The Mochokidae are a family of African catfishes known commonly as "squeakers" and "upside-down catfishes". These common names refer to some unusual habits of certain members of the large genus Synodontis. The name squeaker refers to the fact that, when agitated, many species in the genus are capable of making a squeaking noise by stridulation of the pectoral spine against the pectoral girdle (Jubb, 1967); stridulation has also been observed in Atopochilus, Chiloglanis, Microsynodontis, and Mochokiella.
Even more peculiar is the habit of some species of Synodontis that are known to swim upside-down. This habit seems to be correlated with feeding while upside-down at the water’s surface (Bishai & Abu Gideiri, 1965), but upside-down catfishes will rest and swim in the inverted position on a regular basis. Chapman et al. (1994) showed that an upside-down posture near the surface also facilitates respiration in poorly oxygenated water. While the genus Synodontis presents the most well-known species with their fascinating behaviors and natural histories, the family is actually much more interesting when taken as a whole.
Mochokid catfishes are currently restricted to the freshwaters of Africa, but are nearly ubiquitous in the habitable waters of the continent. A high degree of morphological diversity allows mochokid catfishes to inhabit some of the fastest flowing streams and cataracts to the widest and deepest stretches of the Congo River. Mochokids also inhabit the massive African rift lakes like Tanganyika, Victoria and Nyasa. The greatest diversity of mochokids almost certainly occurs in the Congo River and its numerous tributaries, but they are also found in many of the rivers and lakes of western Africa, southern Africa, eastern Africa and in the Nile. Like a handful of other catfishes, some mochokids are known to swim in mid-water; other members of the family are primarily benthic. Likewise, some mochokids shoal while others are rather solitary. As a rule they are most active during the night, but they can be found hiding amongst plants, logs and other submerged structure during the day.
Fossil mochokids, of the genus Synodontis, have been found in deposits from eastern and northern Africa dating to at least the early Miocene (at least 20 mya) (Stewart, 2001).Interestingly, fragments of pectoral spines of Synodontis dating from the early Oligocene have been found in Oman, an area where mochokids do not exist today (Otero & Gayet, 2001). Fossil mochokids outside of the genus Synodontis are presently unknown.
In all, nearly 250 species of mochokid catfishes have been described, but a large number of those names are now considered junior synonyms. The number of valid, described species is approaching 200 and several undescribed species are known to exist. This makes the Mochokidae one of the largest families of catfishes and certainly the largest family of African catfishes. The nearly 200 species are placed in only nine genera. The largest genus in the family is Synodontis, with approximately 120 species. The genus Chiloglanis follows with about 45 species and numbers for the remaining genera are rather paltry in comparison. Ferraris (2007) provides the most recent list of mochokid species and synonymies (188 listed as valid). Synodontis acanthoperca Friel & Vigliotta (2006), Chiloglanis productus Ng & Bailey (2006), Synodontis grandiops, S. ilebrevis and S. lucipinnis (Wright & Page, 2006), Atopodontus adriaensi(Friel & Vigliotta, 2008), Synodontis kogonensis and S. ouemeensis (Musschoot & Lalèyè, 2008), Synodontis macropunctata (Wright & Page, 2008), Synodontis ngouniensis (De Weirdt et al., 2008), Synodontis orientalis (Seegers, 2008) and Synodontis woleuensis(Friel & Sullivan, 2008) were described too recently to be included in that work. Wright & Page (2006) also make some notable taxonomic changes, including the resurrection of two species of Synodontis.
Little is known about the ecology of most mochokids. What is known pertains mostly to diet and is biased towards species of Synodontis. Stomach contents from Synodontis have included insects, nematodes, crustaceans, mollusks, annelids, seeds, algae, diatoms, fish scales and, incidentally, sand (Bishai &Abu Gideiri, 1965a; Bishai & Abu Gideiri, 1965b; Winemiller & Kelso-Winemiller, 1996; Sanyanga, 1998). From observations of stomach contents, the diet of most mochokids is probably very similar; that is, they are omnivores. For some of the larger sucker-mouthed species (i.e., Atopochilus and Euchilichthys) the stomach contents contain a high proportion of silt, algae and detritus. For these taxa it seems likely that scraping or grazing is the predominant method of feeding.
Beyond information gleaned from captive breeding of certain species of Synodontis, very little is known about reproduction in mochokids. The best studied mochokids in this regard are most likely Nile River Synodontis (including S. membranacea and S. batensoda,previously placed in other genera). Still, details are limited; the studies indicate that spawning occurs from July to October, which coincides with the flooding season, and that pairs swim in unison during spawning bouts (Bishai & Abu Gideiri, 1968). The most interesting and detailed information on mochokid reproduction relates to a species from Lake Tanganyika, Synodontis multipunctatus, which is a brood parasite of mouth brooding cichlids such as Simochromis and Haplochromis(Sato, 1986; Wisenden, 1999). Adults of this species spawn in the midst of spawning cichlids and the fertilized catfish eggs are taken into the mouth of a cichlid. The catfish eggs hatch first and will eat the host eggs before they leave the host mouth. Most amazingly, this species has been able to parasitize mouth brooding cichlids from South America in captivity (Loiselle, 1998).