Table 1. Estimated numbers of farmed aquatic invertebrates besides crustaceans and molluscs (2024).

Taxonomic group Production
(1,000 t)1		 Estimated numbers (millions)2
(Lower) (Upper) (Midpoint)
Sea cucumbers (Holothuroidea) 334 2,100 4,800 3,400
Miscellaneous invertebrates 190 970 2,200 1,600
Sea squirts (Ascidiacea) 23 76 330 200
Sea urchins (Echinoidea) 10 90 90 90
Bristle worms (Polychaeta) 0.5 38 40 39
True jellyfish (Scyphozoa) 83 12 55 34
Total of above 641 3,300 7,500 5,400
1. Source: FAO (2026).
2. Rounded to two significant figures. Full results & notes are available here.

 
We estimated the numbers of farmed aquatic invertebrate animals ‘harvested’ (killed for use as food and feed etc.) in 2024 (Table 1), in addition to the crustaceans and molluscs previously estimated, using FAO aquaculture production tonnages (FAO 2026) and our estimated mean weights. These comprise all other invertebrates not recorded as being crustaceans or molluscs, and their estimated numbers totalled 3.3-7.5 billion individuals (midpoint 5.4 billion or 5.4 * 109 individuals). This includes 0.97-2.2 billion unnamed invertebrates, which may include molluscs and crustaceans. Details of the estimate are available by species and by country.

The largest taxonomic order represented is Echinodermata, comprising an estimated 2,100-4,800 million (midpoint 3,400 million) sea cucumbers (Holothuroidea) and an estimated 90 million sea urchins (Echinoidea).

Sea squirts (Ascidiacea) were the next largest group, with an estimated 76-330 million (midpoint 200 million) individuals. There were also 39 million bristle worms (Polychaeta) and 34 million jellyfishes (Scyphozoa), by estimate midpoint.

Note that these estimates exclude animals caught from the wild (and so exclude sea ranching) and on-farm mortalities. For example, a study of different farming systems for Japanese sea cucumber (Apostichopus japonicus) (discussed below), in a region of China, reported mortalities of 20% for indoor tanks and 10% for semi-intensive ponds during grow out (Wang et al. 2015). Another example of on-farm mortality is the unwanted growth of sea squirts that are discarded at sea (Gao et al. 2023).

Global production of these invertebrates, totalling 641 thousand tonnes in 2024, represented a 22% increase from 525 thousand tonnes in 2020 (FAO 2026).


Sentience, natural behaviours and animal welfare implications

We briefly consider the animal welfare implications of farming these several billion other invertebrate aquatic animals, firstly by looking at the question of sentience. Invertebrate animals are not generally recognised as sentient beings, in law or voluntary codes, but this is beginning to change in some cases.

 
Invertebrate sentience

Birch et al. (2021) developed a framework to evaluate the existing evidence on pain in cephalopod molluscs and decapod crustaceans, and to inform animal welfare legislation. This framework used eight criteria for assessing evidence relevant to pain in animals:

  1. possession of nociceptors;
  2. possession of integrative brain regions;
  3. connections between nociceptors and integrative brain regions;
  4. responses affected by potential local anaesthetics or analgesics;
  5. motivational trade-offs that show a balancing of threat against opportunity for reward;
  6. flexible self-protective behaviours in response to injury and threat;
  7. associative learning that goes beyond habituation and sensitisation;
  8. behaviour that shows the animal values local anaesthetics or analgesics when injured.

Each criterion adds to the case for pain: the more criteria fulfilled, the higher the likelihood. Based on an assessment of evidence against these criteria, Birch et al. (2021) recommended that all cephalopod molluscs (e.g. octopuses) and decapod crustaceans should be regarded as sentient animals for the purposes of UK animal welfare law.

Adopting this framework, researchers have concluded that at least some insects might also feel pain (Crump et al. 2023). Importantly, sentience (i.e. the capacity of an animal to have feelings that matter to them) is not just about pain, and there are ways to make a case for insect sentience that do not proceed via responses to noxious stimuli (Birch 2024). Huge numbers of insects, totalling over a trillion, are farmed for food and feed each year (Barrett & Fischer 2023).

