Ophiuroidea, Gray, 1840

Goharimanesh, Mona, Stöhr, Sabine, Mirshamsi, Omid, Ghassemzadeh, Fereshteh & Adriaens, Dominique, 2021, Interactive identification key to all brittle star families (Echinodermata; Ophiuroidea) leads to revised morphological descriptions, European Journal of Taxonomy 766 (1), pp. 1-63 : 15-19

Conventional identification key to the families of the Ophiuroidea with respect to the species listed in Table 1

1. Longest arm spines shorter than half a segment ........................................................................ 2

– Longest arm spines between half and one segment long ........................................................ 12

– Longest arm spines between one and two segments long ....................................................... 20

– Longest arm spines longer than two segments ........................................................................ 32

2(1). Arm spines tapering (conical) ( Fig. 5 ) ...................................................................................... 3

– Arm spines flat ( Fig. 5 ) ........................................... Ophiodermatidae Ljungman, 1867 [p. 34]

– Arm spines cylindrical ( Fig. 5 ) .................................................................................................. 4

– Arm spines pointed ( Fig. 5 ) ....................................................................................................... 6

3(2). Arm combs present ( Fig. 1F ); dorsal arm plate present; spines all similar in shape ( Fig. 5 ); arm spines mainly lateral ......................................................... Ophiopyrgidae Perrier, 1893 [p. 25]

– Arm combs absent; dorsal arm plate absent; spines not all similar in shape ( Fig. 2F–H ); arm spines at proximal segments only on ventral side of arms .... Asteronychidae Ljungman, 1867 [p. 19]

4(2). Radial shield scalene (oblique) triangular ( Fig. 1E ) ......... Astrophiuridae Sladen, 1879 [p. 24]

– Radial shield isosceles (mirror-symmetric) triangular to pear-shaped ( Fig. 1F )....................... 5

– Radial shield bar-like ( Fig. 1B ) .................................................. Euryalidae Gray, 1840 [p. 20]

5(4). Arm combs present ( Fig. 1F ); arm spines mainly lateral; tentacle scales more than two times as long as wide; dorsal disc scale/plate without tubercles ( Fig. 2B–D ) .......................................... ........................................................................................... Ophiopyrgidae Perrier, 1893 [p. 25]

– Arm combs absent; arm spines at proximal segments only on ventral side of arms; tentacle scales nearly as long as wide; dorsal disc scale/plate with tubercles ( Fig. 2A ) .................................... .......................................................................................... Hemieuryalidae Verrill, 1899 [p. 40]

6(2). Tentacle scales nearly as long as wide ....................................................................................... 7

– Tentacle scales slightly longer than wide .................................................................................. 8

– Tentacle scales more than two times as long as wide .............................................................. 10

7(6). Tentacle scales shorter than the spine nearby; tentacle scales cross section round; disc pentagonal; dorsal disc with spines ( Fig. 2B–D ) ................... Ophiosphalmidae O’Hara et al., 2018 [p. 22]

– Tentacle scales as long as the spine nearby; tentacle scales cross section oval; disc circular; dorsal disc without spines ( Fig. 1A–B, D–F ) .............................. Astrophiuridae Sladen, 1879 [p. 24]

8(6). Accessory dorsal arm plate present; dorsal disc with obvious primary plates ( Fig. 2D ); adoral shield meeting in front of the oral shield; distance between spine articulations dorsalwards increasing .................................................................... Ophiolepididae Ljungman, 1867 [p. 39]

– Accessory dorsal arm plate absent; dorsal disc with concealed primary plates ( Fig. 1A–D ); adoral shield separated ( Fig. 4 ); distance between spine articulations equal ....................................... 9

9(8). Arm spines predominantly parallel to arm axis (adpressed); integument obscuring plates ( Fig. 1D ); dorsal disc without scales; dorsal disc with spines ( Fig 2B, D ) .................................................. ................................................................................... Ophiopezidae O’Hara et al., 2018 [p. 35]

