Daphnia lumholtzi has been very successful in colonizing North America since its appearance in Texas in 1990. Although previous studies have sought to link its success as an invasive species with various aspects of its population biology, there is little experimental data linking the invasion success of D. lumholtzi with its autecology, specifically its reproduction strategy. In this study we sought to link food quality and quantity to diapause in D. lumholtzi through a variation in phosphorus (P) content of algae, food quantity, and light level. We also assessed the effect of Daphnia peak population densities on reproductive rates and production of resting eggs. We found that when food is abundant, per capita ephippia production may be limited by P, but under food limitation conditions, there is no significant effect of food quality on ephippia production. Our results suggest that a combination of food quality/quantity and population density may work together to induce the production of resting eggs in this invasive species.

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To the Editor: We read with interest the article by Cunningham et al.' They reported a patient with Parkinson's disease (PD) who, during the substitution of a controlled-release formulation of Ldopa, developed a symptom complex of severe hyperthermia, rigidity, and autonomic dysfunction resembling neuroleptic malignant syndrome (NMS). NMS is a fatal and life-threatening condition. It is characterized by elevation of serum CK.2 Although the pathophysiology of NMS has not been fully clarified, it is believed to be associated with a large and sudden decrease in dopaminergic transmission. There has recently been considerable attention given to the relationship between serum iron and the maintenance of normal dopaminergic function in the brain.3 Rosebush and Mazurek4 measured serum iron in 26 episodes of NMS. In 25 episodes, serum iron concentration was significantly lower than 2 SD below the mean normal value. With improvement of clinical status, serum iron returned to normal.We have seen a patient with PD who subsequently developed NMS. He had been treated with Sinemet and bromocriptine, but these medications were withdrawn because of no improvement in his clinical state. Our case showed a significant reduction of plasma iron in the acute stage and an increase with clinical improvement.Rosebush and Mazurek4 suggested that serum iron may be a useful biochemical marker for NMS, and a helpful adjunct in the diagnosis of the disorder. We definitely agree with their opinion. It is assumed that the large decrease in serum iron during NMS plays a part in the pathophysiology of the disorder. Iron is present in relatively large quantities in the brain and its distribution roughly parallels that of dopamine, with the highest concentrations in the basal ganglia.5 Finally, we emphasize that serum iron should be routinely measured in patients with PD.Reply from the Authors: We thank Iwasaki et a1 for their comments regarding serum iron as a biochemical marker for neuroleptic malignant-like syndrome associated with withdrawal of dopaminergic medications in parkinsonian patients. These are of interest and highlight the complexities of the dopaminergic neurotransmitter system. That serum iron is an early biochemical marker of NMS in nonparkinsonian patients4 is a n a s t u t e observation. If confirmed in further parkinsonian patients, like the one of Iwasaki e t al, it would appear to contradict the hypothesis advanced by Rosebush and Mazurek4 that suggests the reduction in serum iron precedes and is responsible for the dysfunction of iron-dependent D2-dopaminergic receptors. Why serum iron should fall when central dopamine deficiency is the presumed primary event is difficult to explain in this conception because of the presumed massive peripheral shifts in serum iron level that would be required consequent to small central changes. An epiphenomenon appears to be a more likely explanation.Unfortunately, we did not measure the serum iron in our patient' and are not able t o comment further on its relationship to his NMS or its pat...