Chromis viridis is an evergreen staple of the reef aquarium paradigm. Its ubiquity in the field is reflected almost equally in the trade, and the species plays host to a veritable plethora of hobbyists both novice and pro. The specific epithet “viridis” is of latin origin and means “green”. This name is aptly given to the Blue-green Chromis, whose coloration is reminiscent of springtime freshness.
Interestingly, this fish is a complex of two species with highly overlapping distributions. They are Chromis viridis and Chromis atripectoralis. Both species are found in the same habitat of dense Acropora in shallow water. While both species occasionally intermingle, larger adults tend to stick with their own kind.
Both species were collected in a Coral Sea trip this week at Cairns Marine, and despite their immense ubiquity, it’s interesting to know that C. atripectoralis remains incognito under the guise of C. viridis. While both species aren’t much to shout about, how many hobbyists actually know their Chromis?
C. viridis is found throughout the Indo-Pacific, from the east coast of Africa to the Line Islands and the Tuamotu Archipelago; north to Ryukyu Islands, south to the Great Barrier Reef and New Caledonia.
The type specimen of C. atripectoralis was described from Bikini Atoll in the Marshall Islands. Paratypes were subsequently collected in Fiji, Marianas and the Philippines. This species is mostly Oceanic, and is found throughout Oceania except Hawaii, Marquesas and the Pitcairn group. It is found in Australia and north to Japan. It remains absent in most of the Indian Ocean, except for sparse records in Western Australia, Thailand and Seychelles.
Based on the distribution records of both species, it appears that C. atripectoralis is completely sympatric within the range of C. viridis, and does not venture out of the latter’s boundaries. Sympatric speciation is uncommonly seen in coral reef fish, but this appears to be a rather good example. Although, it cannot be ruled out that these species were perhaps allopatric, later merging in distribution due to range expansions.
The behavioural differences in both species also appear to differ slightly. Chromis viridis prefers sticking closer to coral heads and bommies, while C. atripectoralis favour slightly more open environments. These are anecdotal, of course, and it’s probably best not to rely upon them too heavily.
Chromis atripectoralis can be easily separated from C. viridis by the presence of a large black spot on its pectoral fin axil. In C. viridis, this region is rarely marked, and even if, only the dorsal part (fin side) of the axil may be spotted very sparsely in black, fading to almost none on the ventral part (body side). In C. atripectoralis, this axil is prominently marked in numerous black dots which converge to form a large, distinct spot. A metallic turquoise ring usually surrounds this. The two can also be separated by a difference in pectoral ray count; 17 or 18 in C. atripectoralis vs 15 or 16 in C. viridis.
The viridis vs. atripectoralis confusion is just the tip of the iceberg though. In examining regional populations of both species, we see an inconsistent display of courtship colors by the males. Whether or not these are just geographical variants, or whether or not these are cryptic species hidden by a homogenous phenotype is unknown.; but it’s perhaps better to leave this Chromis can of worms unopened for now.
The humble and pedestrian Green Chromis may be fun to laugh at, but there’s more to it than meets the eye. Goes to show that you can still find joy in the life’s simplest pleasures. I think?