Abstract

Crustaceans are a diverse group of organisms that inhabit a wide variety of environments. These environments can be unstable, fluctuating in conditions such as temperature, salinity, and dissolved oxygen. Because oxygen is necessary to sustain aerobic life, crustaceans need to cope with extreme and sudden changes in oxygen tensions to maintain the delicate balance of internal oxygen homeostasis. Mammals challenged by hypoxia respond by increasing the oxygen carrying capacity of the blood via changes in erythrocyte mass. What is the mechanism of hypoxia tolerance and compensation in crustaceans?

The subject of this study is the Dungeness crab, Cancer magister , a commercially important species that experiences hypoxia in its ambient environment. One hypothesis is that C. magister increases the carrying capacity of its hemolymph in response to hypoxic conditions via increases in the oxygen transport protein hemocyanin. An alternative hypothesis is that hemocyanin responds to decreased oxygen tensions only by allosteric modulation of affinity by effectors such as protons, inorganic ions, or organic cofactors. To test these hypotheses, we measured changes in hemocyanin concentration and subunit composition over the course of moderate hypoxic exposure. We found that C. magister significantly increases hemocyanin concentration and differentially regulates hemocyanin subunits in response to prolonged hypoxia.

Unlike the intracellular mammalian protein hemoglobin, hemocyanin is extracellular. Therefore, changes in hemocyanin quantity could be accomplished by direct regulation of the hemocyanin genes. In other organisms, the oxygen-dependent, heterodimeric transcription factor HIF-1 is directly involved in upregulating genes during hypoxia. Is HIF-1 involved in hemocyanin regulation in C. magister?

To answer this question, we characterized a homologue of the α subunit of HIF-1 (HIF-1α) in the brachyuran crabs Cancer magister and Callinectes sapidus using degenerate PCR. The detection of putative HIF-1 binding sites in the regulatory regions of several hemocyanin genes indicates that HIF-1 may play an important role in crustacean responses to hypoxia. This investigation serves to elucidate the mechanisms by which crustaceans adapt to hypoxia and suggests a possible mechanism for hemocyanin regulation. We propose that the physiological parallels between mammalian and crustacean responses to hypoxia may also be conserved on a molecular level.