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Underwater Photo of the Week - March 18, 2019

Yellowtail Coris Wrasse

POTW March 18, 2019



Hi Gang!


This week’s POTW is a shot of a yellowtail coris wrasse I captured during my recent trip to Maui to dive with Captain Steve. The species is also commonly known by the names tomato wrasse and clown wrasse. Ichthyologists know the species as Coris gaimard.


The bony fishes commonly known as wrasses make up a large family of fishes whose members are present in almost every reef community in tropical and temperate seas with a few species known to occur in the sub-Arctic waters as far north as Greenland. In many settings wrasses are among the most common type of fishes encountered by divers. Worldwide, there are approximately 580 species of wrasses, and all are described in the family Labridae. Wrasses are characterized by their large, easily noticeable scales, large front teeth and tendency to propel themselves by only using their pectoral fins and not their tails as is the case with many fishes. 


On the whole, wrasse are relatively small fishes, typically ranging from three to six inches long. However, the species commonly known as Napolean wrasse provide a notable exception as members of this species commonly reach the astonishing proportions of eight feet long while weighing several hundred pounds.


Perhaps the most remarkable characteristic of wrasses is their ability to change sex over the course of their lives. Different life stages, often signified by a sex change, are referred to as phases. The phases are known as juvenile phase, initial phase and terminal phase. However, not all individual fish change sex during their lives as it is only the females that are transformed into males. Those individuals that are born as males remain male throughout the course of their lives. These fish are known as primary males. 


Some wrasses begin their lives as females and are later transformed into males. These fishes are known as secondary males. Often the largest and most colorful male in a given population is the only male that mates, and it is him, and him alone, that fertilizes the eggs of any and all females in “his” population. Studies have demonstrated that when this male is removed from the population, the male or female closest in size will take his place, changing sex if the next in line is a female. The sex change only takes a few days to complete. 


The juvenile phase is the first stage in the life history of wrasses. Juvenile phase includes males and females. While some of these females continue their lives as females, others eventually transform into males. The phase following juvenile phase is known as the initial phase. The initial phase not only includes sexually mature females, but in certain species, initial phase males include those that have not yet matured as well as those that have reached sexual maturity. Both male and female Initial phase wrasses are usually less brightly colored than terminal phase fish. When mating, initial phase adults do so by gathering together and spawning in large schools.


A wrasse enters the terminal phase when it is the dominant fish within its local population. Dominant wrasses are females as well as males, however the terminal phase is made up entirely of male fishes. Therefore, females entering the terminal phase undergo a remarkable transformation as they turn into males. A terminal phase male mates with one female at a time as opposed to the group spawning that occurs in initial phase Wrasses. The appearance of terminal phase males is often very different from those initial phase males of the same species.


A behavior consistent amongst wrasses is their tendency to constantly feed during the day. Divers often observe wrasses swimming over reef structures as the fishes incessantly stop and move again as they discover prey. At night, these diurnal (active during the day) fishes tuck themselves away into cracks of the reef to rest while some species actually bury themselves in the sandy bottom. 


The characteristic buckteeth of wrasses are useful for plucking invertebrates from cracks and crevices in reefs. Wrasses also have back teeth that are used for crushing the hard exteriors of mollusks, crustaceans and other invertebrates they feed upon. Some smaller species, as well as juveniles of some species, serve as cleaner fishes. These fishes use their sharp teeth to pick tiny parasites from the bodies of larger fishes.


As is the case with many species of their close relatives, the parrotfishes, wrasses feed almost exclusively during daylight hours. When the sun sets, these fishes rest. Most wrasses are opportunistic feeders, often being the first at the scene when a large fish has made a kill as well as being quick to take advantage of areas of sand that have been disturbedin acts that expose a variety of normally concealed invertebrates. Despite being larger than initial phase males, terminal phase males of carnivorous species of wrasses often tend to be more careful and cautious than the smaller fish.


The fish pictured here is transitioning from a juvenile to a mature female. Yellowtail coris wrasse occur in the eastern Indian Ocean and in the western tropical and sub-tropical Pacific Ocean to Hawaii in the central Pacific.


The yellowtail coris wrasse feed primarily upon mollusks, crabs, hermit crabs, and some tunicates.


I hope you enjoy this week’s POTW!


See you next week,






See you next week,