nutrient addition as phosphorus was released to the lake. 3. The response of the animal community to increased plant production was mixed. One of the four macro‐zooplankton species (Daphnia longiremis) increased in number by about twofold in the first 5 years. However, the copepod Cyclops scutifer showed no response during the treatment phase of the study. The benthic invertebrate response was also mixed. After a 2‐year lag time the snail Lymnaea elodes increased in the treated lake sector but chironomids did not. 4. Ecosystem response to fertilisation was not controlled solely by nutrient addition because phosphorus was not recycled from the sediments until the last 2 years of nutrient addition. Phytoplankton still showed the effects of nutrient addition in the recovery period and the hypolimnion of the treated sector was still anaerobic starting at 6 m in 1996.     

The Feeding of Amoeba proteus on Paramecium aurelia*

SYNOPSIS. Density of prey (Paramecium aurelia) and predator (Amoeba proteus) were varied while volume of inorganic medium was kept constant. Variations in density of prey had little effect on the rates of feeding and reproduction of the amoebae; but with increasing predator density the amoebae captured the paramecia less rapidly and ingested fewer before dividing, altho division size did not change appreciably. Therefore, amoebae of a low density population with a constant food supply carry more nutritional reserves from generation to generation than do those in a denser population.