Blue Chromis

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blue chromis

Blue chromis (Scientific name: Chromis cyanea) are a damselfish found in Bermuda, southern Florida and the Caribbean Sea. They are found in lagoons and reef communities at depths of of three to 60 meters (9.84 to 196.85 feet) at an average depth of 10 to 20 meters (33 to 65 feet), where food and shelter are most abundant. They prefer water temperatures of 21° to 27°C (70° to 81°F). Their lifespan in captivity is around five years. [Source: Natasha Perrine, Animal Diversity Web (ADW) /=]

Blue chromis are common reef fish and their survival depends on healthy coral as reefs provide a location for feeding, breeding, and protection from predators. Males maintain small territories, typically comprised of a flat nesting area and an overhang or crevice with a small (approximately seven centimeters in diameter) opening. They are also known to hide in fields of staghorn coral (Acropora cervicornis). Juveniles tend to school above and hide beneath or within a shelter-providing home structure, generally a coral head. They prefer yellow finger coral (Madracis mirabilis), Lamarck's sheet coral (Agaricia lamarcki) and smooth flower coral (Eusmilia fastigiata).

Blue chromis are a bright, iridescent blue in color, with a black dorsal stripe and black margins on the dorsal fin, upper and lower lobes of the caudal fins, and front of the anal fin. According to Animal Diversity Web: They can be distinguished from similar species, such as juvenile black snapper (Apsilus dentatus) and blue hamlet (Hypoplectrus gemma), by their dark eyes and the continuation of their black dorsal stripe to the upper and lower margins of the tail. Males may exhibit any one of five color morphs, including dark grey-blue and light blue shades, with accompanying variations in dorsal stripe thickness. Sexual Dimorphism (differences between males and females): occurs only during spawning, when females can be identified from light blue or dark-colored displaying males by their swollen bellies and protruding white genital papilla.

Blue chromis are hardy inhabitants of saltwater aquariums and are common in the pet trade. Blue chromis are abundant throughout their geographic range, and are listed as a species of least concern by the International Union for the Conservation of Nature and Natural Resources. /=\

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal ; Woods Hole Oceanographic Institute ; Cousteau Society ; Monterey Bay Aquarium ; MarineBio ; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Wikipedia article Wikipedia ; Coral Reef Alliance ; Global Coral reef Alliance ;Global Coral Reef Monitoring Network Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Blue Chromis Characteristics and Behavior

Blue chromis reach a length from 15 centimeters (5.91 inches), with their average length being 13 centimeters (5.12 inches). Both sexes are roughly equal in size and look similar except they are patterned differently According to Animal Diversity Web: The body of the blue chromi is compressed and oval in shape. These fish have a small, terminal mouth, continuous dorsal fin (12 dorsal fin spines and 12 dorsal soft rays) and deeply forked tail (2 anal spines and 12 anal soft rays). [Source: Natasha Perrine, Animal Diversity Web (ADW) /=]

Blue chromis are diurnal (active mainly during the daytime), feeding during the day and hiding at night, territorial (defend an area within the home range), and have dominance hierarchies (ranking systems or pecking orders among members of a long-term social group, where dominance status affects access to resources or mates). Males defend territories that vary in size from three square meters up to 20 square meters, extending upward from the substrate to about one meter. Territory size is related to population density, with males in low density areas maintaining larger territories. Females swim and feed freely between and above these territories. /=\ Juveniles often school and feed together in loose aggregations. Adult males are solitary and highly territorial (defend an area within the home range). They perform distinct behavioral displays to maintain their territories, including chasing, pushing, butting, or ramming intruders out of nests, and swimming in parallel with neighbors to reinforce boundaries. These tactics may be accompanied by color changes, with brighter colors signifying aggression or dominance. If kept in captivity, dominance hierarchies develop; in small schools (less than six fish), individuals on the bottom of the social hierarchy may actually be harassed to death. /=\

Blue chromis communicate mainly through visual channels but also employ touch and use vibrations to communicate. They sense using vision, touch, vibrations and chemicals usually detected with smell. Interactions between competing males, residents and intruders, and potential mates are all marked by variations in swimming patterns and/or physical contact. Specific color changes often accompany behavioral patterns, providing the audience with additional visual clues and communicating intent. Blue chromis also use olfaction and mechanoreception (lateral line) to detect water movement and vibration, and can also hear using their well-developed inner ears. /=\

