Evolution of Sensory Systems: A Comparison of Antarctic and Deep-Sea Ichthyofauna
335
morphological variation seen in the Antarctic fishes. At one extreme the
superficial neuromasts of deep-sea angler fishes are elevated on papillae.
At the other extreme the entire head surface of some deep-sea fish is
covered with wide lateral line canals with membranous coverings. Less
elaborate semimembranous canals have been reported in some icefish.
A final note on mechanosensory camouflage is that the repeated
development of a tapered profile at the posterior end of deep-sea fish
prompts the suggestion that this body form and swimming mode is a
mechanosensory camouflage [1]. It is likely that the wake generated by
this tail form will be much less than the vortex trail generated by a
standard homocercal tail. Notothenioid fishes typically swim using the
labriform mode. There is a possibility that this generates less of a wake
than standard subcarangiform swimming, but this adaptive hypothesis has
to be considered against the alternative that labriform swimming is simply
a retained ancient phylogenetic trait in notothenioid fishes.
Touch
Little is known of touch or somatosensory systems in Antarctic or deep-sea
fish. In general terms it would be surprising if, as the possibilities for
vision declined, touch did not become relatively more important. In the
deep-sea fauna, hints of this are seen in the elaborate extended fin rays of
many species, such as tripod fishes, and in the common occurrence of
mental (attached to the chin) barbels. Somatosensory mediated behaviors
do not necessarily require obvious morphological specializations. For
example, several Antarctic benthic feeders show stereotypical responses to
prey touching their pelvic or anal fins [20]. In response to a touch, the fish
repositioned the head to above where the prey collided with the fin. This
would bring the lateral line sense organs into a position where they could
detect hydromechanical stimuli from the prey. Mental barbels occur in all
Antarctic plunderfish (Artedidraconidae) (Fig. 2C) and Janssen et al. [21]
have shown that the barbel is used as a lure and that touching the lure
initiates a predatory strike.
Although not strictly touch-related, there is a striking parallel between
the icefish Chionodraco hamatus and the tripod fishes with respect to
elongated fin rays. Tripod fishes (Bathypterois spp.) sit motionless,
supported by three stiff elongate fin rays [9], two modified pelvic fin rays,
and one elongate ray from the ventral caudal fin. They face upstream
taking zooplankton brought to them by the current. Chionodraco, which is
piscivorous, could be described as a bipod fish, since it has been observed
sitting on its elongate pelvic fins (Fig. 2D). Marshall [9 p. 305] drew
attention to the similarities between tripod fish and the icefish Pagetopsis
335
morphological variation seen in the Antarctic fishes. At one extreme the
superficial neuromasts of deep-sea angler fishes are elevated on papillae.
At the other extreme the entire head surface of some deep-sea fish is
covered with wide lateral line canals with membranous coverings. Less
elaborate semimembranous canals have been reported in some icefish.
A final note on mechanosensory camouflage is that the repeated
development of a tapered profile at the posterior end of deep-sea fish
prompts the suggestion that this body form and swimming mode is a
mechanosensory camouflage [1]. It is likely that the wake generated by
this tail form will be much less than the vortex trail generated by a
standard homocercal tail. Notothenioid fishes typically swim using the
labriform mode. There is a possibility that this generates less of a wake
than standard subcarangiform swimming, but this adaptive hypothesis has
to be considered against the alternative that labriform swimming is simply
a retained ancient phylogenetic trait in notothenioid fishes.
Touch
Little is known of touch or somatosensory systems in Antarctic or deep-sea
fish. In general terms it would be surprising if, as the possibilities for
vision declined, touch did not become relatively more important. In the
deep-sea fauna, hints of this are seen in the elaborate extended fin rays of
many species, such as tripod fishes, and in the common occurrence of
mental (attached to the chin) barbels. Somatosensory mediated behaviors
do not necessarily require obvious morphological specializations. For
example, several Antarctic benthic feeders show stereotypical responses to
prey touching their pelvic or anal fins [20]. In response to a touch, the fish
repositioned the head to above where the prey collided with the fin. This
would bring the lateral line sense organs into a position where they could
detect hydromechanical stimuli from the prey. Mental barbels occur in all
Antarctic plunderfish (Artedidraconidae) (Fig. 2C) and Janssen et al. [21]
have shown that the barbel is used as a lure and that touching the lure
initiates a predatory strike.
Although not strictly touch-related, there is a striking parallel between
the icefish Chionodraco hamatus and the tripod fishes with respect to
elongated fin rays. Tripod fishes (Bathypterois spp.) sit motionless,
supported by three stiff elongate fin rays [9], two modified pelvic fin rays,
and one elongate ray from the ventral caudal fin. They face upstream
taking zooplankton brought to them by the current. Chionodraco, which is
piscivorous, could be described as a bipod fish, since it has been observed
sitting on its elongate pelvic fins (Fig. 2D). Marshall [9 p. 305] drew
attention to the similarities between tripod fish and the icefish Pagetopsis