Regarding the sentiency of echinoderms, sea squirts, polychaete worms or jellyfishes, there appears to be less published research (Lewbart & Zachariah 2023). While two of the framework criteria (4 and 8) require a suitable analgesic or local anaesthetic, there is little information on these for invertebrates (Wahltinez et al. 2022), though MS-222 and propylene phenoxetol have been used as local anaesthetics in studies involving echinoderm connective tissue (Crespi-Abril and Rubliar 2023). Information is also lacking on the effectiveness of general anaesthetics recommended for the euthanasia (e.g. in science) of echinoderms, jellyfish and polychaete worms (Bakker et al. 2026) and sea squirts (Bay-Nouailhat & Bay-Nouailhat 2015). However, Carter et al. (2024) formally tested the efficacy of four common anaesthetics, using behavioural and other biomarkers, in northern sea cucumber (Cucumaria frondosa) (see next section). Carter et al. (2024) found that MS-222 seemed to be an effective general anaesthetic, i.e. one that blocks pain rather than just immobilising the animal, representing a promising anaesthetic and sedative in echinoderms and other soft-bodied aquatic invertebrates.

It is critical to recognize that the absence of evidence of painful sensations in echinoderms should not be interpreted as conclusive proof of pain absence in this group, according to Argentinian scientists Crespi-Abril and Rubliar (2023), who argue that echinoderm animals should be treated with respect and compassion in science and farming, without waiting for conclusive evidence of sentience. There are inherent difficulties in determining what an animal is capable of feeling, due to our limited understanding of phenomenal consciousness (Browning & Birch 2022).

We discuss these fascinating animals in the sections below, based on literature searches relating to their natural behaviours, nervous systems and learning:

followed by a conclusion.

 
Echinoderms (sea cucumbers and sea urchins)

The top farmed echinoderm species is the Japanese Sea cucumber, which is farmed for food and medicine (Yang et al. 2015), almost entirely in China, with some production in Russia. Sea urchins (Strongylocentrotus spp) are also farmed in these two countries, mainly for their edible roe (gonads), with some Russian sea urchin production used for pharmaceuticals (Rubilar & Cardozo 2021).

Japanese Sea cucumber is a marine bottom-dwelling species whose natural behaviours include crawling (Hu et al. 2021, Kwon et al. 2019) and foraging, using tentacles surrounding the mouth to collect food (Hu et al. 2021); sheltering (Kwon et al. 2019, Li et al. 2026) and aggregating (Hu et al. 2021, Li et al. 2026).

Sea cucumbers and sea urchins move using their numerous tube feet (Lewbart & Zachariah 2023), tiny tube-like projections on their undersides. Some limited research has suggested that the tube feet, tentacles, and papilla (small, soft projections found on the skin surface) of Japanese sea cucumber may be the most important potential sensory organs for light, chemical and mechanical stimulation (Wang et al. 2023).

A study by Hamel et al. (2021) showed that, upon detecting predator scents, the northern sea cucumber prepares itself for potential injury by rapidly increasing levels of coelomocytes (immune cells) and cortisol, suggesting an internal state that might involve anxiety. Carter et al. (2024) found that MS-222 appeared to be an effective anaesthetic in this sea cucumber species, mirroring its prior use in fish. It completely immobilized the individuals and kept cortisol levels low, even during exposure to a predator starfish (Solaster endeca).

The echinoderm nervous system is organized in a central nerve ring and radial nerve cords, along with peripheral nerves (Paganos et al. 2025). The nervous system lacks a ‘brain’, in the sense of any centrally cephalized group of neurons (Freas & Cheng 2022).

When Paganos et al. (2025) reconstructed the cell atlas of a juvenile sea urchin (Paracentrotus lividus) using single-nucleus transcriptomics, more than half of the cell type clusters were neuronal. The large number of neuronal cell type families identified (totalling 29), together with the high diversity of their molecular signatures and use of diverse neuropeptides, suggested a high functional specialization (Paganos et al. 2025). According to these authors, their analysis strongly supports the hypothesis that echinoderms have a ‘head-like’ body-plan, previously proposed by Formery et al. (2023). Paganos et al. (2025) further concluded that the expression of several vertebrate CNS homologs in tissues throughout the sea urchin nervous system, demonstrated by their study, suggests the nervous system has an ‘all-brain’ organization, which may apply to the entire echinoderm clade. In other words, rather than being brainless, the brain is distributed across the entire organism.

There has been some research into echinoderm associative learning, mainly focused on starfish (Asteroidea), according to Freas and Cheng (2022). For example, when given the choice between a rough or a smooth surface to settle on, starfish and brittle stars (Ophiuroidea) prefer a rough texture, but Diebschlag (1938; cited in Freas & Cheng 2022) trained these echinoderms to reverse this behaviour by giving an electric shock to arms that touched the rough surface. Trained starfish and brittle stars would withdraw their arm as soon as it touched the rough texture, i.e. before a shock was applied, including arms that had not previously been shocked, leading Diebschlag (1938; cited in Freas & Cheng 2022) to conclude that the animal as a whole learned, not individual arms. A more systematic reporting of all animals and all phases of experiments would have enhanced the presentation of this 1938 study (Freas & Cheng 2022).

