CONTRIBUTIONS TO THE FUNCTIONAL MORPHOLOGY OF FISHES PART II. THE FEEDING MECHANISM OF THE ANGLER-FISH, LOPHIUS PISCATORIUS LINNAEUS J. G. FIELD Zoology Department, University of Cape Town (Present address: C.S.I.R. Oceanographic Research Unit, U.C.T.) INTRODUCTION Lophius piscatorius Linnaeus is a species of wide distribution. It is commonly trawled from depths of ten fathoms and more around the South Mrican coast and is also trawled off the coast of Europe. Fossils of the genus Lophius date back to the Upper Eocene (Gregory 1951). It is included in the family Lophiidae of the order Pediculati by Regan (Lophiiformes of Berg). The mode of life of Lophius is quite well known.)t lies still on the sea-bed (usually mud) camouflaged by its dark green to dark grey colour, its flat shape and by processes of skin which hang out like pieces of cloth around the edges of its flat ventral surface. By patient angling by means of its 'lure' (il/icium) prey is attracted to its mouth and swallowed. Chadwick (1929) and Wilson (1937) have observed the living fish in an aquarium. The latter author reviews the knowledge of the feeding behaviour up to his time. The only work . ) 0 which directly mentions the feeding mechanism is that of van Dobben (1935). This author 1 0 describes only the principle of the mechanism for the protrusion of the upper jaw. His opinion 2 d of the muscular action bringing about the protrusion is criticised below. Van Dobben mentions e t a the special skull-vertebral articulation. The only other aspect of the feeding mechanism dealt d ( r with by him is the .principle behind the sucking-in of water during opening of the mouth. e h s This is dealt with fully below and criticised. i l b u P MATERIALS e h t y b Two trawled Lophius were used for dissection. The first, 70 em. long, was presumably adult as d e only the largest reach four feet (120 em.) (Smith 1953). This was dissected fresh in order to t n a judge the extent of movement of the parts. The second fish, only 45 ~. long was preserved r g in formalin and then dissected to confirm the observations made on the first. All measurements e c n given are those of the larger specimen. e c i l r e GENERAL DESCRIPTION d n u y Lophius has a wide dorso-ventrally flattened head, about as wide as it is long. Medially are a w e borne three modified dorsal fin rays, the anterior one being called the 'lure' or illicium (Fig. O. t a G t Zo%gica Africana 2 (I): 45-67 45 e n i b a S y b d e c u d o r p e R 46 ZOOLOGICA AFRICANA VOL 2 - . ) 0 1 0 2 d e scale. 5cm t a d ( 1. r Fig. Lophius piscatorius in its 'angling' position. Note the limb-like pectoral fins and the opening e h of the opercular tube which can be seen behind the left pectoral fin. s i l b u P e h The flattened and widened skull forms a dorsal boundary to the head. Wide upper and t y lower jaws form the antero-lateral border. The eyes are dorsally directed and set about 5 em. b d apart. The mouth is also directed upwards and when the mouth is fully opened the animal e t n cannot see directly ahead. a r g The opercular apparatus forms the dorso-Iateral border of the head. Behind this lies the e c dorsal and posterior part of the pectoral girdle. Behind the girdle and in the axis of the fin n e c i is situated the opercular opening. This is the aperture of an elongated opercular chamber l r e passing backward from the gills under the medial surface of the opercular bones, across the d n lateral surface of the cleithrum and below the pectoral fin. Its muscular wall is supported u y ventrally by greatly elongated branchiostegal rays projecting from the hyoid arch. a w The ventral surface of the fish is completely flat. All the lower portions of the arches e t a (mandibular, hyoid and branchial) and also the lower part ofthe pectoral girdle lie in forwardly­ G t directed U's flat on the rsubstratum. The pelvic girdle is attached to the medial, anteriorly- e n i b a S y b d e c u d o r p e R 1966 FIELD: FEEDING MECHANISM OF LOPHIUS 47 directed part of the pectoral girdle. It supports two muscular, very leg-like pelvic fins. The lateral extremities of the pectoral girdle bear pectoral fins which