Rhomb-shaped crystal cells

Crystal cells with a single rhomb-shaped crystalline structure have been documented in asteroids and holothuroids (Table 1). The shape of this crystal has been described as rhomboid(al), rhomb(ic), lozenge-shaped, or diamond-shaped (Table 1). In particular, cells have been described with two distinct rhomboidal crystals (Endean, Reference Endean1958), three distinct crystals (Endean, Reference Endean and Boolootian1966; Smith, Reference Smith, Ratcliffe and Rowley1981; Smiley, Reference Smiley, Harrison and Chia1994), and an arrangement of three individual crystals overlayed at offsetted angles that can give an appearance of a six-pointed star-shaped morphology (Hetzel, Reference Hetzel1963).

Table 1. Summary of the diversity of morphological descriptions of crystal cells identified as a type of coelomocyte in echinoderms, organised by class and then by year of publication

The sizes of these crystal cells in holothuroids vary, with studies reporting lengths of 2–4 μm (Smiley, Reference Smiley, Harrison and Chia1994) and larger sizes of 7–8 μm in length and 4–5 μm in width (Chia and Xing, Reference Chia and Xing1996; Xing, Reference Xing1997; Xing et al., Reference Xing, Leung and Chia1998) as well as 5–6 μm in length (maximum of 24 μm), 4–5 μm in width (maximum of 16 μm), and 1–2 μm in thickness (maximum of 11 μm) (Hetzel, Reference Hetzel1963, Reference Hetzel1965) (Table 2). Species-specific measurements include lengths of 12–30 μm in Paraleptopentacta elongata and Panningia hyndmani, 15 μm in Parastichopus tremula, and 22–28 μm in Ocnus lactea (Théel, Reference Théel1921; Table 2). In Molpadia roretzii, they measured 6–8 μm in length, 3–5 μm in width, and 2–8 μm in thickness (Ohuye, Reference Ohuye1936; Table 2). In Holothuria glaberrima, they had an average length of 5 μm (Ramírez-Gómez et al., Reference Ramírez-Gómez, Aponte-Rivera, Méndez-Castaner and García-Arrarás2010; Table 2). In Holothuria leucospilota, they measured 9–11 μm long, 7–9 μm wide, and 4–5 μm thick (Endean, Reference Endean1958; Table 2). In Cucumaria frondosa, they ranged between 15 and 22 μm in length (Caulier et al., Reference Caulier, Hamel and Mercier2020; also see illustrations by Jobson et al., Reference Jobson, Hamel, Hughes and Mercier2021, Reference Jobson, Hamel and Mercier2022; Table 2). Length measurements of crystal cells in holothuroids, likely to closely represent the length of the crystal, may or may not include the cytoplasm and/or nucleus described in some publications; reports have ranged from 2 to 24 or 4 to 24 μm (Smith et al., Reference Smith, Arizza, Barela Hudgell, Barone, Bodnar, Buckley, Cunsolo, Dheilly, Franchi, Fugmann, Furukawa, Garcia-Arraras, Henson, Hibino, Irons, Li, Lun, Majeske, Oren, Pagliara, Pinsino, Raftos, Rast, Samasa, Schillaci, Schrankel, Stabili, Stensväg, Sutton and Cooper2018; Queiroz and Custódio, Reference Queiroz, Custódio, Mercier, Hamel, Suhrbier and Pearce2024; Tables 1 and 2). The size of rhomb-shaped crystal cells in asteroids has not been reported.

