J_ID: JMOR Wiley Ed. Ref. No.: 1697 Customer A_ID: JMOR10551 Date: 15-JUNE-07 Stage: I Page: 1 JOURNAL OF MORPHOLOGY 00:000–000 (2007) 1 61 2 62 3 63 4 Functional Morphology of the Sonic Apparatus in the 64 5 65 6 Fawn Cusk-eel Lepophidium profundorum (Gill, 1863) 66 7 67 8 Michael L. Fine,1* Hsung Lin,1 Brian B. Nguyen,1 Rodney A. Rountree,2 68 9 Timothy M. Cameron,3 and Eric Parmentier4 69 10 70 11 1 Department of Biology, Virginia Commonwealth University, Richmond, Virginia 23284-2012 71 12 2 72 Marine Ecology and Technology Applications, 23 Joshua Lane, Waquoit, Massachusetts 02536 13 3 73 Department of Mechanical Engineering, Kettering University, Flint, Michigan 48504 14 4 74 Laboratoire de Morphologie Fonctionelle et Evolutive, Universite´ de Lie`ge, B-4000 Lie`ge, Belgium 15 75 16 76 17 ABSTRACT Recent reports of high frequency sound man, mormyrids, gurnards and searobins), the production by cusk-eels cannot be explained adequately 77 18 muscle contraction rate sets the fundamental fre- 78 by known mechanisms, i.e., a forced response driven by quency (Skoglund, 1961; Crawford and Huang, 19 fast sonic muscles on the swimbladder. Time to complete 79 20 1999; Bass and McKibben, 2003; Amorim and 80 a contraction-relaxation cycle places a ceiling on fre- Hawkins, 2005); i.e., simultaneous contraction of 21 quency and is unlikely to explain sounds with dominant 81 22 frequencies above 1 kHz. We investigated sonic morphol- paired sonic muscles at 200 Hz will drive a har- 82 23 ogy in the fawn cusk-eel Lepophidium profundorum to monic sound with a fundamental frequency of 200 83 24 determine morphology potentially associated with high Hz. The northern searobin alternates muscle con- 84 25 frequency sound production and quantified development traction so that the fundamental frequency is dou- 85 26 and sexual dimorphism of sonic structures. Unlike other ble the rate of individual contraction (Bass and 86 27 sonic systems in fishes in which muscle relaxation is Baker, 1991; Connaughton, 2004). More commonly, caused by internal pressure or swimbladder elasticity, 87 28 fish sounds are produced as a series of short-dura- 88 this system utilizes antagonistic pairs of muscles: ven- tion wide-band pulses (Winn, 1964). In weakfish 29 tral and intermediate muscles pull the winglike process 89 30 (family Sciaenidae), each sound pulse is driven by 90 and swimbladder forward and pivot the neural arch a single muscle twitch, and the dominant fre- 31 (neural rocker) above the first vertebra backward. This 91 32 action stretches a fenestra in the swimbladder wall and quency appears to be determined by timing char- 92 33 imparts strain energy to epineural ribs, tendons and lig- acteristics of the sonic muscle twitch rather than 93 34 aments connected to the anterior swimbladder. Rela- bladder resonance (Connaughton et al., 2002) sug- 94 35 tively short antagonistic dorsal and dorsomedial muscles gesting a similar mechanism to multicontraction 95 36 pull on the neural rocker, releasing strain energy, and tonal sounds. Rather than waiting for the contrac- 96 37 use a lever advantage to restore the winglike process tion of a muscle antagonist, muscle relaxation and and swimbladder to their resting position. Sonic compo- 97 38 hence the relaxation component of a twitch sound 98 nents grow isometrically and are typically larger in (Fine et al., 2001) is caused by bladder elasticity or 39 males although the tiny intermediate muscles are larger 99 40 pressure built up in the bladder by contraction of 100 in females. Although external morphology is relatively sonic muscles. Dispensing with antagonist contrac- 41 conservative in ophidiids, sonic morphology is extremely 101 42 variable within the family. J. Morphol. 000:000–000, tion removes the inertia that is otherwise involved 102 in a change of muscle direction necessary to com- 43 2007. Ó 2007 Wiley-Liss, Inc. 103 44 plete a sound cycle. Therefore, the timing of the 104 45 KEY WORDS: sonic muscle; Ophidiiformes; swimbladder; muscle contraction–relaxation cycle typically pla- 105 46 sound production; antagonistic muscles; acoustic commu- ces a ceiling on the maximum fundamental in lon- 106 47 nication; pivot joint ger-duration tonal sounds or dominant frequency 107 48 in single-twitch sounds. 108 49 Most fishes that use sonic swimbladder muscles 109 50 The frequency spectrum of fish sounds produced for sound production vocalize at relatively low fre- 110 51 by swimbladder vibration is determined as a forced quencies that fit the forced-twitch model. For 111 52 response to sonic muscle contraction and swim- example, sonic muscles in the oyster toadfish can 112 53 bladder properties (Sprague, 2000; Fine et al., 113 54 2001, 2004). The requirement for sonic muscles to 114 55 drive each sound cycle has selected for extreme *Correspondence to: Michael L. Fine, Department of Biology, 115 56 speed (Fine et al., 2001), and fish sonic swimblad- Virginia Commonwealth University, Richmond, VA 23284-2012. 