– Arm spines predominantly erect, standing perpendicular to arm axis; integument not obscuring plates ( Fig. 1E–F ); dorsal disc with scales ( Fig. 2B, D ); dorsal disc without spines ( Fig.1A–B, D–F ) ............................................................................. Ophiomyxidae Ljungman, 1867 [p. 36]

10(6). Arm combs present ( Fig. 1F ); disc circular; genital papillae present; distance between spine articulations dorsalwards increasing.................................. Ophiopyrgidae Perrier, 1893 [p. 25]

– Arm combs absent; disc pentagonal; genital papillae absent; distance between spine articulations equal .........................................................................................................................................11

11(10). Arm spines predominantly parallel to arm axis (adpressed); tentacle scales longer than the spine nearby; tentacle pores visible only on proximal arm; dorsal disc with obvious primary plates ( Fig. 2D ) ................................................................. Ophiomusaidae O’Hara et al., 2018 [p. 23]

– Arm spines predominantly erect, standing perpendicular to arm axis; tentacle scales shorter than the spine nearby; tentacle pores visible along most of the arm; dorsal disc with concealed primary plates ( Fig. 1B–D ) ....................................................... Ophiomyxidae Ljungman, 1867 [p. 36]

12(1). Arm spines predominantly parallel to arm axis (adpressed) ................................................... 13

– Arm spines predominantly erect, standing perpendicular to arm axis .................................... 16

13(12). Tentacle scales nearly as long as wide ..................................................................................... 14

– Tentacle scales slightly longer than wide ................................................................................ 15

– Tentacle scales more than two times as long as wide ....... Ophiopyrgidae Perrier, 1893 [p. 25]

14(13). Arm spines cylindrical ( Fig. 5 ); radial shield isosceles (mirror-symmetric) triangular to pearshaped ( Fig. 1F ); the ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.9–1 ( Fig. 5 ); arm spines at proximal segments only on ventral side of arms ................................................................................. Hemieuryalidae Verrill, 1899 [p. 40]

– Arm spines pointed ( Fig. 5 ); radial shield scalene (oblique) triangular ( Fig. 1F ); the ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6–0.9 ( Fig. 5 ); arm spines mainly lateral ................. Ophioleucidae Matsumoto, 1915 [p. 38]

15(13). Arm combs present ( Fig. 1F ); spines all similar in shape; arm spine cross section round ( Fig. 5 ); integument not obscuring plates ( Fig. 1E–F ) ...... Ophiuridae Müller & Troschel, 1840 [p. 24]

– Arm combs absent; spines not all similar in shape ( Fig. 2F–H ); arm spine cross section oval ( Fig. 5 ); integument obscuring plates ( Fig. 1D ) ......... Ophiernidae O’Hara et al., 2018 [p. 37]

16(12). Integument not obscuring plates ( Fig. 1E–F ); nerve opening approximately as large as muscle opening .................................................................................................................................... 17

– Integument obscuring plates ( Fig. 1D ); nerve opening smaller than muscle opening ........... 18

17(16). Tentacle scales nearly as long as wide; radial shield length between one third and half of the disc radius; arm spine articulation ventralwards increasing in size ................................................... ........................................................................................ Amphiuridae Ljungman, 1867 [p. 44]

– Tentacle scales slightly longer than wide; radial shield length more than half of the disc radius; arm spine articulation larger in middle of arm ............... Ophiactidae Matsumoto, 1915 [p. 46]

18(16). Arm covering without granules/spines/tubercles; arm spine cross section round ( Fig. 5 ); arm spines mainly lateral; genital slit longer than half the length of an interradius ....................... 19

– Arm covering with granules; arm spine cross section oval ( Fig. 5 ); arm spines at proximal segments only on ventral side of arms; genital slit shorter than half the length of an interradius .............................................................................. Gorgonocephalidae Ljungman, 1867 [p. 21]

19(18). Tentacle scales present; dorsal disc with scales ( Fig. 2B, D ); radial shield without granules/spines ( Fig. 1B ); ventral teeth single ( Fig. 4 ) ..................... Ophionereididae Ljungman, 1867 [p. 42]