Table 2. Size ranges of crystal cells identified as a type of coelomocyte in echinoderms, re-configured from the morphological summary presented in Table 1 and grouped by crystal shape

Cuboidal crystal cells

Crystal cells with cuboidal crystalline structures have been reported in crinoids, echinoids, ophiuroids, and holothuroids (Cuénot, Reference Cuénot1891; Xing et al., Reference Xing, Yang and Chen2008; Chen et al., Reference Chen, Wang, Xing, Zhu and Zhang2015; Queiroz et al., Reference Queiroz, Arizza, Vazzana and Custódio2022a). Among holothuroids, these crystals are less frequently observed compared to rhomb-shaped ones (described above). In the literature, this morphology has been described variously as cuboid(al), cubic, or cube-shaped (Table 1). In one instance, the shape was referred to as cuboid prismatic (Cuénot, Reference Cuénot1891), but it is unclear if other mentions of prisms/prismatic forms also refer to the cuboidal shape. Notably, the surfaces of some cuboidal crystals were slightly curved (Cuénot, Reference Cuénot1891). In the echinoid Paracentrotus gaimardi, live cells with this type of crystalline structure, including the surrounding cytoplasm, measured approximately 5 μm in diameter (Queiroz et al., Reference Queiroz, Arizza, Vazzana and Custódio2022a; Table 2). The sizes of those observed in echinoids by Cuénot (Reference Cuénot1891) were only described as variable. Based on a published image, the cuboidal crystal in the holothuroid Apostichopus japonicus was ∼6 μm in length (Xing et al., Reference Xing, Yang and Chen2008; Chen et al., Reference Chen, Wang, Xing, Zhu and Zhang2015; Table 2).

Other morphologies of crystal cells

Star-shaped/stellate, rosette-shaped, cross-shaped, hexagonal (also referred to as polyhedrons), rod-shaped/elongated/styloid/needle-like, rectangular, squarish, and spherical/round crystalline structures have been reported in asteroids and holothuroids (Table 1). Regular prisms and prismatic shapes in crinoids, echinoids, and holothuroids are too generic to distinguish them from or classify them with the other named morphologies (Cuénot, Reference Cuénot1891; Xue et al., Reference Xue, Peng and Wang2024; Wambreuse et al., Reference Wambreuse, Guillaume, Eeckhaut, Borrello, Bureau, Fievez and Delroisse2025). Théel (Reference Théel1921) remarked that rectangular crystals may be the side view of rhomb-shaped crystals and that cubic crystals may be the side view of star-shaped crystals. There are also reservations of the actual morphology of quadrilateral crystals in Apostichopus japonicus, as they may be either rhombic or rectangular crystals (Liang et al., Reference Liang, Ren, Wang, Li, Yang and Li2019). There are no instances of multiple crystals within a cell reported for any of these shapes (Table 1).

Star-shaped/stellate crystalline structures are generally not as common compared to rhomb-shaped ones (Endean, Reference Endean and Boolootian1966). They have been reported in the holothuroid genera Psolus and Psolidium with diameters up to 70 μm (Hetzel, Reference Hetzel1963, Reference Hetzel1965; Endean, Reference Endean and Boolootian1966; Table 2). In Psolus chitonoides, this crystalline structure, based on an illustration, measures 12 μm in diameter (Hetzel, Reference Hetzel1963, Reference Hetzel1965; Table 2). Additionally, this crystal shape is known in other holothuroid taxa – namely, Panningia hyndmani, Paraleptopentacta elongata, and Parastichopus tremula – with diameters ranging from 11 to 19 μm (Théel, Reference Théel1921; Table 2). The cross-shaped crystal isolated from a holothuroid and described by Semper (Reference Semper1868) may be a less elaborate form of the star-shaped crystal.

