Non-destructive morphological observations of the fleshy brittle star, Asteronyx loveni using micro-computed tomography (Echinodermata, Ophiuroidea, Euryalida)

Abstract

The first morphological observation of a euryalid brittle star, Asteronyx loveni, using non-destructive X-ray micro-computed tomography (µCT) was performed. The body of euryalids is covered by thick skin, and it is very difficult to observe the ossicles without dissolving the skin. Computed tomography with micrometer resolution (approximately 4.5-15.4 µm) was used to construct 3D images of skeletal ossicles and soft tissues in the ophiuroid's body. Shape and positional arrangement of taxonomically important ossicles were clearly observed without any damage to the body. Detailed pathways inside the vertebral ossicles, lateral arm plates, and arm spines for passage of nerves and water vascular structures were observed. Inter-vertebral muscles were also observed. Forms and 3D arrangements of many important taxonomical characters of the euryalids were scrutinized by µCT in high enough resolution for taxonomic description of ophiuroids.

Keywords: Anatomy; Asteronychidae; Euryalida; computed tomography; soft tissue; taxonomy.

Figure 1.

Microscopic (A, D, G), µCT volume rendered (B) and µCT surface rendered (C, E, F, H) images of the entire body of Asteronyx loveni (NSMT E-6986). A–C whole animal, oral view. Black arrow heads indicate longitudinal dorsal midlines of vertebrae and white arrows indicate external ossicles embedded in skin D, E whole animal, aboral view F mouth frame, virtually dissecting aboral view, peristomial plates are colored green G, H disc, aboral view. White arrow heads indicate external ossicles and black arrows indicate positions where components of plate shaped ossicles of the radial shields are layered. Oral side of bodies were virtually dissected (H). Abbreviations: RS, radial shields; V, vertebra.

Figure 2.

Microscopic (A, C, E) and µCT surface rendered (B, D, F) images of the entire body of Asteronyx loveni (NSMT E-6986). A, B a part of disc, oral lateral view C, D basal part of an arm, oral lateral view E oral disc, oral view. An arrow in E indicates the orientation of observation of F; F two jaws, lateral view. The other jaws are virtually dissected. Parallel teeth are indicated by numeral with apostrophe or double apostrophe. Abbreviations: ABG, abradial plate; ADG, adradial plate; ASH, adoral shield; ASP, arm spine; GSL, genital slit; LAP, lateral arm plate; M, madreporite; OPA, oral papilla; OPL, oral plate; RS, radial shield; TC, tentacle; TO, tooth; TP, tentacle pore; V, vertebra; VAP, ventral arm plate.

Figure 3.

Micro CT surface rendered (A) and 2D section (B–D) images of the entire body of Asteronyx loveni (NSMT E-6986). A oral view, section planes of figures B–D are shown by dotted tetragons B disc and basal part of an arm, horizontal section, a white arrow head indicates vertical crack of a dental plate, a black arrow head indicates the indistinct border of a 1st vertebrae and an oral plate C disc and basal part of an arm, vertical section of central disc D disc and basal part of an arm, vertical section of abradial disc. Abbreviations: AMD, adradial muscular depression; ABG, abradial plate; ADG, adradial plate; ASH, adoral shield; DP, dental plate; ES, external skin; LAP, lateral arm plate; OPL, oral plate; RNC, radial nerve canal; RS, radial shield; RWC, radial water canal; S, stomach; TO, tooth; V, vertebra; VAP, ventral arm plate.

Figure 4.

Microscopic (A), µCT volume rendered (B) and surface rendered (C, D) images of basal part of arm (7–13th arm segments) of Asteronyx loveni (NSMT E-5638). Oral view. Upper and lower sides indicate distal and basal orientation, respectively. Oral side is virtually dissected in D Inter-vertebral muscles are colored purple in D Black arrow heads in B and D indicate identical vertebra, and ST and AS labels also indicates the identical characters in A, B and C. Abbreviations: ASP, arm spine; LAP, lateral arm plate; ST, secondary teeth.

Figure 5.

Micro CT surface rendered (A) and 2D section (B–G) images of the basal part of arm (7–13th arm segments) of Asteronyx loveni (NSMT E-5638). A basal lateral view. Section planes of figures B–G are shown by dotted tetragons B vertical longitudinal section, adradial side of the arm C, D horizontal longitudinal section, aboral view C is set on more aboral side than D horizontal longitudinal section, oral view E–G transverse vertical sections from basal (E) to distal (G) arm, basal view. Vertebrae, lateral arm plates, arm spines and muscles are colored red, blue, yellow and purple, respectively. Red and blue arrow heads (B–G) indicate radial nerve canals and radial water canals, respectively. Arrows indicate the orientations (ab, aboral; ba, basal; dis, distal; or, oral). Abbreviations: E, epidermis; EO, external ossicle.

Figure 6.

SEM (A), µCT surface rendering (B, C) and 2D section (D–O) images of the isolated vertebral ossicles of Asteronyx loveni (NSMT E-5638). A, B basal view, arrow heads indicate oral groove C oral view D–O transverse vertical sections from basal (D) to distal (O) arm, basal view. Arrow heads indicate radial water canals (D–I) and radial nerve canals (K–O). Arrows indicate the orientations (ab, aboral; ba, basal; dis, distal; or, oral). Abbreviations: RNC, pore of radial nerve canal; RWC, pore of radial water canal.