Ophioderma occultum, Humara-Gil & Granja-Fernández & Bautista-Guerrero & Rodríguez-Troncoso, 2022

Humara-Gil, Karla J., Granja-Fernández, Rebeca, Bautista-Guerrero, Eric & Rodríguez-Troncoso, Alma P., 2022, Overlooked for over a century: Ophioderma occultum sp. nov. (Echinodermata), a new species of brittle star from the Eastern Pacific, Journal of Natural History (J. Nat. Hist.) 56 (5 - 8), pp. 365-384 : 369-380

publication ID	https://doi.org/ 10.1080/00222933.2022.2071179
publication LSID	lsid:zoobank.org:pub:2F736F8D-E18E-4979-9259-BAFAE9C47542
DOI	https://doi.org/10.5281/zenodo.6772313
persistent identifier	https://treatment.plazi.org/id/03DF230B-0A0A-947E-FE01-6824FD7EB54D
treatment provided by	Plazi
scientific name	Ophioderma occultum
status	sp. nov.

Treatment

Ophioderma occultum View in CoL sp. nov.

( Figures 1 – 3 View Figure 1 View Figure 2 View Figure 3 ; Table 1 View Table 1 )

Ophiura panamensis . – Ives 1889: 76–77 (partim)

Ophioderma panamense View in CoL . – Ziesenhenne (1955): 194, 197 (partim). – Brusca and Smith (1973): 318, 320, figs 12.15–12.16 (partim). – Brusca 1980: 407, fig. 26.15a–b (partim). – Kerstitch and Bertsch (2007): 113 (partim)

Ophiura teres . – Ives (1889): 77 (partim)

Material examined

Type material. Holotype: ICML-UNAM 18429; Mexico, Gulf of California, Baja California Sur, Punta Pichilingue (24.285°N, 110.334°W); 0.5–2.5 m; sand under rocks; 7 November 2019; coll. K.J. Humara-Gil, R. Granja-Fernández, B. Maya-Alvarado, A. Hernández-Morales, M.A. Medina-López; preserved in 96% ethanol; COI sequence: OK076949 View Materials (GenBank); 16S sequence: OK078012 View Materials (GenBank). GoogleMaps Paratypes: ICML-UNAM 18430, 5 spec.; same data as holotype; preserved in 96% ethanol (1 used for DNA analysis, labelled as ‘OO4’; COI sequence: OK076948 View Materials [GenBank]; GoogleMaps 16S sequence: OK078011 View Materials [GenBank]). ICML-UNAM 18431, 3 spec.; GoogleMaps same data as holotype; dry. ICML-EMU 12867 , 5 spec.; GoogleMaps same data as holotype; preserved in 96% ethanol. ICML-EMU 12868 , 3 spec.; same data as holotype; dry GoogleMaps .

Additional material. See the Supplemental material.

Diagnosis

Radial shields and adoral shields covered with granules. DAPs divided into two (typically) or more irregular pieces for about 2/3 of the arm. Colour pattern dark grey to brown (fixed specimens), or dark olive green to brown (in vivo specimens); arms with subtle lighter transverse bands, and parallel longitudinal white bands over some median and distal arm segments.

Description

Holotype. DD = 15.9 mm, AL = 49.3 mm. Disc nearly rounded, covered with rounded granules slightly separated from each other; size of the granules increasing from the disc centre towards the periphery. Dorsal disc granule density 52 per mm2. Radial shields covered with granules ( Figure 1 View Figure 1 (a)). Interradii covered with granules similar to those on the dorsal side. Four genital slits per interradius. Proximal genital slits oval, in contact with the distal section of the oral shield and the 1st LAP; reaching up to the 2nd VAP. Distal genital slits oval, twice longer than proximal genital slits, placed between 5th and 7th arm segments; surrounded by granules and, only next to the arm, imbricated scales ( Figure 1 View Figure 1 (b)).

