Prionocidaris, A. Agassiz, 1863

Tan, Leonard Yi Herng, Woo, Sau Pinn & Kajihara, Hiroshi, 2025, A taxonomic review of the genus Prionocidaris (Echinodermata: Echinoidea: Cidaridae), with molecular phylogeny and emended diagnosis, Journal of Natural History 59 (17 - 20), pp. 1395-1416 : 1406-1412

Prionocidaris, A. Agassiz, 1863

Prionocidaris A. Agassiz, 1863, p. 18 ; type species, Cidarites pistillaris Lamarck, 1816 , fixed by original designation.

Schleinitzia Studer, 1876: 463 ; type species, Schleinitzia crenularis Studer, 1876 , fixed by original designation.

Plococidaris Mortensen, 1909: 51 ; type species, Cidarites bispinosa Lamarck, 1816 , fixed by original designation [ Mortensen (1928a) subsequently ‘designated’ Cidarites verticillata Lamarck, 1816 as the type for Plococidaris , but this was invalid].

Actinocidaris Mortensen, 1928a: 411 ; type species, Phyllacanthus thomasii A. Agassiz and H.L Clark, 1907a , fixed by original designation.

Emended diagnosis

Genital plates similar in size. Ambulacral pores on peristome forming single series. Interambulacral primary tubercles non-crenulate but may have hint of crenulation at upper side of adapical ones. Areoles transversely elongated, non-confluent except two or three adoral ones. Scrobicular and secondary tubercles hardly differentiated, extrascrobicular densely covered with heterogeneous secondary tubercles and granules. No sutural pits or furrows. Ambulacral pore pairs conjugate or subconjugate, with regular, contiguous marginal tubercles. Inside marginal series, inner zone of smaller tubercles present. Primary spines fusiform or cylindrical, tapering or extended at tip. Collar and neck short; shaft coarsely ornamented with thorns arranged in whorls or longitudinal series that can both be more or less distinct. Adoral primaries slightly flattened and slightly curved. Secondary spines flattened, smaller ones spiniform. [Partially emended from Smith and Kroh (2011)].

Species composition

The following 10 extant species are herein considered members of Prionocidaris : Pr. australis , Pr. baculosa , Pr. bispinosa , Pr. callista , Pr. glandulosa , Pr. hawaiiensis , Pr. pistillaris (type species), Pr. popeae , Pr. thomasii and Pr. verticillata ( Table 2).

The following six extinct nominal species, established based on incomplete specimens or an ambiguous assembly of fragments, were provisionally included in our concept but with reservations, pending the acquisition of more complete material: † Pr. cookei , † Pr. haasti , † Pr. katherinae , † Pr. malindiensis , † Pr. marshalli , † Pr. praeverticillata .

The following two extinct species do not belong to Prionocidaris in the sense of this study, although they are considered valid by by Kroh and Mooi (2025): † Pr. neglecta Smith and Wright, 1989 and † Pr. scoparia Chapman and Cudmore, 1934 .

Remarks

Apart from our molecular results, morphological characteristics of Plococidaris verticillata (now classified in Prionocidaris , but referred to as Pl. verticillata for clarity in this section) show some resemblance to the species included in our concept of Prionocidaris . According to Mortensen (1928a), the main features distinguishing Prionocidaris from Plococidaris are that the pores of the latter are not distinctly conjugate, the periproct has fewer plates and is not conically raised, and the primary spines are verticillate, with the test being conspicuously mottled with green. Aside from the primary spines and colour, we agree with H.L. Clark (1946) that the other features mentioned are largely dependent on the size, age, and condition of the specimen, which does not justify a separate generic classification. Furthermore, Mortensen (1928a) noted that one of the larger specimens of Pl. verticillata exhibited distinctly conjugated pore pairs, a point that Hoggett and Rowe (1986) also highlighted. Whether Pl. verticillata possesses conjugated or subconjugated pore pairs is now irrelevant, as our emended diagnosis of Prionocidaris accommodates both types. Additionally, Schultz (2015) notes that the test sculpture between Pl. verticillata and Prionocidaris is indistinguishable.

