Volunteers are taking part in a one-day, coast-wide survey to gauge green crab populations along Maine’s coast. The number of green crabs has grown sharply in some coastal areas, posing threats to clams, mussels and other marine species. The Department of Marine Resources is holding a press conference Wednesday to discuss the survey, which will provide a snapshot of the crab population. To determine their numbers, volunteers are using baited traps in areas where soft-shell clams are abundant. The department will use the information to raise public awareness and help towns develop plans to control the predators. Green crabs are native to Europe and were first spotted in Maine waters in 1900. The increase in the crab population has coincided with the increase in ocean temperatures in recent years.