Oral shields broader than long, subtriangular; proximal edges convex forming a rounded apex; rounded lateral edges; straight distal edge. Madreporite with oval, central depression. Adoral shields covered with scattered granules, larger than those on the interradii. Jaws with 7–8 oral papillae: LyOs two times longer than broad, angled upwards; AdShSp the largest, subquadrangular; 2°AdShSp similar to AdShSp, smaller; LOPas 3–4 (rarely two), conical, pointed and elongated, slightly separated; IPa similar in shape to LOPas; TPa 1–2 at the apex of jaw, robust and pointed. Teeth, five: vT robust, subtriangular, larger than TPa; median teeth quadrangular; the dorsalmost the smallest, triangular and pointed. One OPRSp at each side of jaw, large, prominent. Oral plates covered with elongated, separated granules, larger than those on the adoral shields ( Figure 1 View Figure 1 (b–c)).

Five arms flattened, tapering distally; one cut for molecular analysis, one complete probably regenerated from the midsection, one regenerating close to tip, two broken near tip. Dorsal arm base covered with granules and scales located laterally over the first three DAPs ( Figure 1 View Figure 1 (d)). DAPs wider than long, divided typically into two and up to five (only one DAP in all five arms) irregular pieces in approximately 2/3 of the arm; the number of pieces decreases distalwards ( Figure 1 View Figure 1 (d–f)). Distalmost DAPs entire, small, trapezoidal ( Figure 1 View Figure 1 (f)). First VAP small, 1.5 as wide as long, with rounded edges ( Figure 1 View Figure 1 (b)). Subsequent VAPs quadrangular, as long as wide; proximal edge straight, distal edge convex ( Figure 1 View Figure 1 (g–h)). Distalmost VAPs pentagonal, longer than wide ( Figure 1 View Figure 1 (i)). A pair of pores are visible between the two proximalmost VAPs in all five arms ( Figure 1 View Figure 1 (b)). LAPs conspicuous, wider than long. LAP with up to eight arm spines, rarely nine. Arm spine sequence of the longest arm (right side): first 10 segments, 2, 2, 3, 3, 4, 4, 5, 6, 8, 8; 11th– 20th, eight arm spines; 21st–30th, 7–8; 31st–40th, 6–8; 41st–50th, 6–8; 51st–60th, 5–6; 61st– 70th, 3–5; 71st–75th, two. Arm spines conical with blunt tips, length 2/3 of LAP. Dorsalmost arm spine the shortest; ventralmost the longest and more robust, in contact with abradial tentacle scale of the following segment ( Figure 1 View Figure 1 (d–j)). Two tentacle scales; adradial tentacle scale oval, elongated, 2/3 the length of VAP; abradial tentacle scale shorter, 2/3 the length of adradial one, subtriangular ( Figure 1 View Figure 1 (g–h)).

General colouration dark grey (specimen in ethanol; Figure 1 View Figure 1 (k)). Dorsal side: disc dark grey, with scattered white granules ( Figure 1 View Figure 1 (a)). Arms grey with subtle lighter transverse bands; 1–3 adjacent DAPs and LAPs with a pair of parallel white blotches every 3–15 segments in the median and distal arm sections, macroscopically perceived as parallel longitudinal white bands ( Figure 1 View Figure 1 (e, j–k)). Ventral side: interradii dark grey with scattered white granules ( Figure 1 View Figure 1 (b)). Jaws and arms light grey, oral papillae beige ( Figure 1 View Figure 1 (b–c)). Some VAPs with a white spot located randomly ( Figure 1 View Figure 1 (b, h)). LAPs grey, some with a dorsal white blotch. Dorsalmost arm spines grey, becoming clearer to the ventralmost side, with a white spot on the tip ( Figure 1 View Figure 1 (j)).

