In oceanic zoology, a creature that lives on the ‘benthos’, such as an anglerfish or a stingray, generally feeds, breeds and completes its entire life history on, or extremely close to the bottom of the sea. In contrast, pelagic animals, like tuna, dolphins or anchovies, never go near the ocean floor, spending their entire lives in the open ocean.

These concepts of ‘benthic’ and ‘pelagic’ are also applied to rivers and freshwater species. In freshwater systems, there are species that have similarly evolved to occupy either the top or the bottom of the water column, and even though a river like the Lachlan contains only a tiny fraction of the diversity of species that live in the sea, it’s no surprise that these ecological niches have been filled by native Australian species over time.

Gudgeons – both the small varieties and the larger Flathead gudgeon – are clearly substrate-dwellers that live in the benthic zone. Like anglerfish, they prefer to sit and wait for their prey to pass by. As a consequence, these fish are not known for their swimming prowess. They drift and hover, then dart a little bit, but sustained swimming isn’t their thing. The are also fairly well camouflaged, usually a mottled combination of browns and greys, so that they can sit well hidden on the river floor.

To be pelagic – to live in the open water – requires a totally different set of attributes, and these are very similar regardless of whether the species lives in the ocean or an inland river.

• First, a slender, compressed body shape is important, because this means the animal can swim consistently without expending a massive amount of energy.
• Colour is also important; the golden rule being dark on top and white underneath. This allows pelagic species to be difficult to see from above (when potential aerial predators are looking into a dark ocean or river), but just as difficult to see from below (when potential aquatic predators are looking up into a lighter-coloured sky). Choose any pelagic animal you like – marlin, sharks, dolphins, sardines – and think about it for two seconds: they all exhibit this colouration.

• The third, and possibly the most important attribute a pelagic species requires, is the ability to school: there’s safety in numbers, and in the open water there’s nowhere to hide from predators. Most of us – at some stage – have been amazed when David Attenborough documentaries show underwater footage of massive schools of pilchards darting and weaving as if they are one connected organism, to foil the attacks of tuna or sharks or seals. A few get eaten, but the vast majority survive, escape and continue living in their giant, fast-moving commune. The same thing happens in our rivers.

The following video shows a school of pelagic Australian smelt found in the Cann River in Victoria.

Un-specked hardyhead (Craterocephalus stercusmuscarum fulvus) is a pelagic species of the Murray-Darling Basin, found commonly in the Lachlan River. As expected, they are:

• silvery, but darker on top and lighter underneath
• bullet-shaped
• live in schools
• occupy the mid and surface water, feeding on zooplankton and insect larvae.

Why are they called “un-specked” hardyheads?

The reason for their slightly unusual common name (and triple-barrelled Latin name) is because there are hardyheads all over Australia, and mostly all from the same genus (ie: their first name is Craterocephalus). The fly-specked hardyhead (Craterocephalus stercusmuscarum) was described fairly early (1867) and was named due to the broken, longitudinal, parallel lines along its flanks. The species has a range that includes coastal drainages all the way up to North Queensland and the Gulf. Later on, the fish experts worked out that the Murray-Darling hardyheads were similar but didn’t have the ‘specks’, so ‘un-specked’ was coined and ‘fulvus’ was added to the Latin name.

The second pelagic species common to the Lachlan and other Murray-Darling rivers is the Australian smelt, Retropinna semoni. Smelt possess the same attributes as the hardyhead: bullet-shaped, slender, silver, schooling. Indeed, the main differences between smelt and hardyhead are that the former is a little more elongated, and they breed in winter rather than spring.

Both hardyhead and smelt are large at 10 centimetres long, and seem to be reasonably common in the mid-Lachlan, despite the imposition of dams and weirs and the threat of introduced species. It’s a sure bet that the big schools of smelt and hardyhead play an important role in the Food webs of the lakes, wetlands and channels of the Lachlan, given the diversity of fish-eating birds and the presence of aquatic predators like bigger fish, turtles and rakali.

The pelagic fishes of the Lachlan have adapted over time to survive by having a colour pattern that protects them from predators above and below, having a body designed to swim fast over distance and sticking together in schools. These iconic native fish are being monitored by the Lachlan Flow-MER team to ensure their habitat is healthy and able to continue supporting native fish populations in the future (probably not for dolphins or sharks anytime soon though).

Featured Photo: The Lachlan River. Photo credit: Finterest.