The operculum or gill cover of a bony fish contains a series of four bones, the preoperculum (also known as the preopercle), suboperculum, interoperculum and the operculum, that provide facial support structure and a protective covering for the gills. It bears the preoperculo-mandibular lateral line sensory canal that is utilized for respiration and feeding. The preoperculum is a crescent-shaped structure that has a series of ridges directed posterodorsally to the organisms canal pores. The preoperculum can be located through an exposed condyle that is present immediately under its ventral margin. It is posterior of the the operculum, suboperculum, and interoperculum. It is typically boomerang or L-shaped, with the horizontal limb overlying the interopercle and the vertical limb overlying the opercle. The preoperculum may bear spines or fine teeth.

There are 8 known Sand Perches of the Diplectrum Genus, found in Mexican waters of the Pacific Ocean.  7 are presented in this website and can be found via links within the Bass (Sea Bass) Family or Serranidae Family. They are exceedingly difficult to identify, however, the shape of their preoperculum can greatly help with this task. The preoperculum is 1 of 4 fused bones of the operculum or gill cover, which is found just in front of the operculum opening and is readily apparent in Sand Perches. The rear edge of the operculum marks the division between the head and the body and has a flexible, ribbed structure that can be sealed to prevent reverse water flow during respiration.

Sand Perch Preoperculums for the ONE known Sand Perch of the Atlantic and SEVEN known Sand Perches of the Pacific are as follows:

FROM THE ATLANTIC (1):

FROM THE PACIFIC (7):