Paratype and non-type variations

Specimens in the type series varied from 3.7 to 21.1 mm in disc diameter size. The smallest specimen (DD = 3.7 mm, Figure 2 View Figure 2 ) presented a disc granule density of 329 per mm2 ( Figure 2 View Figure 2 (a)). Its jaws were covered with granules, including oral shields ( Figure 2 View Figure 2 (b–c)). Only one oral shield, presumably the madreporite, was partially uncovered; it presented a dark grey colour and a minute depression on the centre ( Figure 2 View Figure 2 (b)). This is consistent with what Ziesenhenne (1955) observed in O. panamense (DD = 4 mm), in which the first oral shield to be uncovered was the madreporite. Distal genital slits only presented granules ( Figure 2 View Figure 2 (b)), in comparison with those of larger specimens which also presented scales ( Figure 1 View Figure 1 (b)). Similarly, the specimen presented only granules on the dorsal arm base, extending over two arm segments ( Figure 2 View Figure 2 (d)). All the DAPs were entire and surrounded by granules ( Figure 2 View Figure 2 (d–e)); distal ones were completely covered with granules ( Figure 2 View Figure 2 (f)). Some proximal VAPs were partially covered with granules ( Figure 2 View Figure 2 (b)), the rest were naked ( Figure 2 View Figure 2 (g–i)). All the LAPs were partially covered with granules and presented up to five arm spines ( Figure 2 View Figure 2 (j)). The presence of granules on the disc, including jaws, and arms (DAPs, VAPs, LAPs) is common among Ophioderma juveniles (e.g. Ophioderma brevicaudum Lütken, 1856 , Clark 1939; O. hendleri Granja-Fernández et al. 2020 ; O. panamense Ziesenhenne 1955 ; Ophioderma rubicundum Lütken, 1856 , Hendler 2018). One arm segment presented three tentacle scales.

As individuals became larger (DD = 5.7 mm), some scales on the dorsal arm base and on the distal genital slits were visible; oral shields started to uncover but still presented some granules, and the maximum number of arm spines increased to six. A specimen of DD = 11.2 mm showed oral shields totally naked, but less elongated than those of the holotype, and its DAPs were mostly entire as in smaller specimens. This specimen, as well as the holotype (DD = 15.9 mm), presented 1–6 abnormal arm spines in some segments along the arms; these were twice as wide as typical and variable in shape (e.g. bifurcated, flattened), mainly present in the proximal and median section of the arm. A specimen of DD = 13.3 mm presented most DAPs divided into two pieces; some median and distal DAPs were entire. Specimens of similar sizes to the holotype or larger (DD = 15.1– 21.1 mm) presented a maximum of nine arm spines, and sporadically three tentacle scales in 1–3 arm segments ( Figure 3 View Figure 3 (e)). One specimen (DD = 17.5 mm) presented some median DAPs entire, which were 2.5 times as wide as long. Regarding colouration, the extent of white granules on the dorsal disc varied in every specimen; they could be: (1) a few scattered on the disc as in the holotype ( Figure 1 View Figure 1 (a)), (2) arranged in larger groups at the margin and centre of the disc ( Figure 2 View Figure 2 (a)), or (3) not present at all; the smallest specimens presented a greater extension of white granules on the dorsal disc ( Figure 2 View Figure 2 (a)). Some specimens presented white-spotted oral shields ( Figure 3 View Figure 3 (b)).

Additionally, some variations were noticed in non-type specimens (DD sizes up to 26.7 mm): (1) 16 specimens presented 1–3 partially naked radial shields ( Figure 3 View Figure 3 (a)); (2) one specimen presented white oral shields with a grey madreporite ( Figure 3 View Figure 3 (c)); (3) 13 specimens (DD = 20.8–25.5 mm) presented up to 11 arm spines; (4) one large specimen (DD = 26.7 mm) presented few granules over the six proximalmost DAPs ( Figure 3 View Figure 3 (d)); (5) eight specimens (DD = 12.9–18.5 mm) presented some abnormal arm spines such as those observed in the holotype and one paratype ( Figure 3 View Figure 3 (f)); (6) some fixed specimens presented a brown general colouration instead of dark grey ( Figure 3 View Figure 3 (g)), while live specimens showed a dark olive green to brown colour ( Figure 3 View Figure 3 (h–i)); and (7) the parallel white blotches on the arms of some specimens were present in up to 18 contiguous DAPs and LAPs.