– Tentacle scales absent; dorsal disc without scales; radial shield with granules ( Fig. 1A ); ventral teeth several (tooth papillae) ( Fig. 4 ) ........................ Ophiobyrsidae Matsumoto, 1915 [p. 29]

20(1). Integument not obscuring plates ( Fig. 1E–F ) .......................................................................... 21

– Integument obscuring plates ( Fig. 1D ) .................................................................................... 26

21(20). Radial shield length between one third and half of the disc radius ......................................... 22

– Radial shield length more than half of the disc radius ............................................................ 25

22(21). Arm combs present ( Fig. 1F ); arm spines predominantly parallel to arm axis (adpressed); tentacle scales slightly longer than wide; ventral teeth several (tooth papillae) ( Fig. 4 ) .......................... ............................................................................. Ophiuridae Müller & Troschel, 1840 [p. 24]

– Arm combs absent; arm spines predominantly erect, standing perpendicular to arm axis; tentacle scales nearly as long as wide; ventral teeth single ( Fig. 4 ) ..................................................... 23

23(22). Accessory dorsal arm plate present; accessory ventral arm plate present; spines not all similar in shape ( Fig. 2F–H ); dorsal and/or ventral edge(s) of LAPs with constriction ( Fig. 2I ) .......... ................................................................................. Ophiopholidae O’Hara et al., 2018 [p. 47]

– Accessory dorsal arm plate absent; accessory ventral arm plate absent; spines all similar in shape; dorsal and/or ventral edge(s) of LAPs without constriction ( Fig. 2J ) .................................... 24

24(23). The number of scales on H line ( Fig. 2C ) of an interradius about 7–14; arm spine articulation larger in middle of arm; dental plate ventral half widest; dental sockets less than 50% of the width .................................................................... Ophiothamnidae O’Hara et al., 2018 [p. 46]

– The number of scales on H line ( Fig. 2C ) of an interradius about 14–21; arm spine articulation ventralwards increasing in size; dental plate dorsal half widest; dental sockets equal to or more than 50% of the width .................................................... Amphiuridae Ljungman, 1867 [p. 44]

25(21). Tentacle scales present; dorsal disc with spines ( Fig. 2B, D ); arm spine articulation larger in middle of arm; nerve opening approximately as large as muscle opening .................................. ........................................................................................ Ophiactidae Matsumoto, 1915 [p. 46]

– Tentacle scales absent; dorsal disc without spines ( Fig. 1A–B, D–F ); arm spine articulation ventralwards increasing in size; nerve opening smaller than muscle opening ............................ .................................................................................. Amphilepididae Matsumoto, 1915 [p. 45]

26(20). Tentacle scales nearly as long as wide ..................................................................................... 27

– Tentacle scales slightly longer than wide ................................................................................ 28

– Tentacle scales more than two times as long as wide .............................................................. 30

27(26). Arm spines flat; the number of scales on H line ( Fig. 2C ) of an interradius more than 21; arm spine cross section oval ( Fig. 5 ); ventral teeth several (tooth papillae) ( Fig. 4 ) ......................... .......................................................................................... Ophiotomidae Paterson, 1985 [p. 30]

– Arm spines pointed ( Fig. 5 ); the number of scales on H line ( Fig. 2C ) of an interradius about 7–14; arm spine cross section round ( Fig. 5 ); ventral teeth single ( Fig. 4 ) ................................. .................................................................................... Ophiacanthidae Ljungman, 1867 [p. 33]

28(26). Arm spines predominantly parallel to arm axis (adpressed); radial shield absent ( Fig. 1C ); ventral interradii with spines; ventro-distal tip of LAP projecting ventralwards ( Fig. 2K ) .................... .............................................................................................. Ophiohelidae Perrier, 1893 [p. 27]

– Arm spines predominantly erect, standing perpendicular to arm axis; radial shield present; ventral interradii with granules; ventro-distal tip of LAP not projecting ventralwards ( Fig. 2L ) ....... 29