Hexagonal/polyhedron crystals are well documented in a few holothuroids in the genus Apostichopus (Dybas and Fankboner, Reference Dybas and Fankboner1986; Xing and Yang, Reference Xing and Yang2008; Xing et al., Reference Xing, Yang and Chen2008; Chen et al., Reference Chen, Wang, Xing, Zhu and Zhang2015; Li et al., Reference Li, Chen, Li and Park2009, Reference Li, Ren, Liang, Qiao, Wang, Ye and Li2018; Liang et al., Reference Liang, Ren, Wang, Li, Yang and Li2019; Xue et al., Reference Xue, Peng and Wang2024). Notably, polyhedrons are hexagonal crystals when viewed from the top and rectangular from the side (Li et al., Reference Li, Chen, Li and Park2009; Xue et al., Reference Xue, Peng and Wang2024). In A. japonicus, this is the more commonly encountered crystal shape (Xing et al., Reference Xing, Yang and Chen2008; Chen et al., Reference Chen, Wang, Xing, Zhu and Zhang2015; Li et al., Reference Li, Ren, Liang, Qiao, Wang, Ye and Li2018). The diameter of these cells/crystals measure between ∼5 and 10 μm (Xing et al., Reference Xing, Yang and Chen2008; Xing and Yang, Reference Xing and Yang2008; Li et al., Reference Li, Chen, Li and Park2009, Reference Li, Ren, Liang, Qiao, Wang, Ye and Li2018; Chen et al., Reference Chen, Wang, Xing, Zhu and Zhang2015; Xue et al., Reference Xue, Peng and Wang2024; Table 2).

Biconvex rhomboid/fusiform/spindle-shaped crystals are known in asteroids (e.g., Patiria minata) and holothuroids (Théel, Reference Théel1921; Andrew, Reference Andrew1962; Johnson and Beeson, Reference Johnson and Beeson1966). An image taken by Wambreuse et al. (Reference Wambreuse, Guillaume, Eeckhaut, Borrello, Bureau, Fievez and Delroisse2025) may be depicting the fusiform morphology that was referenced by Andrew (Reference Andrew1962). This morphology of crystalline structure, isolated from Holothuria scabra, measures ∼18 μm long and ∼9 μm wide (Wambreuse et al., Reference Wambreuse, Guillaume, Eeckhaut, Borrello, Bureau, Fievez and Delroisse2025; Table 2).

Crystal cells with rod-shaped/elongated/styloid/needle-like crystals have been noted in asteroids (e.g., P. minata) and holothuroids (e.g., the genera Apostichopus and Holothuria) (Dybas and Fankboner, Reference Dybas and Fankboner1986; Johnson and Beeson, Reference Johnson and Beeson1966; Xing et al., Reference Xing, Yang and Chen2008; Chen et al., Reference Chen, Wang, Xing, Zhu and Zhang2015; Queiroz et al., Reference Queiroz, Mauro, Arizza, Custódio and Vazzana2022b; Queiroz and Custódio, Reference Queiroz, Custódio, Mercier, Hamel, Suhrbier and Pearce2024; Wambreuse et al., Reference Wambreuse, Guillaume, Eeckhaut, Borrello, Bureau, Fievez and Delroisse2025). In A. japonicus, crystal cells with this shape are 16 μm in length (Xing et al., Reference Xing, Yang and Chen2008; Chen et al., Reference Chen, Wang, Xing, Zhu and Zhang2015; Table 2). Rectangular crystalline structures have been observed in the holothuroids A. japonicus and H. scabra (Eliseikina and Magarlamov, Reference Eliseikina and Magarlamov2002; Wambreuse et al., Reference Wambreuse, Guillaume, Eeckhaut, Borrello, Bureau, Fievez and Delroisse2025). Eliseikina and Magarlamov (Reference Eliseikina and Magarlamov2002) reported crystal cells with this form as having lengths of 6–7 μm and widths of 4 μm in holothuroids (e.g., A. japonicus, Cucumaria japonicus; Table 2). Possibly related to either the rod-shaped or the rectangular morphology, the rectangular rod/quadrangular prism crystal isolated from Mesothuria intestinalis measured 8–15 μm long (Théel, Reference Théel1921; Table 2). Furthermore, squarish crystals are found in the genus Holothuria (Queiroz et al., Reference Queiroz, Mauro, Arizza, Custódio and Vazzana2022b). This morphology could be a variant of the rectangular crystal or another term referring to the cuboidal shape (e.g., side view of the crystal).

