Abstract
The discovery of Serolis tropica sp. nov. in the shallow shelf zone of the Pacific of Panamá, just south of 9°N latitude, establishes the first occurrence of the Serolidae in tropical American waters. Since gravid females were found in cool as well as warm thermal seasons, it is assumed that this species is adapted to live under tropical conditions. Serolis yongei, known from the Great Barrier Reef, was collected at a depth of 200 m. It is not presently known if this species, like S. tropica, also inhabits the shallow shelf where warm-water conditions prevail. Serolis tropica, a member of the North American ‘carinata’ group, is related to the ‘marplatensis’ group of South America. While it is argued that these two groups must have shared a common history, the remaining region of approximately 40° latitude between the northern and southern forms greatly complicates the relationship. Whether or not this gap is real or due to a lack of fine, inshore sampling can only be determined by further field work in this region.