 
Sea squirts

Red oyas (Halocynthia roretzi) is a solitary sea squirt farmed in South Korea and Japan, for food and bioactive compounds in the tissues (Lee et al. 2020). Some farmed production of unnamed species of sea squirts (‘Ascidiaceans NEI’) is also reported for the former.

Sea squirts, together with other tunicates, are the closest living relatives of vertebrates (Sasakura 2026); tunicates and vertebrates being the two sister groups in the clade Olfactores, within the phylum Chordata.

In their tadpole larval stage, sea squirts possess a central nervous system which is homologous to that of vertebrates, though with smaller numbers of cells (Sasakura et al. 2012). Once the swimming larvae find a suitable settlement location, they attach themselves to the substrate and undergo metamorphosis, to become sessile adults.

Adults are barrel-like and furnished with two chimney-like siphons (inhalant and exhalant) for water circulation inside the body, on which both respiration and feeding relay (Manni et al. 2025). The adult CNS is reorganised, as well as the body plan, as it no longer needs to regulate swimming but now needs to regulate adult organs such as the heart (Sasakura et al. 2012).

Despite it often being said that the metamorphosing tadpole larva ‘eats its own brain’, based on the notion that the adult brain is degraded, adult ascidians have perfectly good brains (Mackie & Burighel 2005). The adult brains (cerebral ganglia) are an order of magnitude larger than those of their larvae, and their behaviour is as finely adapted to sessility as that of the larvae to motility (Mackie & Burighel 2005).

Sea squirt adults possess primary sensory cells that are sensitive to water movement, vibration, and direct tactile contact (Varello et al. 2023). Their stimulation can evoke the squirt response, a strong, synchronous contraction of both siphons that causes a violent ejection of water from them (Varello et al. 2023). Secondary sensory cells located on the oral tentacles, which comprise the coronal organ, are considered homologues of the hair cells of the vertebrate internal ear and lateral line system (Varello et al. 2023). These, like hair cells, allow hearing and vibrational sensing (Varello et al. 2023).

Tolstenkov et al. (2025) found that the sessile adults of the sea squirt species Ciona intestinalis can exhibit different behavioural states, distinguished from each other by various postural and motion features, which can be induced by external stimuli or spontaneously.

Little research exists on learning and memory in adult solitary ascidians (Gasbo et al. 2025). While Gasbo et al. (2025) recently found non-associative learning (sensitisation and habituation) in these animals, this type of learning does not meet the Birch et al. (2021) criteria (see above). Specifically, Gasbo et al. (2025) showed that the solitary sea squirt Polycarpa mytiligera exhibits non-associative learning; its siphon contraction habituated to a weak mechanical stimulus (brush bristles), and this memory lasted for at least one day.

 
Bristle worms

King ragworm (Alitta virens or Nereis virens) and blow lugworm (Arenicola marina) are the two species of bristle worms reported in FAO aquaculture production statistics for 2024, reared in the Netherlands and France respectively. Polychaete worms are presently cultivated in both Europe and Asia, on a relatively small scale, and mainly sold as aquaculture feed or fishing bait, often as live animals (Standal et al. 2024).

Wild king ragworm is omnivorous and found in intertidal mud flats and sand beaches (Du Clos et al. 2013). It builds U-shaped burrows, often 30 cm or deeper, that it expands over the course of a few days and maintains for its semi-sessile lifestyle, generally remaining in its burrow at high tide and emerging at low tide to scavenge for food (Du Clos et al. 2013). King ragworm will burrow under the sand in the direction of food, in order to reach food without leaving the burrow (Copeland & Wieman 1924). Wild blow lugworm lives in 20 to 40 cm deep J-shaped burrows in lower shore sediment, and feeds by swallowing sediment (Riisgard & Banta 1998).

Wild king ragworm defends its burrow from conspecifics, resulting in the expulsion of the intruder or resident, or in cohabitation (Miron et al. 1992). Clark (1959) observed that a closely related species, Nereis pelagica, seemed capable of distinguishing individuals with which it had previously fought. A further observation was that, on one occasion, such a worm anticipated its invasion of a tube (artificial burrow) by fighting a neighbouring worm outside the tube (Clark 1959).

Polychaetes have a brain or cerebral ganglion that originates and usually resides in the head (Verdonschot 2015). There are six major kinds of sensory structures found in polychaetes, which include palps, antennae and eyes (Verdonschot 2015).