Spherical/round crystalline structures inside crystal cells are among the least known morphologies. They are found in asteroids and holothuroids (Smith, Reference Smith, Ratcliffe and Rowley1981; Chia and Xing, Reference Chia and Xing1996; Xing, Reference Xing1997; Xing et al., Reference Xing, Leung and Chia1998; Liang et al., Reference Liang, Ren, Wang, Li, Yang and Li2019). To our knowledge, there are no images or information on their size (Table 1).

The sizes of unspecified morphologies of crystalline structures (perhaps several shapes pooled together) have been documented. Smith (Reference Smith, Ratcliffe and Rowley1981) reported the lengths of crystal cells to be 2–24 μm with typical sizes between 6 and 10 μm in length (Table 2). This was probably referring to the rhomb-shaped crystals because a later study by a different investigator reported the same range for this morphology (Smith et al., Reference Smith, Arizza, Barela Hudgell, Barone, Bodnar, Buckley, Cunsolo, Dheilly, Franchi, Fugmann, Furukawa, Garcia-Arraras, Henson, Hibino, Irons, Li, Lun, Majeske, Oren, Pagliara, Pinsino, Raftos, Rast, Samasa, Schillaci, Schrankel, Stabili, Stensväg, Sutton and Cooper2018). In holothuroids, crystal cells with crystals of unknown morphology measure 6 μm in diameter on average (up to 7 μm) in Holothuria grisea, 7 μm in diameter on average (up to 9 μm) in Holothuria arenicola, and 12 μm in diameter on average (up to 17 μm) in Holothuria tubulosa (Queiroz et al., Reference Queiroz, Mauro, Arizza, Custódio and Vazzana2022b; Table 2). In H. scabra, prismatic crystals with lengths 7–14 μm (up to 25 μm) are reported (Wambreuse et al., Reference Wambreuse, Guillaume, Eeckhaut, Borrello, Bureau, Fievez and Delroisse2025; Table 2). Liang et al. (Reference Liang, Ren, Wang, Li, Yang and Li2019) reported crystal cells of A. japonicus to be 5–10 μm in length, which was most likely referring to hexagonal crystals (Table 2). Moreover, images of undetermined morphologies of crystalline structures, isolated from A. japonicus, ranged from 4 to 7 μm in length (Li et al., Reference Li, Ri, Wang, Ye, Qiao and Li2013; Dolmatov et al., Reference Dolmatov, Ginanova, Eliseikina and Frolova2023; Table 2).

Cytoplasm and cell membrane of crystal cells

The hyaline cytoplasm and cell membrane surrounding the crystalline structure have been reported by several authors (Semper, Reference Semper1868; Théel, Reference Théel1921; Eliseikina and Magarlamov, Reference Eliseikina and Magarlamov2002; Xing et al., Reference Xing, Yang and Chen2008; Queiroz et al., Reference Queiroz, Mauro, Arizza, Custódio and Vazzana2022b). Hetzel (Reference Hetzel1963, Reference Hetzel1965) further described the cytoplasm as a thin layer around the crystal and only visible when viewed from a ‘favourable angle’. Moreover, the cytoplasm of cells on a prepared microscope slide may have a tendency to spread a little (Hetzel, Reference Hetzel1963; Xing et al., Reference Xing, Yang and Chen2008; Queiroz et al., Reference Queiroz, Arizza, Vazzana and Custódio2022a; Queiroz and Custódio, Reference Queiroz, Custódio, Mercier, Hamel, Suhrbier and Pearce2024). Crystal cells with pseudopodia were noted by Hetzel (Reference Hetzel1963, Reference Hetzel1965).

