Friendly Damsels? It Can't be Possible!... The Genus Chromis

Last month I discussed one of the four subfamilies of Pomacentridae, the always-lovable Clownfishes. For November, I figured I'd keep going with this theme and cover a second subfamily of Pomacentridae, Chrominae, or the Chromis. Few members of this genus are regularly imported for the marine aquarium trade, despite their overall vast numbers and species in the wild. Most often, hobbyists purchase these fish in large numbers to replicate schools or harems, but should it always be this way? Join me in the November edition of 'Fish Tales' as I explore the world of the Chromis.

Chromis verater (above & below) is one of the largest Chromis,
and is endemic to Hawaiian waters. Photos courtesy of Keoki Stender.

One of the 28 genera of Pomacentridae is Chrominae. Chromis are members of the subfamily Chrominae, which contains a total of four genera (see below). In the early 1900's, however, some disagreement persisted. Fowler (1918) described the first subgenus of Chromis, Hoplochromis. In addition, Fowler and Bean (1928) went a step further in naming two additional subgenera, Lepidochromis and Dorychromis. Most authors have largely ignored these three subgenera, including Norman (1957). Fowler was not finished, however, and in 1941 he described the genera Pycnochromis and Thrissochromis followed by Serrichromis and Lepicephalochromis (1943) and Siphonochromis (1946). None of these genera are recognized today; all have been regarded as synonyms of Chromis, but further research of molecular phylogenetic evidence may yield the need for recognized subgenera, which in that case it is likely that some or all of Fowler's five described genera would be used at the subgenus level (Randall and Swerdloff, 1973).

Chromis is a rather large genus, the largest of Pomacentridae, with 86 described species (see below). They are distinguished from other Damsels in a number of ways, most notably by having a forked caudal fin, small conical teeth with an enlarged front row, and a fully scaled head except for around the mouth and nostrils. In addition, nearly all the species have two or three projecting spiniform procurrent caudal rays (Randall and Swerdloff, 1973).

Distribution of Chromis is nearly worldwide in tropical or subtropical seas. From Hawaii to the Red Sea, the Philippines to the Florida Keys, if there is a warm water reef, chances are very good that Chromis can be found nearby. In fact, chances are good that Chromis will also be the most numerous fish present on a given reef. Chromis multilineata and C. cyanea are referred to as the Caribbean's "most abundant reef fish" (DeLoach, 1999) and C. viridis as "one of the most abundant reef fish" (Randall, 1985). The depths at which they can be found are highly variable. Most species are found between the 40 to 100 feet range, but some species extend well out of this "normal" range. Species can be located on coral reef heads in only 10 - 15 feet of water (C. vanderbiliti), while others are plentiful below 400 feet (C. analis). If I had to sum up this paragraph in a single sentence, it would be, "Chromis species are extremely numerous most anywhere a coral reef occurs."

A very attractive Atlantic species of Chromis is C. cyanea, also
called the Blue Chromis. This photo was taken of an aquarium
resident. Photo courtesy of Greg Rothschild.

Most Chromis are located along steep reef slopes and drop-offs with strong water currents where they swim several feet above the coral heads, generally facing the current, and feeding upon zooplankton. The brisk water movement provides an endless supply of their favorite food, zooplankton. The term "zooplankton" can mean a lot of things, but for Chromis, this is generally defined as copepods (up to 60% of the diet), malacostracan larvae (up to 10%), pelagic tunicates (up to 17%), larval polychaetes (2%) and fish eggs (less than 1%) (Randall and Swerdloff, 1973; 1989).

Chromis agilis is located in Hawaiian waters often associated with C. hanui, though C. agilis can also
be located throughout much of the Indo-Pacific where it prefers the leeward sides of islands.
Photos courtesy of Keoki Stender.

When predators threaten, the Chromis will disappear in a flash into the coral heads, remaining tucked tightly into the crevices until the perceived danger is no longer present. Other than their schooling behavior and its benefit of "safety in numbers," this is their only means of defense. Although most members will also spend the evening buried into these same coral heads, some species (C. ovalis) will simply lie upon the substrate and adapt a coloration that blends with the sand, most often a mottled gray.

