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Detection of BCD infections in snow crabs Jeffrey Shields Virginia Institute of Marine Science Dave Taylor Department Fisheries and Oceans, Newfoundland

What is BCD? Bitter crab disease is caused by a parasitic dinoflagellate, Hematodinium, that lives in the blood of the snow crab. The disease causes a aspirin-like, bitter flavor in cooked crabs. Late stages of the disease cause snow crabs to turn pink as if partially cooked. Early stages can only be detected microscopically.

Detection methods Visual examination Heart dissection Microscopic determination wet smears neutral red fixed preparations histology

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Heart dissection Onboard visual determinations can sometimes be questionable. In such cases gross examination of the heart can confirm diagnosis of heavily infected crabs. Remove the carapace off the animal in question. Locate and examine the heart. If the heart is milky white, not translucent beige or white, then the crab most likely has Hematodinium.

Microscopic determination 1 Trained personnel can make determinations using wet smears of hemolymph. However, more reliable results can be obtained by making permanent, stained preparations.

Microscopic determination 2 Neutral red is a useful vital stain for Hematodinium. Mix one drop 0.25% neutral red (in filtered seawater) with one drop hemolymph.  Observe at 400x. Vacuoles in Hematodinium are bright red. Vacuoles in hemocytes rarely stain light orange.

Histological determination 1 Histology is the best method for diagnosis of infections. Hematodinium is like other dinoflagellates. The chromatin is condensed within the nucleus. Nuclei appear as if in arrested metaphase.

Histological determination 2

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