Of the variations observed in O. occultum sp. nov., features such as partially uncovered radial shields, granules on DAPs of large specimens, abnormal arm spines, and arm segments with three tentacle scales are considered rare, as they were observed only in <7% of the total specimens examined.

Habitat and distribution

Ophioderma occultum sp. nov. is distributed in shallow waters, from intertidal to 6 m depth, and is typically found in rocky reefs and tide pools, on sediment under mediumsized rocks. The species has only been found in the northern Mexican Pacific , particularly in the Gulf of California (in the States of Baja California, Baja California Sur, and Sonora) and islas Revillagigedo (isla Roca Partida). Its northernmost record corresponds to Polilla, Bahía de los Ángeles (29°N) and the southernmost to bahía Eclipse , isla Roca Partida (19°N).

Etymology

The species name occultum (-us, -a) is a Latin singular neuter adjective in the nominative case, meaning ‘hidden’. Ophioderma occultum sp. nov. has been collected and recorded since the nineteenth century in the north of Mexico; however, it was mistakenly considered a colour phase of O. panamense . It remained hidden in plain sight, hence its name.

Remarks

Ophioderma occultum sp. nov. most resembles O. panamense , with which it has been confused for over a century ( Ives 1889; Ziesenhenne 1955; Brusca and Smith 1973; Brusca 1980; Kerstitch and Bertsch 2007). Both species present covered adoral shields, scales on the base of the arms, scales and granules ornamenting the distal genital slit, and banded arms. However, O. panamense has naked radial shields, DAPs usually entire, can display different colours on the dorsal disc (green, brown, grey mottled with yellow, orange, or pink), and its bands on the arms are conspicuous ( Figure 4 View Figure 4 (c); Table 1 View Table 1 ; Granja-Fernández et al. 2014, 2020), while O. occultum sp. nov. presents radial shields usually covered with granules, DAPs divided into two (typically) or more pieces, subtle lighter bands on the arms, and characteristic parallel white blotches on some DAPs and LAPs in the median and distal arm sections, giving the appearance of parallel longitudinal white bands.

Some authors noticed the aforementioned characters but considered them intraspecific variations of O. panamense . Ives (1889, 76–77) mentioned the following:

“Mr. Lockington records a variety with a brown disk and olive green, arms without bands except at the tips” and “The darkest variety of O. teres closely resembles the darkest variety of O. Panamensis [sic]. In both forms some of the upper arm-plates of the outer portion of the arm have white markings, which form segments of two broken parallel longitudinal lines, one on each side of the arm. Each segment occupies one or more plates and is separated from the next segment by an also variable number of plates without any such markings”.

This coincides with the colour pattern observed in O. occultum sp. nov. Likewise, Ziesenhenne (1955, 194) mentioned that O. panamense showed one colour combination

In which the disk is brown to olive and the arms greenish, with the arms banded distally with white... Some specimens have broken upper arm plates.. .. The majority of the specimens of this color phase have concealed, or partially concealed, radial shields.

All of the above are now considered diagnostic characters of O. occultum sp. nov.

Moreover, O. panamense and O. occultum sp. nov. differ in their distributions; the former is widely distributed from southern USA to Peru ( Granja-Fernández et al. 2020), while O. occultum sp. nov. is restricted to the northern Mexican Pacific. In the Gulf of California both species appear to be the most common and abundant Ophioderma , along with O. hendleri . However, O. occultum sp. nov. is most often found in shallower waters (i.e. intertidal and tide pools; RGF, KJHG pers. obs).