Aside from the unique colouration of its test, the primary spines of Pl. verticillata are the only characters that distinctly differentiate it from other members of Prionocidaris and are useful for species differentiation. Notably, some specimens of Pr. baculosa observed by Clark (1925, p. 12–14) exhibited verticillate primary spine structures that were sometimes as pronounced as those of Pl. verticillata . Given the tree topology ( Figure 2 ), the presence of verticillate primary spines may be synapomorphic for the two species. Additionally, the distal ends of the spines in Pl. verticillata form a small terminal crown, a characteristic that resembles some species of Prionocidaris (e.g., Pr. bispinosa ) ( Figures 3 , 4 (C–D)).

Among the extant species, the monotypic Actinocidaris has been considered a subjective junior synonym of Prionocidaris by Smith and Kroh (2011) and Kroh and Mooi (2025). Mortensen (1928a) distinguished Actinocidaris from his concept of Prionocidaris based on the nature of the pore pairs, which were purported to be subconjugated in the former and conjugated in the latter. However, this distinction is no longer considered relevant, as mentioned previously. Furthermore, Smith and Kroh (2011) noted that Actinocidaris exhibits ‘clearly conjugated pore-pairs’, contradicting Mortensen’s (1928a) interpretation. In agreement with Smith and Kroh (2011), there is little that distinguishes Actinocidaris from Prionocidaris , aside from the ‘rather stouter and more fusiform spines’. Therefore, we adopt the stance of Smith and Kroh (2011) and include the nominal species Phyllacanthus thomasii (the type species of Actinocidaris ) within our concept of Prionocidaris .

Among the extinct species considered valid by Kroh and Mooi (2025), two nominal species, Pr. neglecta and Pr. scoparia does not belong to Prionocidaris in the sense of this study. Both species possess non-conjugated pore pairs based on the descriptions of the type material ( Philip 1963; Smith and Wright 1989) and does not agree with the diagnosis of Prionocidaris in the present study. On this basis, we recommend referring Pr. neglecta and Pr. scoparia [as suggested by ( Philip 1963) for Pr. scoparia ] to the genus Stylocidaris with reservation until more material is available for us to confidently identify their association.

Prionocidaris bispinosa ( Lamarck, 1816)

( Figures 3 (A–F), 4(A–D))

Cidarites bispinosa Lamarck, 1816: 57 . Prionocidaris bispinosa : in part, Mortensen (1928a): 468 –475; pl. 44, fig. 2; pl. 53, figs. 1–3;

pl. 73, fig. 19; pl. 87, fig. 12.? Leiocidaris bispinosa var. ramsayi Döderlein, 1902: 689 .? Leiocidaris bispinosa var. chinensis Döderlein, 1902: 689 .? Prionocidaris bispinosa var. aruana Döderlein, 1911: 240–243 .? Prionocidaris bispinosa var. elegans Mortensen, 1918: 6–7 .? Prionocidaris bispinosa var. nigrobrunnea Mortensen, 1928b: 74 ; Mortensen (1928a), pl.

47, fig. 1.? Prionocidaris bispinosa var. laevis H.L. Clark, 1938: 371–372 ; pl. 26, fig. 1.

Material examined

USMCRC-Echi0039; total DNA was extracted from the base muscle of the primary spine; hand-collected at a depth of 0.0– 0.5 m on a seagrass bed off Tanjung Pelepas , Johor, Malaysia (1.3316°N, 103.5987°E) on 15 February 2024 .

Diagnosis

A Prionocidaris with no observable spots or stripes on collar of primary spines; sculpture of primary spines highly variable even within same specimen but never shows arrangement in distinct whorls along shaft. The bright pink mamelon of the denuded tests in this species seems valuable to differentiate it from Pr. baculosa and other members of Prionocidaris ( Mortensen 1928a) . The conspicuously distinct whorls present in the primary spines of Pr. verticillata distinguish it from this species.