King ragworm is able to learn to avoid one arm of a ‘T’ maze in which they receive a slight electric shock, providing ‘correct’ choices (i.e. the arm in which they receive no shock) are reinforced by permitting the worms to remain undisturbed in a darkened chamber for at least 5 minutes before the next trial (Evans 1963). In this study (Evans 1963), the arbitrary learning criterion of 10 consecutive correct choices was achieved after an average of 68.2 maze trials. A period of 48 hours separated the first and last trials in a series of 30 consecutive ‘correct’ choices by one individual king ragworm. During initial training, and once trained, the worms moved rapidly through the maze, but during the later part of training they tended to crawl very slowly and often paused at the junction. Frequently, a worm with some experience in the maze made an ‘incorrect’ choice at the junction, halted before reaching the electrodes, retreated and then continued along the ‘correct’ arm.

 
Jellyfishes

One species of jellyfish, the flame jellyfish (Rhopilema esculentum), was reported in FAO aquaculture production statistics for 2024, which a scyphozoan or ‘true jellyfish’. This species is reared in China, where it is a popular food and also used in traditional medicine, with other potential uses for farmed jellyfish including feed (Duarte et al. 2022). Cultured Rhopilema esculentum is fed on wild zooplankton and brine shrimp (Artemia) nauplii (Duarte et al. 2022).

Jellyfishes belong to the phylum Cnidaria, which is the sister group of Bilateria (Zapata et al. 2015), and are therefore our more distant relatives compared to bilaterian animals discussed here (sea squirts, echinoderms and polychaete worms).

While jellyfishes lack a single centralized brain, it is a common misunderstanding that they have only a diffuse, homogenous nerve net for a nervous system (Katsuki & Greenspan 2013). In most jellyfish, an argument can be made for the presence of centralized nervous systems that interact with the more diffuse nerve nets (Satterlie 2011). Katsuki and Greenspan (2013) describe three main components of the ‘true jellyfish’ (Scyphozoa) nervous system, briefly summarised as follows:

  • the rhopalia, sensory structures located round the bell margin (numbering eight or more, and representing integrative centres for sensory inputs and motor outputs (Satterlie 2011));
  • the motor nerve net, which activates swimming muscles (in response to signals from pacemaker neurones in the rhopalia) and
  • the diffuse nerve net, believed to relay sensory info. to the musculature (both directly (i.e peripherally) and indirectly via the pacemakers).

In a paper whose title begins ‘What’s on the mind of a jellyfish?’, Albert (2011) described what he calls the ‘central nervous system’ of the moon jellyfish genus Aurelia (also Scyphozoan) as a functionally effective brain that deals with sensory input from several modalities (e.g. light, touch, gravity, chemicals, sound pressure waves, direction) simultaneously, generating complex behaviours (e.g. horizontal directional swimming, staying below turbulence and away from rock walls, aggregating) that are not simple reflexes.

A review by Cheng (2021) found no published studies on learning in jellyfish, besides habituation. More recently, Bielecki et al. (2023) showed that a jellyfish in the class Cubozoa (box jellyfish), namely Tripedalia Cystophora, is capable of associative learning that changed its behaviour. Unlike Scyphozoans, Cubozoans possess image-forming eyes, and they have a higher degree of neuronal condensation (Katsuki & Greenspan 2013). This learning process was localised within the rhopalia (Bielecki et al. 2023).

 
Conclusion

Cultivation of these invertebrate species has the important advantage, compared with some fed aquaculture species, that they do not require (inhumanely caught) wild finfishes as feed. Red oyas are farmed in Korea on suspended ropes (Goa et al. 2023) and obtain food by filtering seawater. However, while natural food supply (such as microalgae) is often sufficient for growing Japanese sea cucumber, artificial feed containing fish powder is sometimes used in farms with high stocking densities (Yang et al. 2015).

For better welfare in aquaculture, farming conditions should promote animal health and permit natural behaviours, while minimising stress (including during rearing, breeding and slaughter). Meeting these behavioural needs should be easier for sessile and slow moving animals, like sea squirts and sea cucumbers, than it is for motile finfishes (Jacquet 2017), which is another advantage for animal welfare.

However, methods of killing for food, such as live gutting followed by boiling for sea cucumbers (Yang et al. 2015) and using live ragworms to bait fishing hooks (Morris-Webb et al. 2025) could cause considerable suffering if these animals are sentient.

For the ethical treatment of millions, and in some cases billions of animals, the sentience and welfare needs of all farmed animals should be scientifically investigated. This should include the development of more humane methods of slaughter.

Invertebrates currently farmed in the EU27 (besides insects) include molluscs, decapods and bristle worms; while sea cucumbers are under consideration for future culture (AAC 2024). The Aquaculture Advisory Council (AAC) has argued for the EU to organize a programme of research into the potential sentiency of all farmed aquatic animals (AAC 2024).

 

A. Mood, July 2026.

___________________________________________________________________________

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