Nucleus of crystal cells

Although a single nucleus enclosed within the crystal cell was first observed by Semper (Reference Semper1868), later investigators remarked that it is not always visible, describing it as altogether absent, often absent, or sometimes absent from the cell (Smith, Reference Smith, Ratcliffe and Rowley1981; Chia and Xing, Reference Chia and Xing1996; Xing, Reference Xing1997; Xing et al., Reference Xing, Leung and Chia1998; Queiroz et al., Reference Queiroz, Arizza, Vazzana and Custódio2022a, b). Furthermore, Théel (Reference Théel1921) noted that the nucleus was detectable after fixation and staining but not in freshly prepared samples. Hetzel (Reference Hetzel1963, Reference Hetzel1965) further added that the nucleus was difficult to see even after fixation. When detected, the nucleus is described as small, heterochromatic/hyperchromatic, crescent-shaped, round, and or flattened (Ohuye, Reference Ohuye1936; Hetzel, Reference Hetzel1963, Reference Hetzel1965; Smith, Reference Smith, Ratcliffe and Rowley1981; Eliseikina and Magarlamov, Reference Eliseikina and Magarlamov2002; Xing et al., Reference Xing, Yang and Chen2008; Li et al., Reference Li, Chen, Li and Park2009; Chen et al., Reference Chen, Wang, Xing, Zhu and Zhang2015; Smith et al., Reference Smith, Arizza, Barela Hudgell, Barone, Bodnar, Buckley, Cunsolo, Dheilly, Franchi, Fugmann, Furukawa, Garcia-Arraras, Henson, Hibino, Irons, Li, Lun, Majeske, Oren, Pagliara, Pinsino, Raftos, Rast, Samasa, Schillaci, Schrankel, Stabili, Stensväg, Sutton and Cooper2018; Caulier et al., Reference Caulier, Hamel and Mercier2020; Dolmatov et al., Reference Dolmatov, Ginanova, Eliseikina and Frolova2023). Several authors have commented that the nucleus was often located on one side of the crystalline structure, i.e., ex-centrical position (Ohuye, Reference Ohuye1936; Eliseikina and Magarlamov, Reference Eliseikina and Magarlamov2002; Li et al., Reference Li, Chen, Li and Park2009; Liang et al., Reference Liang, Ren, Wang, Li, Yang and Li2019; Dolmatov et al., Reference Dolmatov, Ginanova, Eliseikina and Frolova2023). Moreover, nuclei of crystal cells have been identified on SEM imagery (Xing et al., Reference Xing, Yang and Chen2008; Wambreuse et al., Reference Wambreuse, Guillaume, Eeckhaut, Borrello, Bureau, Fievez and Delroisse2025).

Other organelles of crystal cells

Organelles other than the nucleus have only occasionally been documented in crystal-cell coelomocytes (Smith, Reference Smith, Ratcliffe and Rowley1981). For instance, rare small vacuoles between 0.03 and 0.25 μm in diameter, occurring in the cytoplasm and next to the cell membrane, were reported by Eliseikina and Magarlamov (Reference Eliseikina and Magarlamov2002). A rectangular vacuole (presumably containing a crystalline structure of the same shape) was reported to take up most of the space within the cell (Eliseikina and Magarlamov, Reference Eliseikina and Magarlamov2002; Smith et al., Reference Smith, Arizza, Barela Hudgell, Barone, Bodnar, Buckley, Cunsolo, Dheilly, Franchi, Fugmann, Furukawa, Garcia-Arraras, Henson, Hibino, Irons, Li, Lun, Majeske, Oren, Pagliara, Pinsino, Raftos, Rast, Samasa, Schillaci, Schrankel, Stabili, Stensväg, Sutton and Cooper2018). One to two yellow refractile granules, as large as 4 μm in diameter, were observed in crystal cells studied by Hetzel (Reference Hetzel1963, Reference Hetzel1965).

Colour of crystal cells

The colour appearance of crystalline features in the crystal-cell coelomocytes of holothuroids was described as white by Théel (Reference Théel1921). In the holothuroid Holothuria leucospilota, the ‘central part’ of the cells was reported as green (Chia and Xing, Reference Chia and Xing1996; Xing, Reference Xing1997; Xing et al., Reference Xing, Leung and Chia1998). In the asteroid Patiria minata, crystals were pale, golden, and somewhat opalescent (Johnson and Beeson, Reference Johnson and Beeson1966). Cubic crystals of crinoids were described as refringent and colourless or slightly yellow, those of echinoids (e.g., Strongylocentrotus lividus) were yellow, and those of ophiuroids were refringent yellow (Cuénot, Reference Cuénot1891).

