Abstract

Sea stars in the echinoderm class Asteroidea possess a complex endoskeleton composed of calcified ossicles.¹ Radiography is an excellent modality for detecting skeletal abnormalities that may not be externally apparent though there are limited published references.² Possessing radial symmetry emanating from a central disk makes labeling of the patient important to accurately identify areas of concern. As some species possess different number of arms, a flexible system of arm enumeration is required. The body surface possessing the oral opening is referred to as the oral (O) surface while the top surface is the aboral (A) surface. Radiographs taken with the sea star in normal body orientation on the radiograph plate would be referred to as an aboral-oral (AO) view, comparable to dorsal-ventral (DV). To identify and enumerate the arms, a label ("M") should be placed between the arms nearest the madreporite, or water pore, which is found on the aboral surface of the central disk. Arms can then be enumerated in a clockwise fashion starting at the madreporite. To identify a specific arm location, use both the absolute arm number and relative position in relation to adjacent arms. For example, a lesion near the central disc on arm 2 on the side closest to arm 1 would be "proximal arm 2 - side 1/2" and if it was near the terminus of arm 2 on the side closest to arm 3 it would be "distal arm 2 - side 2/3". This system is also useful for describing gross lesions.

Acknowledgements

The authors wish to thank the John G. Shedd Aquarium and specifically the dedicated members of the veterinary and husbandry teams in the Departments of Animal Health and Fishes for their support of this work.

References

1.  Blake DB, Elliott DR. Ossicular homologies, systematics and phylogenetic implications of certain North American carboniferous asteroids (Echinodermata). J Paleontol. 2003;77(3):476–489.

2.  Lewbart GA. Invertebrate Medicine. Ames, IA: Blackwell Publishing; 2006:254–255.