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20 PRODUCTION Standing crop and production are two sides of the same coin. For instance, if we put 100 head of cattle, each weighing 500 pounds, into a pasture, we would have a standing crop of 50,000 pounds. Eight months later, we remove the cattle and find they weigh 800 pounds for a total of 80,000 pounds. The production was 30,000 pounds. This concept applies to crayfish but the catch is that this isn't so easy to measure in a stream so there aren't many estimates of annual production out there. One worker estimated that total annual production of crayfishes in an Ozark stream was 20 times that of fish. 195 In Michigan lakes, the mean standing crop of Northern crayfish varied from 9.4 to 30.3 kg/ha while annual production ranged from 71.9 to 169.7 kg/ha. 166 COMMENSALS, PARASITES AND DISEASES Commensals, parasites and diseases – Oh My! First, some definitions: a commensalism is where one benefits and other doesn't; mutualism is where both benefit; parasitism is where one benefits and the other is harmed. All of these are forms of symbiosis. There is a whole group of commensal organisms (some 150 species worldwide) called branchiobdellidans (say that three times fast) that live only on crayfish. In a nutshell, these are small worms (1 to 10mm long) that live on crayfish and cannot reproduce or survive without their crayfish host (obligate ectosymbiotic annelids). What the branchiobdellidans do for the crayfish is to eat the bacteria, algae, diatoms and protozoans that accumulate on their exoskeleton or in the gill chamber. What the crayfish get out of this is a clean exoskeleton and clean gills. Actually, no one is really sure if the relationship is commensal, mutual or parasitic but may be all three, depending on conditions. 214 Parasites of crayfish include flukes (digeneans), tapeworms (cestods), roundworms (nematodes) and spiny headed worms (acanthocephalans) though these seldom affect the health of a crayfish. There Normal Northern crayfish cheliped Regenerated Northern crayfish cheliped Branchiobdellidans on crayfish