Sharks have hogged the spotlight this summer. The top-order predators have muscled in and made headlines by doing little more than swimming past a beach.
But what about the smaller creatures doing things you wouldn't believe were even possible. Fish that can go from being female to male or sea cucumbers that can reorganise the position of their vital organs before splitting in two. They all live in Australian waters: in our bays, harbours and deep oceans.
More Science Videos
Australia's seven strangest sea creatures
There are some weird and wonderful things going on beneath our salty waters.
Take for example the tiny male Argonaut. The size of a five-cent piece, this open-ocean loving octopus will literally give his third left arm for a not-so-romantic encounter with the larger female. The mating ritual involves him brushing past her but as he does, his arm breaks clean off.
Accounting for half his bodyweight the oversized arm contains his sperm and after being detached it slithers, caterpillar-like, into the female's mantle cavity.
![Dr Julian Finn explores the water at Williamstown for the weirdest marine creatures.](/web/20170128135352im_/http://www.smh.com.au/content/dam/images/g/t/z/u/u/l/image.related.articleLeadwide.620x349.gtpdf8.png/1485585096991.jpg)
The male meanwhile struggles along as a seven-armed octopus – but not for long. His once-in-a-lifetime chance at sex finishes him off shortly afterwards.
![The deep-sea loving Argonaut octopus.](http://web.archive.org./web/20170128135352im_/http://www.smh.com.au/content/dam/images/g/t/q/c/a/i/image.imgtype.articleLeadwide.620x349.png/1484200724939.png)
The deep-sea loving Argonaut octopus. Photo: Julian Finn Museums Victoria
Museums Victoria senior curator of marine invertebrates Julian Finn has studied hundreds of Argonauts during his research.
What little is known about their mating is learnt from the football-sized females that are found washed up on Australian beaches. Dr Finn said while the males mate only once in their brief lifetimes, one popular female had been found with more than 30 male arms inside her.
"It is really strange," he said. "It's an unusual way to reproduce but it's clearly a strategy that works for these animals."
The Argonaut is not alone when it comes to mind-boggling behaviours occurring beneath the waters. Here are some of the strangest:
The Gobbleguts fish: The female lays her eggs and the male covers them in sperm. He then gobbles them up in his mouth where they stay for two or three weeks. Unable to eat, he looks generally grumpy as he waits for his offspring to develop. "It's the ultimate daddy day care," Dr Finn says.
![A hungry male gobbleguts fish incubating his eggs.](http://web.archive.org./web/20170128135352im_/http://www.smh.com.au/content/dam/images/g/t/q/a/a/s/image.imgtype.articleLeadwide.620x349.png/1484198737182.png)
A dutiful dad incubating his eggs. Photo: Julian Finn Museums Victoria
Pygmy squid: The pygmy squid eats shrimp from the inside out, using a mouth that extends on a stalk. The stalk penetrates the shrimp's body, allowing the pygmy squid to feed through a beak-like macerator. All that is left behind at the end of the meal is a hollow shell.
![A southern pygmy squid in Victorian waters.](http://web.archive.org./web/20170128135352im_/http://www.smh.com.au/content/dam/images/g/t/q/b/1/1/image.imgtype.articleLeadwide.620x349.png/1484199621100.png)
A southern pygmy squid in Victorian waters. Photo: Julian Finn Museums Victoria
Bluethroat wrasse fish: All bluethroat wrasse start their lives as females. The one that grows to be the largest or the most aggressive in the group becomes a male. When he goes, the next biggest female steps up and becomes a male.
![A bluethroat wrasse, photographed at Popes Eye in Port Phillip Bay.](http://web.archive.org./web/20170128135352im_/http://www.smh.com.au/content/dam/images/g/t/q/b/j/m/image.imgtype.articleLeadwide.620x349.png/1484199953532.png)
A bluethroat wrasse, photographed at Pope's Eye in Port Phillip Bay. Photo: Julian Finn Museums Victoria
![A southern bobtail squid is a type of dumpling squid. This one was photographed at Jawbone Marine Sanctuary, off Williamstown.](http://web.archive.org./web/20170128135352im_/http://www.smh.com.au/content/dam/images/g/t/q/b/r/y/image.imgtype.articleLeadwide.620x349.png/1484200036514.png)
A southern bobtail squid is a type of dumpling squid. This one was photographed at Jawbone Marine Sanctuary, off Williamstown. Photo: Julian Finn Museums Victoria
Sea cucumbers: Some sea cucumbers can make clones of themselves. But before they multiply, they reorganise their vital organs by moving those they need to survive to either end of their body. Once they have split themselves in half, each half re-grows the parts needed to live separate lives.
![A sea cucumber photographed off Mornington, Victoria.](http://web.archive.org./web/20170128135352im_/http://www.smh.com.au/content/dam/images/g/t/q/c/0/g/image.imgtype.articleLeadwide.620x349.png/1484200268703.png)
A sea cucumber photographed off Mornington, Victoria. Photo: Julian Finn Museums Victoria
Barnacles: The humble barnacle lives a solitary life stuck on a rock. The only way it can mate is with the world's longest penis, which is 30-times its body length. It sits there and literally gropes around in the dark in the hopes of getting lucky.
![Barnacles near Flinders on Western Port Bay in Victoria.](http://web.archive.org./web/20170128135352im_/http://www.smh.com.au/content/dam/images/g/t/q/c/8/d/image.imgtype.articleLeadwide.620x349.png/1484200544148.png)
Barnacles near Flinders on Western Port Bay in Victoria. Photo: Julian Finn Museums Victoria