David Muirhead observed these cuttlebones on Normanville Beach, SA on 6th May 2025: –
He posted his photos of the cuttle bones to iNaturalist. The cuttlebones were soon identified as being from the New Holland Cuttlefish, Ascarosepion novaehollandiae.
David’s initial notes read: –
“Three of the mostly similar sized (mostly quite small) cuttlebones that are currently present in very large numbers on this beach (Three within about a metre square was almost the norm along the mid beach at the most recent high tide mark).None of these small ones had bite or tooth marks, AFAIK, although I only checked a tiny percentage.”
David later added, “Probably my imagination but the cuttlebones were virtually all intact, yet seemed more brittle than usual when handled even gently. I don’t expect that it will be possible to prove any causal relationship between the many dead cuttlefish that generated these cuttlebones and the current very extensive toxic algal blooms affecting many parts of coastal SA (especially Encounter Bay, Fleurieu Peninsula, eastern KI and southern Yorke Peninsula), but I note the brittleness in case it’s actually relevant. That said, none of these cuttlebones had any pink or red colour, they were the usual white colour. I don’t know if cuttlefish dying from toxic algal bloom exposure tend to develop discoloured cuttlebones. I guess it is possible, but I doubt it would be inevitable. Much would depend on the leading cause of death, i.e. if primarily hypoxia, no discoloured cuttlebones, but if primarily toxin accumulation, perhaps discoloured tissues and thence cuttlebones?? Wild speculation only, with apologies for not checking the web first.”
David then sent this message to me – “Perhaps you might like to look into the matter of whether cuttlebones can become discoloured, brittle, or both, when the animals are dying from a red tide event?”
All I could do at the time was to ask the Microsoft Copilot AI Assistant. The response from the Assistant was, “Short answer: yes — cuttlebones can become brittle during or after a red‑tide mortality event, but true discolouration of the cuttlebone itself is unlikely, and when it does occur it’s usually indirect, caused by tissue decay, pigments, or environmental staining, not by the toxins altering the aragonite structure.
Below is the detailed, evidence‑based breakdown you like, Steve — and I’ll anchor it to what’s actually known about cephalopod physiology, algal‑toxin pathology, and post‑mortem processes. I’ll also connect it to the iNat page you’re viewing, including the observer’s comments about brittleness and lack of pink/red staining .
🧬 1. Can red‑tide toxins change the cuttlebone while the animal is alive?
The cuttlebone is made of aragonite arranged in a highly ordered, chambered structure. It is not vascularised and has no direct blood supply, so toxins circulating in the bloodstream do not reach the cuttlebone in a way that would chemically stain or alter it.
Therefore: algal toxins do not directly discolour the cuttlebone.
However, toxins can indirectly affect it by:
- a) Disrupting buoyancy control before death
Neurotoxins (saxitoxin, domoic acid) can cause:
loss of motor control
inability to regulate gas/water in the cuttlebone
osmotic imbalance
This can lead to:
over‑hydration of the cuttlebone
weakening of the septa
post‑mortem brittleness
This aligns with the iNat observer’s impression that the bones “seemed more brittle than usual” .
- b) Hypoxia
If the primary cause of death is hypoxia, the cuttlebone remains structurally normal. The observer speculated the same: hypoxia → no discolouration .
🎨 2. Can red‑tide events cause discolouration of cuttlebones?
Direct biochemical staining from toxins:
No — there is no known mechanism for toxins to pigment aragonite.
Indirect staining can occur, but only via:
Decomposing tissues
When the mantle rots, pigments (especially melanin and hemocyanin breakdown products) can leach into the cuttlebone surface.
This can produce yellow, brown, grey, or pinkish surface staining.
Pigmented algal cells coating the bone
Karenia and Noctiluca blooms can leave rust‑red or pink films on stranded material.
This is surface staining, not internal discolouration.
Iron‑rich sediments
Common in SA’s Gulf beaches.
Can produce orange or reddish staining.