Staining of crystal cells

Unlike the nucleus and other cellular components, crystalline structures are reported to be resistant to vital/cytochemical staining (Théel, Reference Théel1921; Ohuye, Reference Ohuye1936; Hetzel, Reference Hetzel1963, Reference Hetzel1965; Eliseikina and Magarlamov, Reference Eliseikina and Magarlamov2002; Queiroz et al., Reference Queiroz, Arizza, Vazzana and Custódio2022a, b; Xue et al., Reference Xue, Peng and Wang2024).

Micrography of crystal cells

Although some investigators report difficulties (Queiroz et al., Reference Queiroz, Arizza, Vazzana and Custódio2022a), crystal cells have been successfully imaged using SEM (Xing et al., Reference Xing, Yang and Chen2008; Xing and Yang, Reference Xing and Yang2008; Li et al., Reference Li, Chen, Li and Park2009; Chen et al., Reference Chen, Wang, Xing, Zhu and Zhang2015; Queiroz and Custódio, Reference Queiroz, Custódio, Mercier, Hamel, Suhrbier and Pearce2024; Xue et al., Reference Xue, Peng and Wang2024). Using SEM, crystal cells appear to have notches on their surfaces (Li et al., Reference Li, Chen, Li and Park2009; Xue et al., Reference Xue, Peng and Wang2024). To date, attempts to image crystal cells using TEM procedures have been largely unsuccessful (Xing et al., Reference Xing, Yang and Chen2008; Li et al., Reference Li, Chen, Li and Park2009; Chen et al., Reference Chen, Wang, Xing, Zhu and Zhang2015; Wambreuse et al., Reference Wambreuse, Guillaume, Eeckhaut, Borrello, Bureau, Fievez and Delroisse2025), with successful visualisation reported in only two studies (Eliseikina and Magarlamov, Reference Eliseikina and Magarlamov2002; Dolmatov et al., Reference Dolmatov, Ginanova, Eliseikina and Frolova2023).

Mobility of crystal cells

The lack of mobility of crystal cells has been widely documented (Ohuye, Reference Ohuye1936; Chia and Xing, Reference Chia and Xing1996; Xing, Reference Xing1997; Xing et al., Reference Xing, Leung and Chia1998), though some observations of slight movements have also been reported (Théel, Reference Théel1921).

Osmoregulation by crystal cells

The formation of crystals inside a vacuole in living cells of holothuroids under conditions of increasing osmotic pressure was described by Eliseikina and Magarlamov (Reference Eliseikina and Magarlamov2002). They concluded that this process was reversible, supporting the role of crystal cells in osmoregulation, i.e., normalising the osmotic pressure in the body. Under slight osmotic stress, the crystalline structure and the cell containing it, however, had been observed to dissolve and collapse, respectively (Hetzel, Reference Hetzel1963). Under laboratory conditions, the concentration of crystal cells did not change when the holothuroid Cucumaria frondosa was exposed to low salinities; however, there was an uptick in cell numbers when the holothuroid was exposed to cold air (Jobson et al., Reference Jobson, Hamel, Hughes and Mercier2021). These experimental results downplay the role that these cells play in regulating osmotic pressure (Jobson et al., Reference Jobson, Hamel, Hughes and Mercier2021).

Presence of crystal cells in response to stress

The abundance of crystal cells increased by 100 ± 122% in the Polian vesicle in response to exposure to cold air of 5 °C experienced by the holothuroid Cucumaria frondosa (Jobson et al., Reference Jobson, Hamel, Hughes and Mercier2021). Furthermore, the number of crystal cells increased slightly in sea cucumbers exposed to cold air of 17 °C and low salinity of 22 psu, but not to low salinity of 15 psu (Jobson et al., Reference Jobson, Hamel, Hughes and Mercier2021).