Blue Chromis Eating Behavior and Predators

blue chromis being chased by a sperm whale

Blue chromis feed mainly on plankton suspended in the water column. When feeding, they remain stationary in the water column and rely on the current to bring food particles to them. Feeding is accomplished by rapid extension of the jaw, which causes prey to be sucked into the buccal cavity, due to the negative pressure generated. Feeding is frequently interrupted when blue chromis dart for cover from a perceived threat. The threat of predation increases under conditions of low prey density, when these fish must distance themselves from protective structure in order to feed. Examination of the stomach contents of blue chromis revealed that their diet is composed of about 52 percent copepods and 34 percent tunicates by volume, with the remainder comprised of shrimp larvae, siphonophores, fish eggs, and ostracods. Juveniles may also eat planktonic algae. [Source: Natasha Perrine, Animal Diversity Web (ADW) /=]

As a moderately small reef fish, blue chromis are a food source for a variety of larger fish species such as Trumpetfish (Aulostomus maculatus), Bar jack (Carangoides ruber), Nassau grouper (Epinephelus striatus), Yellowfin grouper (Mycteroperca venenosa) and Cero (Scomberomorus regalis). Predators can be resident or transient fish eaters. To avoid predation, blue chromis hide under or within shelter and may also adopt a darker color when closer to the reef to camouflage their bodies.

Blue chromis share the reef habitat with many other species of planktivorous fish. Juvenile black snapper (Apsilus dentatus), which feed alongside blue chromis, mimic them in order to attack unsuspecting crustaceans. The territorial nature of these fish may often reduce the number of hiding places for other similarly-sized non-territorial fish, such as brown chromis (Chromis multilineata), increasing predation rates on these species./=\

Blue Chromis Reproduction and Offspring

Blue chromis are iteroparous (offspring are produced in groups) and oviparous (young are hatched from eggs). Reproduction is external, meaning the male’s sperm fertilizes the female’s egg outside her body. Blue chromis engage in year-round breeding. Female blue chromis have synchronized ovarian cycles and spawning occurs once a month, generally during the full moon. The spawning period lasts three to seven days, during which time females both actively seek out males and are solicited. Reproduction peaks in June and October., with the average number of offspring being 40,000. The time to hatching ranges from two to three days and the age in which they become independent ranged from 17 to 47 days. Both females and males reach sexual or reproductive maturity at 17 to 47 days.[Source: Natasha Perrine, Animal Diversity Web (ADW) /=]

According to Animal Diversity Web: Blue chromis are promiscuous, with multiple females visiting and laying eggs at many nests and males mating with multiple females. Male reproductive success may be determined by the condition and location of the nest, as well as the presence of eggs already in the nest. To solicit females, males may perform a maneuver called a signal jump in which they rapidly propel themselves up vertically and then loop back to the starting position. They may also perform "dipping," where they swim in a zig-zag fashion above their territories. Males will present to females, swimming laterally and rotating 180 degrees for inspection. If a female indicates receptivity and the display is not interrupted, the male will lead her to his nest using an exaggerated side to side swimming motion. She will "nip" at the floor of the nest, and the male will respond by quivering his fins and skimming over the subtrate. He may nudge the female's genital papilla with his snout to encourage oviposition (known as butting).

Once eggs are laid, a male will release milt (seminal fluid); it is not known precisely when the male releases milt, but it is hypothesized that it is dispersed by tail wagging. A male may begin to solicit another mate while one female is laying eggs in his nest. Spawning ends when the female is spent; she will either leave the nest immediately or be chased out. Other males may interfere in breeding, since displays take place up to one meter above a nest (outside territorial (defend an area within the home range), borders). Males may also sneak into nests while their owner is occupied in a display, preemptively fertilizing deposited eggs.

Blue Chromis Offspring and Development

blue chromis

Eggs hatch into independent larvae within two to three days of spawning and are cared for by males. Males maintain territories throughout the year, though increases in the number of larvae observed during the months of June and October indicate that reproduction may have a seasonal component. /=\

Parental care, pre-fertilization protection, pre-birth protection and pre-independence protection are provided by males. Males assume responsibility for all parental care, from nest preparation to caring for eggs. Small areas of sand, coral, or algae-covered rock are cleared by "nipping" (removing material orally) or generation of water currents by tail fanning. Males care for eggs and larvae by chasing away similarly-sized predators, nipping away debris, and moving water over the eggs.

Blue chromis go through a development and life cycle characterized by indeterminate growth (they continue growing throughout their lives). They begin life as soft, translucent, demersal eggs, adhered to the substrate. The eggs are tended to by the male for two to three days before hatching into fast-swimming larvae. These larvae stay within the cover of the reef surrounding the nest until they reach about one centimeter in length; they then join other juveniles in aggregations above the reef to feed on zooplankton. Juveniles, which have a more compact body shape than adults, school together until reaching 5-6 centimeters in length. After this they become sexually mature, at which point males begin to defend territories and maintain nests of their own. /=\

Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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