The new species was also misidentified as O. teres in the scientific collections and literature ( Ives 1889). Since the holotype of O. teres is lost, we based our comparisons on the original description of the species, as well as on a specimen identified by Lyman as O. teres (MCZ OPH-112). Although this specimen is not a topotype (i.e. it is not from Panama, the type locality of O. teres , but from Acapulco, Mexico), it agrees with the description of O. teres and was included as additional material in Lyman’s first descriptions of the species ( Lyman 1860, 257, 1865, 38). Ophioderma teres differs from O. occultum sp. nov. in having DAPs divided into multiple pieces (up to five pieces), rounded arms, and a speckled colour pattern on disc and arms ( Figure 4 View Figure 4 (j); Table 1 View Table 1 ). Confusions between the two species may arise from the fact that both present divided DAPs, as well as a misinterpretation of O. teres speckles on the arms ( Figure 4 View Figure 4 (j)) as the white blotches present in the median and distal arm sections of O. occultum sp. nov.

Differences among O. occultum sp. nov. and all its congeners from the Eastern Pacific are summarised and illustrated in Table 1 View Table 1 and Figure 4 View Figure 4 .

Genetic delimitation of the species

COI sequences obtained in this study had a total length of 680 bp; however, they were cut to align them with those retrieved from BOLD and GenBank, resulting in the analysis of 426 bp. Of these, 305 (71.60%) sites were conserved and 121 (28.40%) were variable, all parsimony informative. On the other hand, 16S sequences varied from 489 to 500 bp, and the alignment presented several indels. The latter were removed, and 454 bp were analysed. In this case, 328 (74.25%) sites were conserved and 126 (27.75%) were variable, of which 123 (27.09%) were parsimony informative.

DD = 21.1 mm): (d) close-up of a jaw; (e) dorsal arm. Ophioderma peruanum (holotype: CZA-363, DD = 32.5 mm): (f) portion of dorsal disc and arm. Ophioderma sodipallaresi (holotype: ICML-UNAM 3.24 .3, DD = 19.1 mm): (g) portion of dorsal disc and arm; (h) close-up of a jaw; (i) distal genital slit . Ophioderma teres (non-type specimen: MCZ OPH-112 , DD = 24.8 mm): (j) portion of dorsal disc and arm . Ophioderma teres unicolor (holotype: MCZ OPH-114 , DD = 30 mm): (k) portion of dorsal disc and arm . Ophioderma vansyoci (holotype: LACM 90 View Materials – 391.1 View Materials , DD = 31 mm): (l) portion of dorsal disc and arm; (m) close-up of a jaw . Ophioderma variegatum (lectotype: NHMD-107680, DD = 12.4 mm): (n) portion of dorsal disc and arm; (o) close-up of a jaw. See the Supplemental material for more information.

The topology of the trees was consistent between the two genes (COI, 16S) and the two methods (ML, NJ). The main clade grouped the species of Ophioderma , followed by a subclade grouping O. cf. teres unicolor, O. panamense and O. occultum sp. nov., and another subclade grouping the two latter species; both presented high bootstrap values (81–100). Sequences of O. occultum sp. nov. formed a well-supported clade (bootstrap values: 93/100) for both molecular markers with ML and NJ, separating it from the rest of the species. Particularly for COI, BOLD sequences included in the analysis were grouped with O. occultum sp. nov., which confirmed the hypothesis that those specimens were O. occultum sp. nov. and not O. panamense , as suspected based on photographs. According to the analysed data, the sister taxon of O. occultum sp. nov is O. panamense , whereas O. hendleri is the most distantly related to it ( Figure 5 View Figure 5 ).

The estimated mean genetic distances among species were 8.76–19.95% for COI, and 3.16–22.69% for 16S. The lowest genetic distances were recorded between O. occultum sp. nov. and O. panamense , and the greatest between O. occultum sp. nov. and O. fallax (COI) and O. panamense and O. fallax (16S; Table 2 View Table 2 ).