Society member David Muirhead photographed an unusual skate at Edithburgh jetty in April. He posted the photos to iNaturalist.
At the time, he said, “I think this is the only one of these I’ve ever seen. It might be a common inshore species or perhaps (speculation only) it may have ventured inshore to feed on the infaunal invertebrate feast left by the mass die-off of benthic invertebrates that occurred earlier this same weekend.”
He later added, “FWIW it seemed perfectly healthy. It swam around well, stopping on the bottom frequently. The only potentially significant behaviour was that it seemed to spend more time than you’d expect for a bottom dwelling (demersal) species swimming high in the water column, in a slightly agitated manner. But this might simply have been because we encountered it on the bottom near the exit steps at dive’s end, so it was being disturbed frequently by divers coming and going.”
It was later identified as being a Pygmy Thornback Skate, Dentiraja flindersi. It was said to “Only occur in this area, reported depth range 20-55 m. Biology largely unknown. Looks like a iNat first, possibly a first live in-situ image anywhere.”
It was later suggested that it “might be the first live, in-situ example, because available photos are pickled carcasses. Most records in SA are from trawl grounds in Gulf St Vincent and Investigator Strait / northern KI, and a few from trawls on the west coast / eastern GAB, and in Spencer Gulf (rarely). For example, during a 3-year trawl bycatch program, in which 278 trawls were sampled over 5 sites in Spencer Gulf, less than 5 Pygmy Thornback Skates were recorded (Svane et al., 2007). The holotype was from deeper water in Backstairs Passage / off NE KI. Remarkably, there appear to be no records from anywhere else except SA, and thus it is classed as an endemic. What size would you estimate? Trawl examples are generally less than 30-33cm, but it might grow slightly larger. Apparently matures at a large size.”
David’s response was “My crude estimate of size is a TL (i.e. including tail) of between 30 and 35cm. (The mature Pinna bicolor razor clam in the images is a useful guide).
It was later stated, “33cm is around the maximum known size of D. flindersi. The males mature at a large size (= presumably late age). Very little is known of the biology.”
Mark McGrouther later posted details at Pygmy Thornback Skate: A Rare Inshore Encounter at Edithburgh Jetty, as follows: –
“An intriguing observation was made on April 5, 2025, under Edithburgh Jetty by Australasian Fishes member David Muirhead. What appeared to be a small skate turned out to be something far more interesting.
After submitting the observation a couple of days later, the discussion among iNaturalist users quickly identified the species as Dentiraja flindersi, the Pygmy Thornback Skate. This species is typically found at depths between 20 and 55 metres, making this sighting in shallow water both rare and potentially significant.
As @clinton pointed out, this may be the first live, in-situ image of the species, with most existing records being from preserved trawl specimens. D. flindersi is considered endemic to South Australia, with most records coming from trawl grounds in Spencer Gulf, Gulf St Vincent, and around Kangaroo Island.
@marinejanine added that during a three-year trawl bycatch program in Spencer Gulf, fewer than five individuals of this species were recorded across 278 trawls. Such scarcity makes this encounter noteworthy, especially considering the specimen was alive and appeared healthy at the time of observation.
Despite being a bottom-dwelling species, the skate was observed swimming higher in the water column, possibly due to diver activity near the jetty steps. Its estimated size was around 30–35 cm total length, gauged using a nearby razor clam as a scale reference.
This particular individual may have ventured inshore to take advantage of the dead and dying animals, or was possibly affected itself by the Algal Bloom Crisis in South Australia.
As Clinton clarified, initial thoughts leaned toward Dentiraja lempieri, but the dense white spotting pattern and geographic range did not match. Another similar species, D. cerva, was also ruled out due to differences in snout shape and depth range.
A special thanks goes to @clinton, @marinejanine, @diegoalmendras, and others who contributed their expertise and thoughtful commentary to help confirm the identity of this species.
And of course, hats off to @davemmdave for not only making the observation but also documenting and following up with such detail. Observations like this exemplify the value of citizen science in expanding our understanding of Australia’s unique marine life.”
There is an audio link on the web page. It is suggested that you “listen to a thought-provoking, 6-minute discussion of this journal post. The file was generated by Notebook LM, and although it contains a few ‘truth-stretchers’, it’s definitely worth a listen.”
Comments added to the web page include: –
“Thanks Mark McGrouther. We agree that it is an important record for many reasons:
it was live (and the previous available visual example was a pickled trawl specimen photographed at CSIRO);
it was in shallow water (unexpectedly and possibly due to the algal bloom at Edithburgh that hit the shallows exactly at that time in early April – multiple accounts across Yorke Peninsula and northern KI of deeper water batoids and sharks and bony fishes ‘driving themselves’ to shore at the time).
“Noting that tens of thousands of dives have been undertaken at Edithburgh since the 1970s, and this is the first record there of Dentiraja flindersi. It is clearly not resident in the area.
“Since Dave’s @davemmdave sighting in April, two dead juvenile Dentiraja flindersi have been recorded in the SA Marine Mortalities database that @b_martin_ and myself have been managing since early April.
“One from @johnnybwoodside in Foul Bay (southern Yorke Peninsula) in April that I identified from the shape, thorns and surface appearance, and @clinton and others have agreed with the ID: https://www.inaturalist.org/observations/272816839 (it would have been worthwhile to collect the specimen of that juvenile for certain ID, but it’s too late now). And one from Goolwa Beach in early June, that @clinton ID’d: https://www.inaturalist.org/observations/286436820 .
“NB the 20-55 m range in your article above was not the full depth range of Dentiraja flindersi even before Dave’s record – it is the range of the trawl records that CSIRO staff used in the description summary in previous publications. We had the same situation with the endemic SA coastal stingaree Urolophus orarius – the IUCN Red List listing that utilises published records assumed that it only occurs in a very limited depth range, based on published trawl records, even though divers were seeing it in the shallower depths (e.g. 5-10m), during dives, and taking photos for verification. I mentioned that to IUCN at the time, as I was involved with conservation listing assessments for more than 250 SA fishes, sharks and rays as part of a contract for government. The info was ignored, even though I was listed as a reviewer for IUCN’s U. orarius listing, without any direct contact about the content.”
Those comments drew this response from Mark McGrouther: –
“Thank you for taking the time to write such an informative comment. Interesting to read that 2 dead juvenile D. flindersi have been recently recorded. Is this https://www.inaturalist.org/projects/sa-marine-mortality-events-2025 the database you mentioned? Yes, it’s a shame that the Foul Bay specimen wasn’t retained. It must have been frustrating to have been ignored when you provided advice on the depth range of U. orarius. If it’s possible to send me your more up to date depth range for D. flindersi I can update this information in the journal post. Thank you again, Mark”
(All photos taken by David Muirhead)