29(28). Radial shield scalene (oblique) triangular; the number of scales on H line ( Fig. 2C ) of an interradius more than 21; arm spine cross section oval ( Fig. 5 ); radial shield with granules ..... .......................................................................................... Ophiotomidae Paterson, 1985 [p. 30]

– Radial shield isosceles (mirror-symmetric) triangular to pear-shaped ( Fig. 1F ); the number of scales on H line ( Fig. 2C ) of an interradius about 14–21; arm spine cross section round ( Fig. 5 ); radial shield without granules/spines ( Fig. 1B ) ....... Ophionereididae Ljungman, 1867 [p. 42]

30(26). Tentacle scales shorter than the spine nearby; the number of scales on H line ( Fig. 2C ) of an interradius about 14–21; oral shield covering less than one third of interradius; at least one socket on DP as depression or perforation without septum ................................................................ 31

– Tentacle scales longer than the spine nearby; the number of scales on H line ( Fig. 2C ) of an interradius more than 21; oral shield covering more than one third of the length of an interradius; at least one socket on DP as perforation with septum ................................................................. ...................................................................................... Ophiopsilidae Matsumoto, 1915 [p. 43]

31(30). Arm spine cross section round ( Fig. 5 ); tentacle scales cross section oval; radial shield absent ( Fig. 1C ); ventral interradii without granules/spines .... Ophioscolecidae Lütken, 1869 [p. 28]

– Arm spine cross section oval ( Fig. 5 ); tentacle scales cross section round; radial shield present ( Fig. 1C ); ventral interradii with spines ................ Amphilimnidae O’Hara et al., 2018 [p. 41]

32(1). Radial shield length less than one third of the disc radius ...................................................... 33

– Radial shield length between one third and half of the disc radius ......................................... 35

– Radial shield length more than half of the disc radius ............................................................ 36

33(32). Radial shield scalene (oblique) triangular ( Fig. 1E ); arm spine cross section oval ( Fig. 5 ); integument obscuring plates ( Fig. 1D ); radial shield with granules ( Fig. 1A ) ....................... 34

– Radial shield isosceles (mirror-symmetric) triangular to pear-shaped ( Fig. 1F ); arm spine cross section round ( Fig. 5 ); integument not obscuring plates ( Fig. 1E–F ); radial shield without granules/spines ( Fig. 1B ) ............................................ Ophiomyxidae Ljungman, 1867 [p. 36]

34(33). Adoral shield separated; ventral position of LAP projecting ventro-proximalwards ( Fig. 2K ); disc flat (about same level as arms); lateral oral papillae multiple rows of lateral papillae ........ ...................................................................................... Ophiocomidae Ljungman, 1867 [p. 36]

– Adoral shield meeting in front of the oral shield; ventral position of LAP not projecting ventroproximalwards ( Fig. 2L ); disc high (above arm level); lateral oral papillae single row of lateral papillae ( Fig. 2E ) ............................................................. Ophiotomidae Paterson, 1985 [p. 30]

35(32). Arm covering without granules/spines/tubercles; arm spines pointed ( Fig. 5 ); tentacle scales nearly as long as wide; integument obscuring plates ( Fig. 1D ) .................................................. .................................................................................... Ophiacanthidae Ljungman, 1867 [p. 33]

– Arm covering with tubercles grown out of scales/plates; arm spines cylindrical ( Fig. 5 ); tentacle scales slightly longer than wide; integument not obscuring plates ( Fig. 1E–F ) .......................... .............................................................................. Ophiocamacidae O’Hara et al., 2018 [p. 29]

36(32). Tentacle scales nearly as long as wide ..................................................................................... 37

– Tentacle scales more than two times as long as wide ............................................................. 38

37(36). Accessory ventral arm plate present; tentacle scales cross section round; scales on ventral interradii absent; ventral teeth several (tooth papillae) ( Fig. 4 ) .................................................. .................................................................................. Ophiopteridae O’Hara et al., 2018 [p. 31]