are also muscular and some­ what leg-like. Both sets of fins are believed to lever the fish up during the capture of prey (Wilson 1937; Gregory 1933). The thick, fleshy tail contributes much muscle to the operation of the feeding mechanism. THE STRUCTURE OF THE 'LURE' AND ITS MECHANISM (Fig. 2) Lophius has three dorsal fin rays which have migrated forwards to take up a position on top of the skull. The first two rays share a common supporting plate which is partly bone and partly, posteriorly, cartilage. It is possible that even the larger specimen was not fully adult and the basal plate may become completely ossified in older specimens. The ray used in 'angling', the illicium, the first of the three, is attached at the anterior end of the basal plate called by Gregory (1951) the pterygiophore. The flexible illicium forms at its base an almost complete ring of bone, the bottom of the ring being completed by a ligament. The ring fits through a hole in the median dorsal ridge of the pterygiophore. The articulation provides freedom for almost 180 0 of rotation in an anterior-posterior arc so that the illicium can be moved in any direction from back flat against the head to directly forwards. It also allows considerable lateral movement. The second dorsal ray has a slightly different articlJlation. Its proximal end bears two . ) prongs which fit on either side of the dorsal ridge on the pterygiophore. The ray is held in 0 1 0 position by connective tissue and also the muscles which move it. The same type of articulation 2 d is found in all the other dorsal rays. e t 0 a The third ray can be erected to about 30 only from its position flat on the skull. It is d ( supported on a separate dorsal plate. This suggests that the pterygiophore is formed by the r e h fusion of two basal plates. s i l b Both the pterygiophore and the ascending processes of the premaxillae (see p. 51) rest u P in a longitudinal trough on the dorsal surface of the skull. The trough is bounded laterally by e h serrate ridges of the frontal bones and it is floored by the frontals and the vomer. The anterior t y part of the pterygiophore overlaps the ascending processes above a groove between the right b d e and left processes. The basal plate of the third dorsal ray is situated above the otic region of t n the skull. a r g e c Muscles n e c i The pterygiophore is operated by two pairs of muscles. A posterior pair of retractors l r e originates in the trough of the skull on either side of the third ray and inserts by a tendon onto d n the pterygiophore on either side of the second ray. Each of a very thin lateral pair of muscles u y originates on the skull on the medial side of the ridge formed by the frontal and runs obliquely a w backwards towards the midline to insert on the underside of the pterygiophore about 1 em. e t a behind the second ray. These lateral muscles probably function as protractors. Connective G t tissue strands anchor the anterior end of the pterygiophore to the underlying connective e n i b a S y b d e c u d o r p a.A.4 e R 48 ZOOLOGICA AFRICANA VOL 2 M. a b due t. 2 ILLICIUM SPINA L NERVES pte r. A ) 0 1 0 2 rd I L LI C I U M 2 nd RAY 3 RAY d e t a ~t£:S;:~ d ( r B e '~ h s i PTERYGIOPHORE l b u P e h t y b d e t M. erector i 1. M. retractor i I. n ~-'-~ a C r g e c 5 c m n scale: e c i l r Fig. 2. ILLICIUM AND DORSAL RAYS. e d A. Latero-dorsal view of the angling apparatus. The left pterygiophore retractor muscle has been cut n u to show the nerves of the left side. The third dorsal ray has been erected more than is normal in the y a living fish. w e t B. Dorsal view of the first three rays and their basal plates with all the muscles removed. a G C. Lateral view of the illicial muscles after removal of the other muscles. t e For abbreviations see p. 65. n i b a S y b d e c u d o r p e R 1966 FIELD: FEEDING MECHANISM OF LOPHIUS 49 tissue sheet above the ascending processes of the premaxillae. Movement of the pterygiophore is quite independent of that of the upper jaw. The second and third rays are moved by three pairs of muscles in the same manner as the normal dorsal fin-rays of teleosts (Gregory 1951).