Solubility of crystal cells

Crystal cells were observed to be insoluble in potassium hydroxide, which is a strong base with a pH from 10 to 13; the exposure time, however, is not specified (Semper, Reference Semper1868).

Chemical composition of crystalline structures inside crystal cells

Based on the morphology of crystalline structures, Semper (Reference Semper1868) initially hypothesised that these structures were crystals of uric acid. However, he concluded that their insolubility in potassium hydroxide suggested otherwise. In the more recent literature, the chemical composition of crystalline structures inside crystal cells is reported as being largely unknown (Endean, Reference Endean and Boolootian1966; Smiley, Reference Smiley, Harrison and Chia1994; Queiroz and Custódio, Reference Queiroz, Custódio, Mercier, Hamel, Suhrbier and Pearce2024).

Derivation of crystal cells

Théel (Reference Théel1921) and Ohuye (Reference Ohuye1938) posited that crystal cells were derived from lymphocytes, which in turn were derived from coelomic epithelium (also see Kindred, Reference Kindred1924; Hetzel, Reference Hetzel1965). In the asteroid Patiria miniata, production of ‘crystal bodies’ was observed from tissue representing the coelomic epithelium; however, it is unknown which cell type from which crystal cells were derived (Johnson and Beeson, Reference Johnson and Beeson1966). Unlike other types of coelomocytes, crystal cells did not incorporate any injected carbon particles inside the cell, suggesting that they are not capable of phagocytosis of foreign particles and are already highly differentiated (Hetzel, Reference Hetzel1965; Zavalnaya et al., Reference Zavalnaya, Shamshurina and Eliseikina2020).

Occurrence of crystal cells

In holothuroids, crystal cells are commonly found in coelomic fluids (i.e., in the perivisceral and hydrovascular coeloms) and entirely absent from the connective tissue compartment of the body wall (Smiley, Reference Smiley, Harrison and Chia1994; Eliseikina and Magarlamov, Reference Eliseikina and Magarlamov2002; Caulier et al., Reference Caulier, Hamel and Mercier2020; but see Ramírez-Gómez et al., Reference Ramírez-Gómez, Aponte-Rivera, Méndez-Castaner and García-Arrarás2010). They are also present in aggregates of coelomocytes in the holothuroid Cucumaria frondosa (Caulier et al., Reference Caulier, Hamel and Mercier2020); however, Dybas and Fankboner (Reference Dybas and Fankboner1986) did not find any crystal cells trapped in aggregates (or brown bodies) of the holothuroid Apostichopus japonicus. Crystal cells are also known from the coelomic fluids of the asteroid Patiria miniata (Johnson and Beeson, Reference Johnson and Beeson1966). In crinoids, they are found near the oesophagus and in the part of the visceral sac beneath the oral disc (Cuénot, Reference Cuénot1891).

Proportion of crystal cells among coelomocytes

The proportion of crystal cells relative to other types of coelomocytes is generally low in holothuroids, ranging from 0.4 to 4.6% (Hetzel, Reference Hetzel1963; Queiroz et al., Reference Queiroz, Mauro, Arizza, Custódio and Vazzana2022b; Wambreuse et al., Reference Wambreuse, Guillaume, Eeckhaut, Borrello, Bureau, Fievez and Delroisse2025). In particular, densities are reportedly greater in the perivisceral than in the hydrovascular fluids even though not all individuals contain crystal cells (Wambreuse et al., Reference Wambreuse, Guillaume, Eeckhaut, Borrello, Bureau, Fievez and Delroisse2025). However, in the holothuroid Apostichopus japonicus, no difference in crystal cell abundance was detected between perivisceral and hydrovascular fluids (Li et al., Reference Li, Ri, Wang, Ye, Qiao and Li2013).

