Observation of the Week, 2/23/18

Our Observation of the Week is this multi-colored wonder, an Anoplodactylus evansi sea spider! Seen in Australia by @sascha_shulz.

“I was snorkeling with the Shelly Beach Swim group (many of whom also contribute to iNat), and was photographing a Sand-diver species [See if you can spot it! -Tony]...when I noticed something crawling on my hand,” recalls Australian freediver Sascha Shulz. “And it turned out to be the first sea spider I have ever seen in 24 years of diving in Australia and other locations around the world!”

The sea spider Sascha found, Anoplodactylus evansi, is more colorful than any I’d personally seen (here in California they’re usually pretty drab), and the colors along with its spindly pretty-much-legs-only construction make it a striking organism. In fact, the lack of a “body” means that internal organs like those of the digestive system reside partially in the legs of the beast, and the high surface-area to volume ratio allows respiration to occur directly through the exoskeleton. While sea spider taxonomy is somewhat in flux, they are currently in the subphylum Chelicerata, which also contains arachnids and horseshoe crabs.

Like most arachnids, sea-spiders can only eat liquids, and a long proboscis allows them to pierce and suck out the insides of soft-tissued animals like sponges, gastropods, and hydroids. According to Sea Slug Forum, one of Anoplodactylus evansi’s favorite prey items are juvenile Sea Hares (among other Opisthobranchs), which are young enough to not have accumulated an amount of diet-derived compounds to deter predators. They’ll also eat around an organ where these compounds are kept, like the digestive gland of Aplysia parvula.

Sascha grew up partly in Germany, and says he was “heavily influenced by the German TV series Expeditionen ins Tierreich.” He moved to Australia when he was still a child, and started freediving when he attended the University of Wollongong, where he received a Marine Biology degree. He’s worked for the Australian Museum and still collects specimens for its fish section when he can.

“The amount of valuable data and knowledge that is accumulating due to the people who contribute to iNaturalist is truly stunning,” says Sascha, but “to avoid iNat becoming a ‘problem’” he uses it to record mostly marine species, explaining “I have found myself interrupting conversations to get a photo of a bug crawling past!...’I can stop anytime I want’ I tell myself!”

- by Tony Iwane


- Most sea spiders are tiny, but at the Earth’s poles, there be giants.

- Sascha, among many other iNatters, contributes to the Australasian Fishes and Seaslugs of the World projects, which are amazing.

- Over 150 sea spider observations have been upload to iNaturalist, seeing them all together is pretty amazing.

- Sea spiders don’t just walk, some of them swim!

Posted by tiwane tiwane, February 24, 2018 12:35 AM

Comments

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Amazing......love it !

Posted by judithmarland 4 months ago (Flag)
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Shelly Ocean Swimmers (Shelly Beach Cronulla) thank Sascha for his tireless identifications of our observations at Cronulla, (Sydney Australia) and we were happy to snorkel with him and share his knowledge of marine fish species.The spider photo is excellent and a great addition to our Shelly Ocean Swimmers Project, that has a major aim to gather data for a future SYDNEY MARINE PARK .

Posted by pam_darook 4 months ago (Flag)

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