A trip to the aquarium finds us out of our depth when it comes to life in the deep - The Guardian

A cownose ray arcs its body so that one whole wing is raised out of the tank. A jet of water squirts from an orifice in its belly that we don't want to reflect on for too long, splashing near my son. We're at the aquarium in London's Waterloo and this may be the greatest thing that has ever happened to him. We watch the cownose as it rejoins its siblings, just above the larger southern stingrays patrolling the tank's floor like dour, barb-tailed sentinels.

I wonder if they cohabit without incident, all these rays, and I could probably tell you if I had time to read the copious literature on the walls, but instead I am chasing my daughter into the next room. She's also having a wonderful time, but mainly because she has so much open space to run around, rendering the underwater creatures entirely superfluous. It is my first signal that most of today will be engaged in apologising as she barrels through forests of legs, kisses random bits of wall and, at least twice, comes perilously close to accidentally clambering into one of the tanks.

My son points out his favourites as we walk from room to room, but it's clear that some knowledge has been lost even in those few months since he stopped obsessing over Octonauts quite so much. Those who haven't watched CBeebies' premier nautical adventure cartoon might presume it features only big-ticket aquafauna, your great whites and blue whales. In fact, it centres on a different animal each week, so in over 150 episodes and a dozen or so specials, they've devoted whole episodes to the combtooth blenny, the dwarf lanternshark, the siphonophorae and – of course – Hawaii's official state fish, the humuhumunukunukuāpua'a. With a mild sense of malaise, I realise these are no longer household names to my son. I watch as he pauses at the glass with a look of studied confusion, as if confronted at a wedding by someone whose name he can't quite place.

This has been a fairly regular experience as his brain has developed and the horizons of his childhood memory recede. This was never more poignantly demonstrated than last week, when he told us he'd be doing a reading of The Snail And The Whale for his school assembly – a book I likely read to him 400 times when he was a toddler – prompting him to ask me if I'd ever heard of it.

There's no time for such melancholy, however, for we have reached the sharks, which sail past our eyes, all massive and famous and terrifying. The ocean's true celebrities are mesmerising to look at and we all find ourselves quite starstruck. Even my daughter pauses from trying to climb into one of the bins to come close to the wall of the tank and watch them glide by.

I spot three huddled at the bottom of the tank, motionless. A lifetime spent scrolling quickly past motivational posts by accounts with names like Grind Til You Die and Psychopath Mindset has led me to believe sharks can't stop moving or else they'll die and I wonder aloud if they're OK. 'They're nurse sharks,' my son tells me, with the offhanded elan of a five-year-old zoologist. 'They sleep.' As long as I'm around to be corrected, I realise, he'll be fine.

Did Ye Hear Mammy Died? by Séamas O'Reilly is out now (Little, Brown, £16.99). Buy a copy from guardianbookshop at £14.78

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