Bar Jacks: Bright, Bold, and Beautiful

A barrage of bar jack buddies

Bars of Quicksilver63

Red jack, black jack, crevalle, greenback, neverbite, point-nose, runner and skip-jack are only a few of the many names for Caranx Ruber. Bar jack, however, is probably the most well known and the name I shall use for this post. The bar jack is that constantly moving silver fish with a blue line on its back that you often see passing close by during a dive. They are the ocean’s social butterflies, flitting across the coral reefs and greeting everything around them. So let’s delve into these friendly critters and see what they have to say for themselves!

All That Glitters

This shiny guy is certainly striking in appearance. Boasting the elegant shape of the classical fish symbol, the bar jack casts a silvery gleam in the light. Its blue bar streaks from its head over the top of the body before descending onto its lower tail fin. The slim angled shape of its body, aided by powerful muscles, allow for rapid acceleration and a fast swimming speed which is typified in their darting behavior.

(photo from djscho on inaturalist.org)

The Jack or the Egg?

Love is in the air for these silvery starlets! Bar jacks typically spawn between February and August. They can spawn twice a year, releasing 67 000 - 23 000 eggs per spawn! The eggs will then float to the ocean surface before hatching. As juveniles, the baby bar jacks will hang around shallow waters or hide underneath sargassum mats for protection until they grow to maturity - 25cm (10 inches) for males, 31cm (12 inches) for females.

Dinner Time 

Being primarily predatory creatures, the bar jack uses its speed and agility to hunt down small fish, shrimps, and other invertebrates. Their hunting habits rely strongly on their eyesight, which sees them to have a strictly diurnal lifestyle. But they are not always proud hunters! Especially early on in their life, they are not above being bottom feeders. Bar jacks also tend to change colour to a darker brown when they are bottom feeding, a stark departure from their usual glamour. This darker colour can also be seen when they attach themselves to a stingray to poach whatever the ray churns up from the sand.

Social Butterflies

Just a bar jack and his sting ray pal (photo from djscho on inaturalist.org)

Ray-liable friends

One can never have enough friends! Or so the bar jack seems to think. As mentioned previously, bar jacks can often be seen hovering above sting rays to take advantage of their habit of stirring up sand. Its a sneaky tactic that gives the fish easy access to a constant food supply - think of it as an endless bar jack buffet! The stingrays don’t seem too bothered by this, but as far as we can tell they don’t really benefit from it either.

Barracuda and sharks

Jacks also seem to enjoy gathering around sharks and barracudas, possibly for protection but once again without the larger fish necessarily getting anything in return. In fact, I have personally witnessed multiple times where barracudas get thoroughly annoyed by a juvenile bar jack that has taken a shine to them. They will twist and snap at their unwanted hanger-on, only for the joyful juvenile to showcase its dazzling speed and agility to just stay out of reach of the larger fish’s fangs. After showing off, the ‘chased’ jack will inevitably settle alongside the barracuda once again, much to the larger fish’s consternation. 

Puddingwife Wrasse

Perhaps the most meaningful cross-species relationship a bar jack can form, however, is one with a puddingwife wrasse (Halichoeres radiatus). Jacks will often form a partnership with a puddingwife wrasse to create an epic foraging team. This partnership has shown to result in greater success for both fish during scavenging. Interestingly, despite the two species sharing some similarities in diets, neither partner will steal from the other. Jacks seem to be loyal to their partners, and will even defend their puddingwife wrasse from other bar jacks!

Divers

Bar jacks are not above trying to make friends with humans too, and divers seem much more receptive to their advances than the less friendly barracuda. Whether they are attracted to our bubbles or we are just another large fish for them to garner protection from, bar jacks will appear quite curious and friendly to divers. If you’re particularly lucky, you can even have a shoal form around you during a safety stop!

To sum it up

Divers often only seek out hidden delights or rare sightings. However, bar jacks are one of the more common fish that we find along the Cayman reefs and you will rarely go on a dive without being able to see at least one. But before you dismiss them, the joyful energy and splendour that they bring to our reefs ensures that sometimes it is worth just stopping on a dive and just appreciating our quicksilver friends for who they are.