What we do not see:
Uniform internal red/pink colouring of the cuttlebone
Toxin‑specific colour signatures
Predictable discolouration patterns in red‑tide mortalities
This matches the iNat observation: “none… had any pink or red colour, they were the usual white colour” .
🪨 3. Can red‑tide mortality make cuttlebones brittle?
Yes — indirectly.
Three mechanisms are plausible:
- a) Osmotic imbalance before death
If the animal loses control of buoyancy regulation:
chambers may flood
septa weaken
the bone becomes more fragile
- b) Rapid decomposition after death
If many animals die simultaneously (as in a bloom event), the bodies:
bloat
rupture
release acidic breakdown products
These can etch the outer aragonite, increasing brittleness.
- c) Environmental exposure
Mass strandings often occur during:
rough surf
high‑energy deposition
UV exposure
All of these accelerate drying and fragility.
So the observer’s impression of “more brittle than usual” is biologically plausible and consistent with mass‑mortality conditions.
🧭 4. So what’s the most likely explanation for the Normanville cuttlebones?
Based on the page you’re viewing:
Large numbers of similar‑sized bones present
No bite marks (not predation)
Brittleness noted
No red/pink staining
Observer aware of concurrent toxic algal blooms
The most parsimonious interpretation is:
A bloom‑related mortality event where:
Cause of death = likely hypoxia + possible neurotoxin exposure
Brittleness = post‑mortem dehydration + buoyancy‑control failure
Lack of discolouration = no tissue‑derived staining + no algal coating at that site
This fits the observer’s own speculation that discolouration is “possible but not inevitable” and depends on cause of death .
🔍 5. What would confirm a red‑tide link?
If you want to pursue this for your colleague, the strongest indicators would be:
Multiple size‑classes dying simultaneously
No predation marks
Soft‑tissue residues with reddish or brownish staining
Nearby reports of paralysed or disoriented live cuttlefish
Water samples showing Karenia, Alexandrium, or Noctiluca dominance
Cuttlebones that are intact but unusually fragile
You already have several of these.
If you want, Steve, I can also help you draft a short note for the SA Marine Mortality Events project or prepare a concise summary for your colleague.”
I then asked, “Can you please draft a short note for the SA Marine Mortality Events project or prepare a concise summary for my colleague?”
The Assistant’s response was, “Short Note for the SA Marine Mortality Events Project – Title: Cuttlebone Condition Following Suspected Bloom‑Related Mortality Event
A large number of Sepia cuttlebones were found washed ashore at Normanville following recent toxic algal bloom activity in Gulf St Vincent. The bones were mostly similar in size and showed no predation marks. Several appeared more brittle than usual, but none displayed the pink or red surface staining sometimes associated with decomposing tissues or algal pigments.
Current understanding suggests that algal toxins do not directly discolour cuttlebones, as the aragonite structure is not vascularised. However, bloom‑related mortality (via hypoxia or neurotoxin exposure) can lead to buoyancy‑control failure, chamber flooding, and post‑mortem dehydration, all of which may increase brittleness.
The absence of staining is consistent with hypoxia‑driven mortality or rapid soft‑tissue loss before deposition. The overall pattern is consistent with a bloom‑associated mass‑mortality event.
Further indicators would include reports of disoriented live cuttlefish, water‑sample data, or additional strandings along the same coastline.”
“A cluster of cuttlebones washed up at Normanville during the recent red‑tide activity. The bones were intact, similar in size, and showed no bite marks. Several were noticeably brittle, but none had the pink/red staining sometimes seen when decomposing tissues or algal pigments coat the bone.
From the literature: red‑tide toxins don’t directly alter or stain the cuttlebone, but bloom‑related mortality can cause loss of buoyancy control, chamber flooding, and post‑mortem fragility. Discolouration, when it occurs, is usually from tissue breakdown or environmental staining, not from the toxins themselves.
Taken together, the observations are consistent with a bloom‑related mortality event, likely driven by hypoxia with possible neurotoxin involvement.”
- a longer interpretive version for MLSSA readers,
- a public‑facing paragraph for your website, or
- a technical note suitable for a marine‑science mailing list.
I thought that I would leave it at that for now.