Description

Test. v.d. 24.85 mm; h.d. 42.45 mm; flattened above and below; AmW/IAmW = ca. 25%; IAmPl:col = 8–9; AmPl:IAmPl = ca. 11; ambulacra slightly sinuous ( Figure 3 (D)).

Apical system. Diameter ca. 41% h.d. (17.60 mm); ocular plates all insert; ocular and genital plates densely covered with small tubercles; periproct relatively less densely tuberculated. Ocular plates heart-shaped, truncated at inner apex. Periproct slightly elevated towards middle, forming slightly conical structure ( Figure 3 (B)).

Peristome. Diameter ca. 36% h.d. (15.20 mm); slightly smaller than apical system; with ca. 11 ambulacral plates and ca. 5 interradial plates; interradial plate series not reaching mouth edge.

Ambulacra. Slightly sinuous. Interporiferous area slightly wider than single pore zone at the ambitus; marginal series of tubercles regular, contiguous. Inside marginal series, two (outer larger and inner smaller) internal tubercles present on bottom edge of plate; farther inside, third, smaller internal tubercle sometimes present between top and bottom edges of plate; rest of interporiferous area covered with minute tubercles leaving no naked space. Pore pairs conjugated, well separated from each other ( Figure 3 (D)).

Interambulacra. Extrascrobicular areas generally closely covered with small tubercles decreasing in size towards interradial and adradial sutures, leaving no naked space. Median space nearly as wide as areole at ambitus. Scrobicular tubercles fairly distinct, larger than marginal tubercles, forming ring around primary tubercle; smaller tubercles (ca. half the size of the scrobicular tubercles) roughly encircling scrobicular ring. Areoles shallow, not confluent except for two or three closest to the peristome, and transversely elongated except for adapical ones, the latter appearing subcircular. Primary tubercles perforate and non-crenulated (there may be a hint of crenulation at the upper side of adapical tubercles) ( Figure 3 (C)).

Spines. Primary spines generally shorter than h.d., with longest one being only slightly longer than h.d. (length at 42.60 mm), generally gradually tapering distally; primary-spineshaft ornamentation varying greatly even within same specimen, from being almost smooth with small thorns in indistinct lateral series to bigger, coarser thorns generally scattered with very slight arrangement in whorls or lateral series. Distal end of primary spines generally ending in terminal crown where thorns of crowns arranged in a way resembling whorls, but occasionally ending with branching thorns slightly longer than those on crowns; first primary spine from apical system ending in blunt but pointed tip; shaft more or less covered with anastomosing hairs. Oral primaries flattened, slightly widened and slightly curved with proximal ones being most distinctly flattened, resembling paddle; edges slightly serrated. Second and third oral primaries ending in crown and covered in small knobs on aboral side ( Figures 3 (E), 4(C–D)). Collar of primary spines generally widening towards distinct milled ring ( Figure 3 (F)). Scrobicular spines flattened, surrounding base of primary spines, and not narrowing towards tip. Smaller spines pointed, covering rest of interambulacra and ambulacra. Some primary spines covered by barnacles in the studied specimen.

Pedicellariae. No large globiferous pedicellariae and tridentate pedicellariae found on this specimen. Small globiferous pedicellariae abundant; valves having large opening, ending in distinct end tooth ( Figure 4 (A–B)).

Colour. Denuded test generally cream-coloured with hints of green, most prominent on ambulacra and genital plates; mamelon of primary tubercles having bright distinct pink colour. Primary spines mainly cream coloured to brownish green, mostly banded with dark purplish red bands that can be more or less conspicuous; collars of primary spines uniformly pink without any spots or stripes. Scrobicular spines faintly pink at base and brownish green at top. Smaller spines in interambulacral and ambulacral regions dark green to black on bottom half and appearing slightly brownish at top half.