Factors affecting abundance and size of crystal cells

Density and size of crystal cells were reportedly the same between small (∼1 cm long) and larger individuals of the holothuroid Psolus phantapus (Théel, Reference Théel1921). Moreover, the size of crystal cells reportedly varied within the same individual (Hetzel, Reference Hetzel1963, Reference Hetzel1965), while their density tended to decrease over the course of the day (Hetzel, Reference Hetzel1963). In Apostichopus japonicus, densities of crystal cells did not vary between wild and cultured individuals or among individuals of different sizes (Li et al., Reference Li, Ri, Wang, Ye, Qiao and Li2013; Liang et al., Reference Liang, Ren, Wang, Li, Yang and Li2019).

Morphological properties

The shapes and sizes of uric acid microcrystals are consistent with those of the crystalline structures ascribed to ‘crystal cells’ (Tables 1 and 2). For instance, the lengths of rhomb-shaped microcrystals in Cucumaria frondosa (i.e., 5–29 μm; Ma et al., Reference Ma, Gao, Hamel and Mercier2025) and Psolus fabricii, and the diameter of a thin star-shaped in P. fabricii (a variant of the rhombic shape; e.g., Hetzel, Reference Hetzel1963) remains largely within the reported range of rhomb-shaped crystal-cell coelomocytes (Table 2). Furthermore, the width of these crystals in C. frondosa (i.e., 4–20 μm; Ma et al., Reference Ma, Gao, Hamel and Mercier2025) also aligns closely with the reported range of ‘crystal cells’ (Table 2).

Additionally, the width-to-length ratio of rhomb-shaped microcrystals of uric acid in C. frondosa had a mean of 0.60 and a maximum of 0.69 (derived from data collected by Ma et al., Reference Ma, Gao, Hamel and Mercier2025), in line with the width-to-length ratios of rhomb-shaped crystal-cell coelomocytes (described above) estimated to average 0.66, based on an analysis of published values, i.e., 0.50–0.63 (Molpadia roretzii; Ohuye, Reference Ohuye1936); 0.57–0.63 (Chia and Xing, Reference Chia and Xing1996; Xing, Reference Xing1997; Xing et al., Reference Xing, Leung and Chia1998); 0.60 (P. fabricii); 0.67, using maximum values (Hetzel, Reference Hetzel1963, Reference Hetzel1965); and 0.78–0.82 (Holothuria leucospilota; Endean, Reference Endean1958, Reference Endean and Boolootian1966).

Optical properties

Rhomb-shaped microcrystals of uric acid are colourless in the holothuroid Cucumaria frondosa (Ma et al., Reference Ma, Gao, Hamel and Mercier2025), but they can exhibit some colouration depending on the solution and surrounding material. Accordingly, the variable colours of ‘crystal cells’ reported in the literature (see above) may also be influenced by the tint of the solution in which they were prepared or the lighting conditions during observation. Moreover, rhombic crystalline structures isolated from the holothuroid C. frondosa were optically active under polarised light (Figure 1J), which is a known property of uric acid microcrystals. Finally, difficulties reportedly encountered when trying to image rhombic forms of uric acid crystals with SEM (Figure 1K) find echoes in some studies of ‘crystal cells’ (e.g., Queiroz et al., Reference Queiroz, Arizza, Vazzana and Custódio2022a).

Chemical properties

Solubility tests and Raman spectroscopy of cuboidal–barrel-shaped and rhomb-shaped microcrystals isolated from Cucumaria frondosa (Ma et al., Reference Ma, Gao, Hamel and Mercier2025), which were previously identified as crystal-cell coelomocytes (Caulier et al., Reference Caulier, Hamel and Mercier2020; Jobson et al., Reference Jobson, Hamel, Hughes and Mercier2021), provided more precise information on their chemical properties, which are consistent with those of uric acid. Although chemical analyses have so far focused on these two common morphologies, other crystal forms observed in echinoderms may represent additional morphological variants of uric acid (e.g., rosette-shaped; Kleeberg et al., Reference Kleeberg, Gordon, Kedar and Dobler1981) crystallised under different physicochemical conditions or, possibly, crystals composed of other chemical compounds.