Complex sentence: It is common for gobies to have their pelvic fins fused together this allows them to use them as a suction device to keep them anchored on a hard surface such as a rock or piece of coral.
Split into simpler sentences: It is common for gobies to have their pelvic fins fused together. This allows them to use their pelvic fins as a suction device to keep them anchored on a hard surface such as a rock or piece of coral.