211. A knot of Striped Catfish (Plotosus lineatus)

Subject:

A ball of fish rolling over the bottom of the ocean in surprising synchrony… these are juvenile Striped Eel Catfish (Plotosus lineatus).

A school of Striped Eel Catfish is sometimes called a ball or a knot. These knots form after the eggs of a catfish hatch; the little juveniles then stick together in a dense fish ball. They recognise each other by way of smell and react to chemical cues to move as one. When they grow older, they will become solitary or live in groups of about a maximum of 20 individuals or so. However, juveniles are, luckily for us scuba divers, day-active.

When fish form a dense and socially bound aggregation, they are shoaling. When a shoal moves uniformly in speed and direction, they are schooling. A shoal of fish can suddenly become a school, and a school can suddenly lose its synchronisation and return to the status of a shoal. Normally, all fish in a particular school are of the same species, and thus of the same size, shape, and colour. Predators perceive such a big school as a swirling and flashing mass, making it visually very difficult to single out just one fish, let alone grab one to eat. By doing so, fish apply what is called "safety in numbers”. This means that statistically, the bigger the shoal, the lesser chance each individual has of falling victim to a hungry predator.

The speed and synchronisation of such a school astonish scuba divers and scientists alike. It is poorly understood how these animals manage to move as one. Most fish that school have peculiar body marks, as in the case of these Striped Eel Catfish, where the striped catfish's stripes are most likely visual aids or reference marks. Recent research shows that schooling is most likely embedded in the species’ DNA. This instinct-like behaviour is controlled by the same region in the fish’s brain that also controls the animal’s lateral line, the sensory organ on both sides of the fish that is used to maintain the right distance from each other.

Although these balls or knots look playful, they are extremely dangerous to touch. These Striped Eel Catfish are equipped with three highly venomous spines. One of these hypodermic needle-like spines is situated in their first dorsal fin, and the two others are in each of their pectoral fins. These seemingly innocent-looking fish have the ability to deliver a deadly sting to an adult human.

Technique:

Besides the usual tricks like sharpening the subject slightly, removing as much backscatter as possible, adding a subtle vignette, correcting the colour and increasing the contrast, I changed the speed of the music slightly.

My apologies to the composer of this nice little tune, but by speeding up the music by just five percent, the melody came in almost perfect synchronisation with the sometimes frantic movement of this little school. I could have chosen to slow down the speed of the footage to match the tune’s tempo, but this would have created complications with the frame rate of the video, resulting in a stuttering image. The editing software would have dropped some frames in an attempt to match the number of frames per second of the recording. By changing the speed of the music, the footage can be shown at its original recording frame rate. You will find a detailed description about frame rates in our online Marine Wildlife Videography Course.

Filming location:

This short underwater videoclip has been filmed in Mauritius 🇲🇺

211. A knot of Striped Catfish (Plotosus lineatus)

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