TOP row: razor clam; goose barnacle, sea tulip
BOTTOM row: abalone, surf crab, sea cucumber, sea mouse (a type of bristle worm), sipunculid (a different group of worms), sea urchin.
All marine creatures need to extract oxygen from sea water and eliminate the carbon dioxide that they generate via their metabolism1. As with fish, most larger species of invertebrates have gills of some sort to carry out this function. Invertebrates with gills or gill-like structures include crustaceans (eg crabs, prawns, crayfish, barnacles); molluscs (eg, clams, oysters, pipis, abalones, snails, whelks, squid, octopus, cuttlefish); echinoderms (eg, sea urchins, sea cucumbers, sea stars); ascidians (eg, sea squirts, sea tulips), most annelid worms (eg, sand worms, tube worms, sea mice). Some creatures, such as Sipunculid worms, lack conventional gills: they generally exchange oxygen and carbon dioxide with the water directly through their skin (cuticle). See the video below for a massive wash-up of Sipunculids.
The overall structure and function of the gills of most molluscs and crustaceans, for example, is remarkably similar in basic construction to those of fish. The gills consist of a series of thin-walled lamellae (plate-like flanges) through which the blood flows. Water is moved across the gills either by the action of cilia or by muscular activity2.
Although they have a heart to move blood around the body, most invertebrates do not have a continuous circulatory system of arteries, capillaries and veins as seen in vertebrates such as fish (and humans!). Instead their blood (haemolymph) flows freely around the internal organs. Although some invertebrate species have red haemoglobin to help transport oxygen, just like the haemoglobin in our red blood cells, most invertebrates have a bluish or clear oxygen transporter known as haemocyanin.
Regardless of how they exchange oxygen and carbon dioxide between their bodies and sea water, a fundamental requirement for this to happen is that the barrier between the internal and external environments must be thin. Just as in fish gills, this barrier is usually a thin layer of epithelial cells. And just as in fish gills, this barrier can be catastrophically disrupted by contact with cytotoxins produced by Karenia mikimotoi and other Karenia species. It is also probable that the gills of invertebrates can become physically clogged with dead Karenia cells, again in a manner similar to that seen in fish gills. These effects alone or in combination would severely compromise the respiration of most types of invertebrates, as illustrated in the image above3. The larvae of most marine invertebrates have very different body forms and behaviours compared with adults and, where they have been examined, are even more susceptible to the cytotoxic actions of Karenia mikimotoi as well as other Karenia species.
The salt concentration in the body fluids of most invertebrates is similar to that of sea water, so they only need minimal regulation of salt balance across their gills4. Nevertheless, as in all living creatures, the salt concentration inside their cells is very different to that outside the cells. Different types of invertebrates have a variety organs with kidney-like functions to deal with internal salt balance and metabolic waste products. The degree to which these functions can be affected by Karenia cytotoxicity is not known.
The molecular processes underlying the electrochemical impulses of nerve are remarkably similar across most types of animals. It is likely that brevetoxins would impair neural activity in most types of invertebrates to some degree. Nevertheless, it is well established that molluscs such as oysters can accumulate brevetoxins filtered out from the water, presumably within Karenia cells, with few apparent ill effects.
A relatively small number of dead seabirds and dolphins have washed up on beaches during the bloom. It is probable that the effects of exposure to the cytotoxins of Karenia mikimotoi and brevetoxin from Karenia cristata would have similar effects on creatures like these as they do in humans. These effects would include irritation of the eyes, nose and throat as well as the airways. While such irritation is not in itself fatal, it is distressing and may change the behaviour of affected animals, especially if they are already in poor or compromised condition.
Brevetoxin can accumulate in the tissues of fish, mostly in their viscera, but also in their muscles to a lesser degree5. Mammals and birds that eat such fish can be affected by the brevetoxins, potentially contributing to mortalities.6 There also is evidence that brevetoxins can directly lead to mortality in sea turtles, birds, and dolphins, although the precise mechanisms have not been explicitly identified.7
During late summer 2026, occasional dead sea birds, especially cormorants, have washed up on Adelaide metropolitan beaches under conditions when Karenia counts are effectively zero. As mentioned above, these birds could be affected by eating fish that have accumulated brevetoxins. However, it is also possible that these birds are under-nourished due to the greatly reduced numbers of prey species in the bloom-affected inshore and near-shore reefs where cormorants feed. Malnutrition would also lead to poor general health of the birds making them more susceptible to infection.
The following video shows a small sample of a massive wash-up of Sipunculid worms at the north end of Seaford beach (Trigs). Some of the individuals were still alive but would not survive long exposed to the air. These creatures normally live beneath rocks and rubble, sometimes within burrows, below low tide level. Consequently, they are rarely seen. They do not have typical gills or a vascular system. Some exchange oxygen and carbon dioxide with the water through thin skin. Others probably use a set of thin tentacles located around their mouth for both feeding and respiration. In either case, the thin epithelium on respiratory surfaces would be susceptible to damage from Karenia cytotoxins and/or allow uptake of neurotoxins. As they feed on organic detritus on the sea floor, they also may be affected by Karenia that they forage.