The overall numbers can be confusing to some hobbyists as well. Although "schooling" by large numbers of Chromis is something that can occur regularly, mostly by juvenile fish, this should not be considered commonplace for all adult Chromis. Many, but not all, species prefer to maintain a nest occupied by only themselves or with a small harem of females. Due to the small territories they claim and with their passive nature, unlike most damsels, feeding is done in the close vicinity of their cohorts. Aggregations of these harems, coupled with their profuse abundance on the reef, and their overlapping territories, can easily be confused as a single large school of Chromis. If watched closely, however, members will remain close to the same location, usually within a radius of several meters of their nesting area, and associate primarily with the same members of their harem, even as they forage for food in the water column. When individuals do stray far from home, however, it is always done under the cover of a large school for the purpose of feeding. Schools are a safer option than going at it alone when traveling great distances, and schools can easily overrun single or paired specimens when competing for new feeding grounds. For the species that typically school for the majority of their existence, such as C. atripectoralis and C. viridis, neither nests nor territories are maintained. The only exception to this rule is when spawning occurs. Males of both the schooling species and non-schooling species have similar spawning tendencies (more on this below).

Chromis hanui, another endemic of Hawaiian waters, is perhaps the most secretive of Chromis,
rarely venturing further than several feet away from protective crevices.
Photos courtesy of Keoki Stender.

Coloration in juveniles can vary sharply from that of adults of the same species. For example, Chromis ovalis has three described colorations which originally were the subject of confusion for ichthyologists attempting to classify this species. Chromis ovalis outgrows its juvenile coloration when reaching a size of between 30mm and 60mm, and then sheds its second coloration for its final adult coloration as it reaches sexual maturity around 95mm of total length (see photos below). For comparison, 120mm is considered full grown for this species, although specimens reaching 153mm have been reported.

Sexual dichromatism is present for many species during courtship and spawning only. For species such as Chromis cyanea this results in males darkening their back and sides to a much greater extent than normally present, while the females are noted to turn nearly entirely black. Sexual dimorphism is also present, but not obvious, as the males are only slightly larger than females.

Spawning behavior usually follows one of two patterns. The most common pattern is a single male spawning with a single female. In this instance, males will prepare the nest site by fanning away sand, detritus, and other debris from algae brush, gorgonians, or rock crevices. Unlike most fish, which use their caudal fin to fan away debris, Chromis utilize a fast beating of their pectoral fins to accomplish this feat. The male will attempt to mate with each female of its harem, usually displaying their mating dance to the entire harem. They employ a rather simplistic mating ritual, one where the male makes short, vertical bursts of about three feet, repeating frequently. This is always followed by nipping at the fins of the females until the females are coaxed into the nest of the male. Although the male may display to several females at once, only one at a time will enter and lay eggs. In most cases, the male waits until the female is finished attaching her eggs to the chosen algae, gorgonian, or rock crevice before he follows behind and fertilizes the eggs.

The highly sought after C. vanderbiliti is seen here. Despite the wonderful photograph,
Vanderbilt's Chromis is significantly more impressive in person.
Photos courtesy of Keoki Stender.

The second type of spawning event is less frequently seen, occurring only once per lunar cycle and in areas with high densities of Chromis. For two or three days preceding the event, large adult males will begin nest-building activities on an open sand bottom. On the morning of day three, the adult females have joined the males and with a few traditional spawning jumps, a five or six hour spawning event occurs. This event can include several thousand individuals.

The males guard the fertilized eggs, and this is the lone time that Chromis can become aggressive. Males will often confront potential egg thieves several meters above the nest in an attempt to ward off the intruder. When several intruders approach at once, males will remain defiant and motionless in the typical head down position just above or at the entrance of their nest. As is expected, males will also fan the demersal eggs to remove settling debris and provide aeration, as well as remove unfertilized or fungal infected eggs. Within three days, the eggs hatch and become planktonic larvae for three or four weeks before the newly settled juveniles seek out security in the crevices of the rockwork.

