Article © Heok Hee Ng, uploaded January 01, 2002.
Fig. 1. From left to right: oral suckers of Astroblepus (Astroblepidae), Hypostomus (Loricariidae) and Euchilichthys (Mochokidae)
Catfishes have come to occupy many different kinds of freshwater habitats, including torrential hillstreams. A torrential hillstream is not an easy habitat to reside in. Although the problem of dissolved oxygen (a hillstream is replete with dissolved oxygen) does not exist, the overriding problem is one of maintaining station amidst the strong current.
The body shapes of catfishes in general do not lend themselves well to adapting to living in a strong current. Most other fishes that do so have highly compressed bodies and are strong swimmers. While some major catfish groups (e.g. Siluridae) have laterally compressed bodies, they are not found in fast flowing habitats (the singular exception being Pterocryptis, which has a more eel-like body; more of that later). Nevertheless, there are a few catfishes that have laterally compressed bodies and live in areas of strong current (e.g. Batasio, Gagata). A few catfish groups have evolved an anguilliform (i.e. eel-like) body form for living amongst the crevices of rocks at the bottom of fast flowing streams (e.g. Olyra, amblycipitids, Pterocryptis), very much like that of some loaches. But overwhelmingly, the general depressed body form of the catfishes means that additional help is required in order to maintain station against the current (although it should also be mentioned that the depressed body form in itself facilitates the use of what is known as the boundary effect, whereby the water flow is slower the nearer it approaches the stream bed; it is for this reason that a few catfishes that live in areas of very strong current, e.g. glyptosternines, have very strongly depressed bodies).
Fig. 2. Ventral view of Oreoglanis, showing modifications to paired fins and maxillary barbels to form a large sucker
Fig. 3. Lateral view of Exostoma, showing the depressed body form
Adhesion mechanisms fall into two broad classes: friction type mechanisms and suction type mechanisms. The former involves increasing friction with the surface via numerous tiny projections (unculi) on the surface of the skin, and is usually associated with unique pleats of skin especially modified for this purpose. Suction type mechanisms involve the formation of a sucker (an enclosed area within which water pressure is reduced and the subsequent difference in water pressure holds the fish tightly against the substrate) can be further subdivided into active and passive suction mechanisms. In active suction mechanisms (oral suckers), the pressure is continuously maintained, while in passive mechanisms (paired fin modification), it need not be. A brief overview of adhesion mechanisms used by catfishes to maintain themselves against strong water flow is given below.
One of the more prominent and most common mechanisms utilized by catfishes is the modification of the mouth into an oral sucker (it is the most common because members of the Loricariidae, one of the largest catfish families, all possess oral suckers). The oral sucker is basically formed from the enlargement and fusion of the upper and lower lips into a disc (Fig. 1), and is seen in members of the Astroblepidae, Loricariidae and Mochokidae (Chiloglaninae). Suction pressure is provided from within the oral cavity. Since the mouth is integral to the respiratory movements of the fish (it is through which water is drawn into the gill chamber), a separate inhalant opening through which water enters the gills is generally present (e.g. in the Astroblepidae). This frees up the mouth from involvement in breathing.
Modifications of paired fins and barbels
Paired fins can be modified to form suckers (the most ubiquitous example in fishes being the sucker of gobies formed from modified pelvic fins). This usually involves enlargement and in some cases, fusion. The modification of paired fins into a sucker has independently evolved several times within bony fishes, being seen in gobies, scorpion fishes, catfishes and balitorine loaches, to name a few groups. In catfishes, the modification of paired fins into a sucker is seen only in glyptosternine catfishes (e.g. Euchiloglanis, Exostoma, Oreoglanis), a group than inhabits the mountainous regions throughout southern and eastern Asia. The modifications involve enlargement of the paired fins and the maxillary barbels, such that they enclose an area consisting chiefly of the abdomen and the head (Fig. 2), which is flat (the lower lip is greatly flattened); this area functions like a large suction cup. The first fin rays of the pectoral and pelvic fins are greatly thickened and striated ventrally (Fig. 2) as are the ventral surfaces of the flattened maxillary barbels, which then act as the edge of the suction apparatus (the striations most likely help to maintain friction and a good seal for the suction apparatus). As mentioned previously, glyptosternines are typically greatly depressed (Fig. 3); this makes use of the boundary layer effect and in combination with the suction apparatus, makes for a fish that is able to cling on even in the strongest of torrents.
Fig. 4. Thoracic adhesive apparatus of Glyptothorax (left) and Pseudecheneis (right)
Thoracic adhesive apparatus
One of most extreme manifestations of a friction type adhesive apparatus is the thoracic adhesive apparatus seen in many sisorid (and some erethistid) catfishes. This involves numerous pleats of skin arranged (either longitudinally or transversely) in an elliptical field in the thoracic (breast) region of the fish (Fig. 4). The pleats of skin are densely covered with small projections (unculi), which aid in increasing friction (somewhat like the fin hairs on the feet of a gecko), thus enabling the fish cling on to the substrate. It is interesting to note that the friction type mechanism does not function well when the water is full of suspended particles. This has been duly observed by Indian fishermen, who note that the silty water accompanying the first flush of the monsoons usually yield plenty of sisorids (most notably Glyptothorax). They attribute the silt in the water to cause these "stone-sucking fish" to loosen their grip and be flushed with the current, which makes for easier capture.
Fig. 5. Ventral view of Pseudecheneis sympelvica
While not fully effective adhesion mechanisms per se, dorsal and pectoral spines can sometimes help a catfish in maintaining station against a current. This can be done when the catfish wedges itself tightly against a crevice by using the spines. The serrations on the spines may also aid in snagging the catfish against vegetation (it has been noted in some species of Hara that the fish rest amongst dense thickets of vegetation with the spines extended).
It should be noted that more than one mechanism may be utilized, and this has occurred for at least one species of catfish, Pseudecheneis sympelvica. This species has a thoracic adhesive apparatus consisting of transverse pleats of skin typical for the genus, but also has a suction apparatus formed from the fusion of the pelvic fins (Fig. 5).
As can be seen from the examples above, a strong current is no deterrent to the exploitation of hillstream habitats by catfishes. The adhesion mechanisms they have evolved span practically the entire gamut known in bony fishes, and is further testament to the adaptability of this group.
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