Not all organisms seem to be badly affected by the bloom
As far as anyone can tell, some invertebrates and some fish seem not be be excessively affected by the bloom
Right through the bloom, low tides have revealed that some rock outcrops and pools along the Mid Coast (eg Seaford, Moana North) have had what seem to be more or less normal populations of organisms that live on the rock faces between the tide marks, such as barnacles (eg Chthalamus antennatus) together with molluscs including Austrolittorina unifasciata, Austrocochlea sp, Siphonaria sp, and small mussels (eg Xenostrobus pulex). All these intertidal species are highly adapted to extreme environments. For example, they must face extended exposure to hot sun during a summer low tide, that would kill most other marine life. They do this by tightly closing their shells and / or sealing their contact with the rock surface.
One possible factor contributing to their survival is that these creatures can close up in the same way as they do when exposed to the sun if irritated by the bloom and thereby protect themselves from its toxic effects. Furthermore, since they are exposed to air for significant periods, they may not in the water long enough to be so badly affected by the bloom.
Other species living in deeper water do not have these options. Abalones, for example, cannot fully withdraw into their shell and continuously pump water for respiration through holes in their shell. Similarly, benthic bivalves living in sand or amongst the roots of seagrasses cannot fully close their shells and cease respiration for long periods. Indeed, these species have been commonly observed washed up amongst the weed and dead fish in bloom-affected areas.
Free swimming fish, such as whiting, tommy ruffs (Australian Herring) and salmon trout have means of escaping the bloom which are not available to benthic fish living in restricted habitats.
Siphonaria sp and its egg mass in an inter-tidal rock pool. These gastropods are air-breathers but feed and lay their eggs under water (Moana North, August, 2025).
Defences of fish against the bloom
Like most animals, fish are highly adapted to deal with ever present dangers in the environment. Although the algal bloom has caused massive mortalities of many species of fish, many have survived to varying degrees.
The most obvious strategy for fish to escape the effects of the bloom is to swim away. To do this they must be able to detect the bloom in some manner. We detect the irritant effects of algal cytotoxins on our eyes, nose and throat, or skin via specialised nerve endings that respond to tissue damage and a range of potentially dangerous chemicals such as acids. Fish also have comparable nerves in their eyes, oral cavity and skin8. So it is likely that the cytotoxic effects of contact with Karenia organisms would generate an irritant response in the fish as it does in us. And just like us, this would trigger an aversive behaviour: they would try to move away from the source of irritation.
Fish have well developed olfactory systems9. Unlike mammals, the nostrils are separate structures, not connected to the oral cavity and respiratory system. Olfaction plays an important role in fish behaviour from detecting potential food items through to recognising different marine environments10. It is highly likely that they can detect changes in water quality associated with the bloom, such as metabolites of the algae themselves, or the breakdown products of organisms killed by the bloom, such as hydrogen sulphide.
The amount of oxygen dissolved in water is much lower than that in air, so fish are well adapted to extracting as much oxygen as possible. When oxygen levels in the water drop, as in environments associated with bacterial decay of organic matter or very high density of phytoplankton, fish have only a limited range of physiological regulation that they can implement. Ultimately they must move away or die of asphyxia11.
The skin of fish provides multiple layers of protection. In adult fish, the skin is largely impermeable to most water-borne agents. The scales that most fish have provide protection against physical damage. The outermost layer of protection is provided by a layer of mucus that is generated by cells in the skin. The mucus contains a mix of complex chemicals that form a continuous gelatinous hydrated matrix. A major function of this barrier is to prevent infection. It does this in two ways: blocking access by potentially pathogenic micro-organisms; and by providing a supportive environment for beneficial microbes, many of which can themselves repel or inhibit the activity of pathogens. The composition of the mucus can be altered adaptively in response to different environmental challenges12.
If the superficial defences of the skin or gills are breached, toxic agents or micro-organisms in the water can potentially enter the blood stream. However, fish have a well-developed immune system that can fight immediate threat of infection and raise antibody responses against future infections13.
Despite popular memes to the contrary, fish learn quickly to adjust their behaviour in response to changes in environmental conditions14. If those conditions are sub-optimal, they will actively seek locations where conditions are better. Similarly, they will adapt their foraging and feeding strategies depending on the availability of food items. They are likely to remember locations where conditions were bad or, alternatively, where food sources are more reliable. Unfortunately, some fish, such as those living near the sea floor in weedy habitats, may not have the capability to move far enough away from bloom-affected conditions to survive.
Selected references
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↩︎ - Brusca RC et al (2023) Invertebrates, 4th ed. Oxford University Press, ISBN 9780197554418.
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Natsuike M et al (2024) The lethal effects of the harmful dinoflagellate Karenia mikimotoi on two bivalves, Yesso scallop and Sakhalin
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