Clean and wash the celery and cut it into slices about 3 mm thick. Clean and peel the carrot and cut it into fine cubes. Peel and finely dice the onion. Heat the oil in a pot and sauté the onion until translucent. Add the celery and carrots, sauté briefly and add the stock. Simmer for about 10 minutes.
In the meantime drain the crabs. Wash the chives, dab dry and cut into fine rolls. Stir the starch with 1 tablespoon of water until smooth and thicken the soup slightly. Season with salt and pepper. Bring the soup to the boil again, add the shrimps and let it simmer for about 1 minute. Arrange in soup plates and sprinkle with chives