– Accessory ventral arm plate absent; tentacle scales cross section oval; scales on ventral interradii covering the whole area of an interradius; ventral teeth single ( Fig. 4 ) ...................................... .................................................................................... Ophiacanthidae Ljungman, 1867 [p. 33]

38(36). Arm spines flat ( Fig. 5 ); tentacle scales shorter than the spine nearby; arm spine cross section oval ( Fig. 5 ); tentacle scales cross section oval ........ Clarkcomidae O’Hara et al., 2018 [p. 32]

– Arm spines pointed ( Fig. 5 ); tentacle scales as long as the spine nearby; arm spine cross section round ( Fig. 5 ); tentacle scales cross section round ..... Ophiotrichidae Ljungman, 1867 [p. 48]

Figures

Fig. 5. Typical arm spine shape. A. Tapering (conical): distally ⅓ as thick as proximal diameter with a round cross-section; Pointed: distally half as thick as proximal diameter with a round cross-section; Cylindrical: same thickness at both ends with flat/round tip. B. Flat: distally half as thick as proximal diameter with oval cross-section. Red dashed lines show the ⅓ distal cross-section. The letters ʻaʼ and ʻbʼ illustrate the diameter of ⅓ distal and basal cross-sections, respectively. ʻa/bʼ shows the approximate ratio of these diameters.

Fig. 1. Dorsal disc shapes and radial shields in brittle stars as defined for the purpose of this study. A. Flat (Ophioderma sp.). B. High, with bar-like radial shields (Asteronyx loveni Müller & Troschel, 1842 (dry specimen)). C. Sac-like, radial shields absent (Ophiotholia spathifer (Lyman, 1879)). D. Thick skin obscuring scales and plates (Ophiosparte gigas Koehler, 1922). E. Flat, radial shield scalene triangular to half circle (Amphiura chiajei Forbes, 1843). F. Arm combs, radial shields isosceles triangular to pearshaped (Ophiura ophiura (Linnaeus, 1758)).

Fig. 2. SEM images of various brittle star ossicles. A–E. Disc structures.A. Dorsal disc, radial shield with tubercles (Ophiomusa lymani (Wyville-Thomson, 1873)). B. Dorsal disc (Ophiolimna bairdi (Lyman, 1883)). C. Ventral disc (Ophionereis porrecta Lyman, 1860), the interradius is assumed as a triangle and H gives its height. D. Dorsal disc, triangular radial shield (Ophiactis savignyi (Müller & Troschel, 1842)). E. Ventral disc, mouth (Ophioderma sp.). – F–H. Atypical arm spines. F. Umbrella-shaped arm spine (Ophiotholia spathifer (Lyman, 1879)). G. Hook-shaped arm spine (Asteronyx loveni Müller & Troschel, 1842).H. Bristle spines (Ophiernus vallincola Lyman, 1878). – I–L. Lateral arm plates (LAP). I. LAP with constriction (Ophioscolex glacialis Müller & Troschel, 1842). J. Dorsal and ventral part of LAP without constriction (Ophiura ophiura (Linneaus, 1758)). K. Ventral part of LAP projecting ventro-proximalwards (Ophiopallas paradoxa Koehler, 1904). L. Ventral part of LAP not projecting ventro-proximalwards (Ophiosphalma fimbriatum (Koehler, 1922)).

Fig. 4. Oral papillae sensu lato. A. Ventral side of a schematic brittle star. B. One angle of the oral frame includes a dental plate (DP), two oral plates (= half-jaws) (OP), two adoral shields (AdSh), and one oral shield (OSh), positioned from proximal towards distal. The typical position of teeth, tooth papillae, secondary infradental papillae (2IPa), infradental papillae (IPa), buccal scale, secondary adoral shield spine (2AdShSp), adoral shield spine (AdShSp) on DP, OP and AdSh are illustrated, respectively. The presence/ absence, number and shape of each papilla vary among different taxa.