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Blue Planet II

The ocean’s most dedicated mothers

By Ella Davies

From octopus to orca, our oceans are home to some of the hardest working mothers around.

At some point, all new mothers wish they had an extra pair of hands to help juggle feeding, nappies, toys and tears. Out in the ocean eight-limbed animals are among nature’s most dedicated mums – but it’s less about multi-tasking and more a case of single-mindedness.

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Longest brooding period ever recorded

The longest known brooding period of any animal was reported in 2014 when scientists revealed details about a deep sea octopus (Graneledone boreopacifica) that tended her eggs for a record-breaking 53 months. According to researchers, such a long-term investment was due to the low temperatures the octopus inhabits. In species that live in shallower, warmer waters, octo-moms look after their fertilised eggs until they hatch between 1-3 months later. But in the colder depths, metabolic processes are dramatically slowed prompting G. boreopacifica to care for her eggs for much longer.

The brooding period is brutal for an octopus mother. While caring for her eggs, protecting them from predators and making sure they get enough oxygen, she does not feed. She will eventually die of starvation. The researchers studying the deep sea octopus saw her condition deteriorate over the course of their submarine visits: her colour faded and eyes became cloudy. There’s no doubt she was one tough mother, her 4.5 year brooding period is an unmatched endurance feat. The reason she underwent this is to give her hatchlings the best start in life. They are the largest and most developed of any octopus, which gives them a significant advantage in the survival stakes.

The spiny dogfish's long gestation period has put the species at risk (Credit: Boris Pamikov/Shutterstock)
The spiny dogfish's long gestation period has put the species at risk (Credit: Boris Pamikov/Shutterstock)

Octopods aren’t the only ocean species to invest a lot of time in their embryonic young. Among the longest pregnancies on Earth is that of the spiny dogfish (Squalus acanthias) – a common species of shark found in temperate waters around the world. Gestation can last up to 2 years as the young develop from eggs to pups, with half a dozen in the average litter.

Sadly, this unhurried approach to reproduction puts the spiny dogfish in danger. Conservationists list the species as Vulnerable because it is under tremendous pressure from fisheries. The sharks come together in large aggregations and are often victims of by-catch. The low reproductive rate means populations are slow to bounce back. The IUCN estimates a decline of more than 30% of the global population of S. acanthias in the last 75 years.

Another shark could be the overall record-breaker for the longest vertebrate gestation. To find it we need to plumb the depths again. With the primitive looks of a sea serpent and commonly living 500-1000 metres below the surface, it’s no surprise we’re only just getting to know the frilled shark (Chlamydoselachus anguineus). In 1990, Japanese scientists published research on frilled sharks captured in Suruga Bay, including gravid females. Based on the development rates of the embryos they collected and data from other species, they estimated the frilled shark’s gestation period at up to 3.5 years.

While for some species motherly duties end once eggs are hatched or young are born, there are a few oceanic examples of more long-lasting bonds. It may surprise you to see killer whales on the shortlist for mum of the year. Orcas are best known for their predatory behaviour, but they are not simply black and white killers.

Male orcas stay with their mothers for life (Credit: Tory Kallman/Shutterstock)
Male orcas stay with their mothers for life (Credit: Tory Kallman/Shutterstock)

Around the world’s oceans there are several different ecotypes, each living in a particular habitat with distinctive behaviours and social structures. Among these the southern resident killer whales are an extended family that feed exclusively on salmon and live in the Salish Sea off south-western Canada / north-western US.

The southern resident pods are matrilineal – led by females and made up of their relatives. Within these pods, males stay with their mothers for life with only brief departures to breed with other females. Males live into their thirties so they remain mummy’s boys for decades.

Scientists have found that these male orcas are heavily reliant on their mothers for food - showing them where to find salmon, and even feeding them directly well into adulthood. Males are around 25% bigger than females so need more food but it is the females that have the knowledge of feeding grounds and salmon movement.

“In the resident killer whales that we work with, the sons and daughters never disperse from their mothers. So, they really are dedicated across the entire lifespan of the offspring, particularly to their sons,” explains Professor Darren Croft, an animal behaviour expert at the University of Exeter, UK.

“Sons mate with females from different groups, but return to their mother to live. From evolutionary theory, if you have a daughter that reproduces it’s another mouth to feed but if you have a son that reproduces while another group carries that cost, genetically you get the same benefit.”

“That’s why we think these mothers are dedicated carers of their sons, that’s where they can drive higher lifetime fitness and transfer more of their genes to the next generation.”

Ensuring the health of future generations is imperative for these orca. The southern resident population is considered Critically Endangered, numbering only 77 individuals. Food is in short supply due to fishing pressures and the whales are yet to recover after many were captured for the entertainment industry in the 1960s and 70s.

The lifetime of devotion demonstrated by these orca is matched by a much smaller species that nonetheless takes the mother-child bond to its literal extreme. Scientists at the University of Kansas, US, identified how a species of hydrozoan keeps its children as close as possible. Sometimes known as the ringed tubularia, Ectopleura larynx is a small, predatory species widespread in the North Atlantic that resembles a miniature bouquet (albeit one topped with pink tentacles). While many hydrozoans reproduce asexually to build colonies of clones, E. larynx has a different approach. After the female has intercepted sperm she reproduces sexually and broods her young inside organs called gonophores.

When the juvenile polyps are released they settle on suitable substrate, which can include their mother. If this happens, she physically fuses her offspring to herself. In the competitive environment of the seabed the mother can feed her offspring via a shared digestive system, ensuring access to scarce resources.

The uniting theme for all of these ocean-dwelling mothers is dedication. Whether they’re ensuring a solid foundation or caring for their offspring for life, each species invests time and energy – sometimes at great personal cost. If you’re looking for the toughest sea creatures, mum’s the word.

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By Ella Davies
Featured image by Tory Kallman/Shutterstock