Provided a few basic needs are met, Chromis can be kept in the home aquarium rather successfully. Unlike their cousins, the damsels, who generally dictate the need for aggressive tankmates, Chromis are the exact opposite. Tankmates that would be required for other Pomacentridae would undoubtedly pester or harass Chromis. In fact, these damsels will do best in an aquarium where they are the dominant member(s) with a few other less aggressive species in the same tank. They will flourish if large fish, such as adult angels or surgeonfish, are not present. The presence of the larger fish will not necessarily doom the Chromis, but it will make them work harder to obtain enough food, and will likely discourage natural shoaling with the Chromis instead opting to remain closer to the rockwork. When choosing suitable tankmates, predatory fish (see below) and predatory invertebrates, such as Ophiarachna species, should be avoided as Chromis can make an easy target. Chromis will not harm corals or mobile invertebrates.

Compatibility chart for Chromis:

Fish	Will Co-Exist	May Co-Exist	Will Not Co-Exist	Notes
Angels, Dwarf	X			Good choice.
Angels, Large		X		Some large angels are too aggressive to share an aquarium with Chromis.
Anthias			X	Food competitors. Both share open water column.
Assessors	X			Good choice.
Basses	X			Good choice.
Batfish	X			Good choice.
Blennies	X			Good choice.
Boxfishes	X			Good choice.
Butterflies	X			Good choice.
Cardinals	X			Good choice.
Catfish		X		Adults may consume Chromis.
Comet	X			Good choice.
Cowfish	X			Good choice.
Damsels		X		Some damsels are too aggressive to share an aquarium with Chromis.
Dottybacks		X		Some dottybacks are too aggressive to share an aquarium with Chromis.
Dragonets	X			Good choice.
Drums	X			Good choice.
Eels			X	Adults may consume Chromis.
Filefish	X			Good choice.
Frogfish			X	Adults may consume Chromis.
Goatfish	X			Good choice.
Gobies	X			Good choice.
Grammas	X			Good choice.
Groupers			X	Adults may consume Chromis.
Hamlets			X	Natural predator of juvenile Chromis.
Hawkfish	X			Good choice.
Jawfish	X			Good choice.
Lionfish			X	Adults may consume Chromis.
Parrotfish		X		Most Parrotfish will become too large for Chromis to feel comfortable.
Pineapple Fish	X			Good choice.
Pipefish			X	Pipefish require an aquarium unto themselves.
Puffers		X		Some Puffers are too aggressive to share an aquarium with Chromis.
Rabbitfish		X		Most should mix well, but some Rabbitfish can become aggressive over time.
Sand Perches	X			Good choice.
Scorpionfish			X	Adults may consume Chromis.
Seahorses			X	Seahorses require an aquarium unto themselves.
Snappers			X	Adults may consume Chromis.
Soapfishes			X	Adults may consume Chromis.
Soldierfish			X	Adults may consume Chromis.
Spinecheeks	X			Good choice.
Squirrelfish			X	Adults may consume Chromis.
Surgeonfish		X		Some Surgeonfish may become too aggressive for Chromis.
Sweetlips			X	Adults may consume Chromis.
Tilefish	X			Good choice.
Toadfish			X	Adults may consume Chromis.
Triggerfish		X		Most Triggerfish are too aggressive to share an aquarium with Chromis.
Waspfish			X	Adults may consume Chromis.
Wrasses		X		Many wrasses will coexist very well, but some become very aggressive as well.

Note: While many of the fish listed are good tank mates for Chromis species, you should research each fish individually before adding it to your aquarium. Some of the fish mentioned are better left in the ocean, or for advanced aquarists.

Food is another important consideration for Chromis. They are active fish that are constantly feeding in the wild. Their natural instinct to consume food from the water column usually translates into a fish that is eager to accept prepared foods. Aquarists can expect their newly arrived Chromis to consume prepared foods in only a day, or at the most three days, but this will most likely occur within the same day they are added. Any prepared foods geared towards a carnivorous marine fish will be a suitable substitute for their natural diet. The type of food to feed is easy; the quantity to feed is the more challenging question to answer. A lot of the answer depends on the particular aquarium in which the fish are present. Small aquariums or aquariums that produce small amounts of planktonic life will require more frequent feedings than larger aquariums or aquariums that produce copious amounts of planktonic life. A safe median to shoot for would be three feedings per day unless the aquarium is supplying a portion of their diet.

