been reported in the region. Internal anatomy In many catfish, the "humeral process" is a bony process extending backward from the pectoral girdle immediately above the base of the pectoral fin. It lies beneath the skin, where its outline may be determined by dissecting the skin or probing with a needle. The retinae of catfish are composed of single cones and large rods. Many catfish have a tapetum lucidum, which may help enhance photon capture and increase low-light sensitivity. Double cones, though present in most teleosts, are absent from catfish. The anatomical organization of the testis in catfish is variable among the families of catfish, but the majority of them present fringed testis: Ictaluridae, Claridae, Auchenipteridae, Doradidae, Pimelodidae, and Pseudopimelodidae. In the testes of some species of Siluriformes, organs and structures such as a spermatogenic cranial region and a secretory caudal region are observed, in addition to the presence of seminal vesicles in the caudal region. The total number of fringes and their length are different in the caudal and cranial portions between species. Fringes of the caudal region may present tubules, in which the lumen is filled by secretion and spermatozoa. Spermatocysts are formed from cytoplasmic extensions of Sertoli cells; the release of spermatozoa is allowed by breaking of the cyst walls. The occurrence of seminal vesicles, in spite of their interspecific variability in size, gross morphology, and function, has not been related to the mode of fertilization. They are typically paired, multichambered, and connected with the sperm duct, and have been reported to play glandular and storage functions. Seminal vesicle secretion may include steroids and steroid glucuronides, with hormonal and pheromonal functions, but it appears to be primarily constituted of mucoproteins, acid mucopolysaccharides, and phospholipids. Fish ovaries may be of two types - gymnovarian or cystovarian. In the first type, the oocytes are released directly into the coelomic cavity and then eliminated. In the second type, the oocytes are conveyed to the exterior through the oviduct. Many catfish are cystovarian in type, including Pseudoplatystoma corruscans, P. fasciatum, Lophiosilurus alexandri, and Loricaria lentiginosa. Communication Catfish can produce different types of sounds and also have well-developed auditory reception used to discriminate between sounds with different pitches and velocities. They are also able to determine the distance of the sound's origin and from what direction it originated. This is a very important fish communication mechanism, especially during agonistic and distress behaviors.