Serve Maryland blue crab with other summertime classics like corn on the cob and potato salad. For dessert, make watermelon extra special in a simple way: Cube it and toss with a little chiffonade of fresh mint.
Combine the coriander seeds, mustard seeds, and celery seeds in an 8-inch skillet and heat over medium-low heat until just fragrant. Set aside to cool.
In a 6-quart heavy-duty pot fitted with a steamer basket and a tight-fitting lid bring 1 inch of water to a boil over medium-high heat.
Meanwhile, transfer all the spices, and 2 tsp. of salt to a spice grinder or small food processor and process until finely ground.
Generously sprinkle the crabs with the spice mix and immediately place them in the steamer basket and cover. Steam until the crabs are cooked through and turn bright red, 14 to 16 minutes. Remove the crabs from the steamer basket with tongs.
Garnish with the parsley leaves and serve immediately with the lemon wedges and cider vinegar.