Remarks

Prionocidaris bispinosa is known to exhibit a wide range of intraspecific variation ( Mortensen 1928a; H.L. Clark 1946), to the extent that may mask the existence of a morphologically similar species resulting from convergent evolution and representing the ‘ Prionocidaris bispinosa ’ identified by Smith et al. (2006) (see Phylogenetics above). At least six names have been proposed to refer to different varieties within this species ( Döderlein 1902, 1911; Mortensen 1918, 1928b; H.L. Clark 1938; see the synonym list above). Our material appears somewhat intermediate between two varieties, agreeing partially with the description of Pr. bispinosa var. nigrobrunnea (sensu Mortensen 1928a) in terms of its physical morphometry (i.e., smaller apical system, fewer ambulacral plates) and primary spine sculpture (coarsely thorny and ending in the terminal crown, although more thorny spines are also present), but the colouration of the secondary spines rather conforms to that of the ‘typical form’ of Mortensen (1928a) (i.e., not uniformly of a very dark colour). While our molecular results clearly indicate that the specimen identified as ‘ Pr. bispinosa ’ by Smith et al. (2006) represents a distinct species, the latter may share similarities with one of the previously recognised varieties.

Determining whether subspecies should be recognised within Pr. bispinosa requires additional information, although some have been considered valid ( Kroh and Mooi 2025). The six variety names in the synonym list above ( Döderlein 1902, 1911; Mortensen 1918, 1928b; H.L. Clark 1938) are nomenclaturally available according to Article 45.6.4 of the International Code of Zoological Nomenclature ([ICZN] International Commission on Zoological Nomenclature 1999), as these authors did not provide any statement indicating that these names were intended for infrasubspecific entities. On the other hand, Pr. bispinosa is widely distributed throughout the Philippines and Malayan Archipelago, extending westward to Ceylon and south-west Australia, and eastward to north-east Australia and the Bismarck Archipelago ( Schultz 2015). Therefore, unless the population is genetically structured, with subpopulations that are geographically coherent and morphologically distinct from others, these names should not be used as subspecies. In the modern zoological context, subspecies refers to ‘a collection of populations occupying a distinct geographic range and diagnosably distinct from other such conspecific populations based on heritable population traits’ ( Molinari 2023, p. 2; reworded from Mayr and Ashlock 1991; Patten and Unitt 2002).

So far as we are aware, this is the third record of Pr. bispinosa from Malaysia. The species was first recorded in the Straits of Malacca by Bedford (1900) under the name Rhabdocidaris annulifera ( Lamarck, 1816) , which was later considered a synonym of Pr. bispinosa in Mortensen (1928a). It was subsequently rediscovered by Mok et al. (2023) at the same locality where our specimen was collected. This species has also been recorded in the neighbouring country Singapore ( Schultz 2015).

Figures

Figure 2. Maximum-likelihood phylogenetic tree based on 476 bp of the 16S rRNA gene, showing the position of Prionocidaris bispinosa (Lamarck, 1816) as determined from our specimen (USMCRC- Echi0039), compared to the sequence identified by Smith et al. (2006). Bootstrap values are displayed along the branches. Familial and subfamilial classification follows Kroh and Mooi (2025).

Figure 3. Prionocidaris bispinosa (Lamarck, 1816) (USMCRC-Echi0039). (A) Aboral view, with the apical system partially denuded. (B) Close-up of the denuded apical system. (C) Interambulacral plate. (D) Ambulacrum. (E) Primary spines from a single column, arranged top to bottom (left to right). (F) Primary spine collar. Scale bars: A, B, E = 10 mm; C, D, F = 5 mm.

Figure 4. Prionocidaris bispinosa (Lamarck, 1816) (USMCRC-Echi0039). (Ai) Small globiferous pedicellaria (larger). (Aii) Small globiferous pedicellaria (larger): valve top view, valve side view, stalk (left to right). (Bi) Small globiferous pedicellaria (smaller). (Bii) Small globiferous pedicellaria (smaller): valve top view, stalk (left to right). (C) Close-up of primary spine (top view). (D) Close-up of primary spine (side view). Scale bars: Ai, Bi = 1 mm; Aii = 200 µm, Bii = 500 µm, C, D = 10 mm.