116 57 der muscles are considered the fastest muscles in E-mail: mfi[email protected] 117 58 118 vertebrates (Tavolga, 1964; Rome and Linstedt, Published online 00 Month 2007 in 59 1998; Ladich and Fine, 2006). In fishes that make Wiley InterScience (www. interscience.wiley.com) 119 60 long duration tonal sounds (e.g., toadfish, midship- DOI: 10.1002/jmor.10551 120 Ó 2007 WILEY-LISS, INC. ID: vasanss Date: 15/6/07 Time: 15:48 Path: J:/Production/JMOR/Vol00000/070042/3B2/C2JMOR070042 J_ID: JMOR Wiley Ed. Ref. No.: 1697 Customer A_ID: JMOR10551 Date: 15-JUNE-07 Stage: I Page: 2 2 M.L. FINE ET AL. 121 twitch sounds can have higher dominant frequen- 181 122 cies, and the maximum for the weakfish Cynoscion 182 123 regalis is above 500 Hz (Connaughton et al., 2002). 183 124 Note that high frequency components in fish 184 125 sounds (e.g., those above the peak frequency) rep- 185 126 resent higher order vibrations of the swimbladder 186 127 and not the driving frequency. In many cases, 187 128 these high frequency components may be above 188 129 the fish’s auditory range and therefore irrelevant 189 130 to communication (Ladich and Fine, 2006). 190 131 Ophidiids, the dominant group of benthic deep- 191 132 sea fishes in both numbers and species in tropical 192 133 and subtropical areas (Howes, 1992; Nielsen et al., 193 134 1999; Music personal communication), have swim- 194 135 bladder muscles, implicating them in sound pro- 195 136 duction (Marshall, 1967). Recent descriptions of 196 137 the sounds produced by a shallow-water ophidiid, 197 138 Ophidion marginatum, indicate a dominant fre- 198 139 quency above 1 kHz (Mann et al., 1997; Perkins, 199 140 2001; Sprague and Luczkovich, 2001; Rountree 200 141 and Bowers-Altman, 2002). This frequency is too 201 142 high to be explained by the twitch model because 202 143 no vertebrate muscle is known to complete a 203 144 twitch in less than 1 ms. We examined the sonic 204 145 anatomy underlying potentially high frequency 205 146 sound production in the fawn cusk-eel Lepophi- 206 147 dium profundorum, a species that lives on the 207 148 outer continental shelf in the Northwest Atlantic 208 149 Ocean (Collette and Klein-MacPhee, 2002) and 209 150 whose sounds have not been recorded. Although 210 151 direct evidence is lacking, we are proceeding under 211 152 the hypothesis that other ophidiids, but not related 212 153 carapids (Parmentier et al., 2006b), likewise pro- 213 154 duce high frequency sounds. Several previous 214 155 studies in ophidiids have clarified parts of the 215 156 mechanism and indicate considerable variability 216 157 within the sonic anatomy of the family (Rose, 217 158 1961; Courtenay, 1971; Carter and Musick, 1985; 218 159 Howes, 1992; Casadevall et al., 1996; Parmentier 219 160 et al., 2006a). These studies have noted sexual 220 161 dimorphism in sonic structures, and the present 221 162 study quantifies sexual differences for the first 222 163 time. 223 164 224 165 225 166 MATERIALS AND METHODS 226 167 227 168 Fawn cusk-eels Lepophidium profundorum (Gill, 1863) were Fig. 1. A: Relationship of weight to length for combined 228 169 captured by otter trawl and frozen aboard Albatross IV cruises. males and females in Lepophidium profundorum: Y 5 Fish were collected from Cape Hatteras to the Gulf of Maine at 229 0.01316X 2 170 1.116e , r 5 0.90. B: Relationship of skull weight to fish depths of about 50 fm from spring (March and April) and fall 230 2 171 weight: Males: Y 5264.91 1 21.72 X, r 5 0.80, Females: (September and October) bottom trawl surveys in 2001, 2002, 231 2 172 231.51 1 17.74X, r 5 0.84. C: Relationship of spine length to and 2004. 232 total length for combined males and females: Y 5 1.041 1 Specimens were weighed to 0.1 g and measured for total 173 2 233 0.02118X, r 5 0.60. length (TL) to the nearest millimeter, and gonads were exposed 174 to determine fish sex. Fish were dissected to expose muscles 234 175 attached to the swimbladder or to processes that connect to the 235 176 follow an electrical stimulus at 400 Hz without tet- bladder, and the origin and insertion of these muscles were 236 177 any (Fine et al., 2001) although the highest funda- described. We measured muscle length between the origin and 237 178 insertion and length of the tendons protruding from the ventral mental frequencies of toadfish recorded in nature muscle. Muscles (the right muscle of each pair) were weighed in 238 179 approach 300 Hz (Fine, 1978; Thorson and Fine, milligrams after soaking in 0.9% NaCl for 5 min and blotting to 239 180 2002; Remage-Healey and Bass, 2005).