While swimming across the reef looking for Zanzibar Butterflyfish (Chaetodon zanzibarensis) to photograph ( they are yellow), in the distance some 15 meters away I saw a yellow fish, or so I thought. As I swam closer the fish disintegrated into eight yellow phase Gold Saddle Goatfish (Parupeneus cyclostomus) who started feeding together on the bottom , also in a fairly tight group.
The whole lot then swam off together , with one slightly slower off the mark than the others.
It soon caught up and the group swam off in tight formation looking for another feeding spot.
It common to see many different species of goatfish schooling together, often with other species in fairly loose formation. These groups of fish tend not to compete with each other for food and each species has a separate prey. Gold saddle goatfish prey on small fishes whereas the other fish in these groups will be after crustaceans or other food. Often they will feed with Longbarbel Goatfish and juvenile Parrotfish who do not compete with them for food and who may camouflage them to some extent, providing protection in numbers.
It is also known that Gold Saddle Goatfish collaborate when hunting from research conducted in the Red Sea. The fish were found to form loose groups and when one member of the group started chasing a fish the others assisted by heading off the prey. As the article points out this is similar behavior to that exhibited by lions when hunting.
However these groups are not as tight as those comprised solely of the juvenile yellow phase specimens, which given the fish are all travelling in the same direction is more appropriately described as a school. I have observed this behavior on several occasions and the small schools are always very tightly knit with the fishes touching each other while swimming.
These schools have so far always been of up to eleven yellow phase juveniles approximately 8 to 10 cm in length. Never I have I seen a normal colored specimen in the group and neither have I seen the other colour phase swimming in these tightly knit schools. Interestingly these shoals do not intermingle with other less tightly knit larger shoals of Goatfish. They may swim past them or through them but they do not feed on the bottom together.
From a distance they give one the impression of a larger yellow fish swimming across the reef. One can easily theorize that this behavior provides protection against smaller predators. However it also begs the question as to whether the fish have a sense of their own coloring.
It is quite common to see a single yellow phase adult Goldsaddle Goatfish swimming on the reef with a Yellow phase Cigar Wrasse following closely along as seen below. The two stick together and feed across the reef. I have never seen either of these species following each other in the other colour phases. Again this begs the question as to how aware the fish are of their own colouration. Why do only the yellow specimens of each species exhibit this behavior. Possibly with sufficient research on the subject we may one day know the answer to these questions. But at this stage we really do not know.
One day while observing a group of juvenile yellow phase Goldsaddle Goatfish swimming across the reef in a tightly knit shoal, I noticed a Cigar wrasse with the group. It was still in the initial phase and not yet either yellow or brown, but possibly going to be a yellow phase specimen from the yellow spots on the upper body . This of course begs the question as to whether he knows he is going to be yellow one day. Again another question that we do not yet know the answer to !
I have asked other fellow divers if they have seen similar behavior and one friend Glen Wyness has seen this behavior in Mauritius. The yellow phase specimens are not that common in Tanzania and it is a fairly rare sight to see them swimming together.
Sources
https://www.smithsonianmag.com/science-nature/the-fish-that-hunt-like-lions-479311/