Hydrothermal represents one of the most extreme environments on earth. Chemolithotrophic bacteria are common residents in such ecosystems. These bacteria are believed not only to enable their hosts to adapt the extreme environment but also act as food source to support their host animals. A shallow-water hydrothermal vent near the Turtle Island off the northeast coast of Taiwan provides a unique opportunity to explore the trophic ecology of a vent system influenced by the phototrophic ecosystem. We examined isotopic compositions of various potential food items and crab’s tissues from water column and vent floor, including zooplankton both living and dead, particulate organic matter, bacterial mat, and various tissues of sulfur-tolerant the vent crabs. The similar δ13C and δ15N values of crabs’ stomach content and dead zooplankton mixture suggested that both zooplankton and bacteria are the food sources for the vent crab. This result confirms that the vent crab is an opportunistic, which means that they have a remarkable adaptation to their living surroundings. The vent crab is an amazing creature living in extreme environment. We could data back to the evolving history of the earth living things through our results. Simultaneously, there might exist an implied connection between the deep sea Hydrothermal vent ecosystem and the shallow sea Hydrothermal vent ecosystem.