This column would not be complete without including a paragraph regarding the often-debated topic of aquarium size. When a single individual is kept they can usually do well in aquariums as small as 40 gallons, assuming it shares the aquarium with non-threatening fish. Consider increasing the size of the aquarium if the plan is for the Chromis to share the aquarium with fish larger than itself. For the schooling species, a small shoal of six to eight individuals can do well in the traditional four feet long, 75 gallon aquarium, provided enough food is available to the fish. Also consider that a shoal of this size should be the focal point of the aquarium and the remaining residents should be a less dominant species. Open water swimming fish, like surgeonfish and large angels, should be avoided in aquariums as small as the aforementioned 75 gallon aquariums that house a shoal of Chromis.

With 86 species to the genus' credit, I surely cannot discuss each individual. Thankfully, only a handful of species actually make it into the American market for retail sale. Far and away the most popular species sold in the trade is Chromis viridis, or the Blue/Green Chromis. Actually, it would not surprise me if some readers of this column just realized for the first time that there are more Chromis than just the Blue/Green Chromis. Many retail outlets rarely sell any other species. Confusion existed for quite some time regarding the name of this species. In addition to C. viridis, names such as Heliases cauruleus, Chromis ternatensis and C. caerulea were all used. Once the confusion was sorted, the original name was determined to be derived from a hand painted, color portrait by Ehrenberg in Cuvier (1830) (Randall, 1985). Chromis viridis, possibly the most abundant reef fish, spends the afternoon in large shoals above Acropora sp. in areas stretching from the Red Sea and coast of East Africa and across the entire Indo-Pacific range.

A fish often confused as C. viridis is C. atripectoralis, or the Blackaxil Chromis. Although they have been measured slightly larger than C. viridis (4 ½" compared to C. viridis at 3 ½"), they are nearly identical except for one determining factor. Chromis atripectoralis has a black spot at the base of each pectoral fin. This trait, of course, led to the Latin name it was awarded.

A popular Caribbean import for the hobby is Chromis insolata, or the Olive Chromis. Juveniles are magnificently colored, but as they age into adults, they loose their remarkable coloration for a drab olive green color. They are among the largest Chromis, reaching just over five inches of total length.

The juvenile coloration of Chromis insolata. Adults loose much of their striking
colors and become entirely one color; the same shade as the underside
of the fish seen here. Photo by Henry C. Schultz III.

One of the more sought after Chromis is C. vanderbiliti, or more commonly referred to as Vanderbilt's Chromis. It is rarely imported, although it does have a natural home range covering most of the Central and Western Pacific, including Hawaii. This species remains small, roughly two inches, and can be less outgoing than other Chromis. Because of this, it is best to keep this Chromis in small aggregations in very peaceful aquariums. It prefers shallow water, often less than 20 feet, where it feeds in mid water above large coral heads. Chromis nigrurus is closely related, differing only by having a yellow caudal fin edged with black, and the anal and dorsal fin also edged with black.

Chromis ovalis, having the clever common name of Oval Chromis, is also sometimes referred to as C. velox. However, 23 publications have referred to this species as C. ovalis compared to the four known publications using C. velox, the last some 43 years ago (Randall and Follett, 1989). This species is endemic to the Hawaiian Islands, where it is found in small aggregations. Spawning season is from February through May.

The Oval Chromis is seen here in all three color phases. The juvenile is seen on the top left,
which progresses to the coloration found in the sub-adult (top right), and finally
progresses to an adult seen on the bottom right.
Photos courtesy of Keoki Stender.

Many times over hobbyists receive Damselfish as their first fish to place into a newly setup aquarium. Oftentimes this is the first step backwards that a hobbyist takes, as the belligerent personality of the damsel makes it nearly impossible to add more fish in the future. Furthermore, it's likely that the next step is this same damsel becoming the first fish that the new hobbyist must try to capture - a near impossible feat for even the most seasoned veteran. Chromis, on the other hand, fit this 'first fish' niche quite nicely. They are nearly as bulletproof as their cousins, but arrive without the pugnacious attitude that is so common in other damsels. They are equally as difficult to remove, but as is most often the case, there is no need to remove the Chromis. Supplying the correct type and amounts of food, in conjunction with housing it with equally passive tankmates, will help ensure their health for years to follow.

I would like to extend a special "thank you" to Kevin Kocot for assisting in the collection of the photographs for this column, as well as to all the photographers who granted their permission of photo requests.