FLOUNDERS OF GENUS PARALICHTHYS AND RELATED GENERA 



281 



CHANGE OF FORM WITH SIZE 



In connection with an attempt to use the dif- 

 ferent body proportions for the proper distinction 

 of the species, some interestino; observations bear- 

 ing on change of form with size have been made, 

 which may be proHtably discussed here. Of 

 course, it was not the primary object of this in- 

 vestigation to study in detail the change of form 

 with change in size, and an insufficient number of 

 specimens were measured to describe with exacti- 

 tude tiie form of the curves representing these 

 changes. However, measurements made seeiii to 

 justify certain conclusions which are of interest 

 and importance in these flounders and may per- 

 haps hnd a wider application. Since so much 

 stress is laid on measurements in extant descrip- 

 tions and they are of some use for the distinction 

 of the species in a few cases, it is important to 

 point out some of the changes noted with size. 

 Some tentative conclusions suggested by a close 

 scrutiny of tables 7 and 8 seem to be as follows. 



The curve representing lelative depth appar- 

 ently follows a sinuous course during the life 

 cycle of most species. That is, with growth it 

 alternately increases and decreases, or vice versa, 

 the change taking place more than once. Thus, 

 in aesfuurht^s, woohnan/', and lefhostlgma there is 

 a gradual decrease in depth in the smaller fish up 

 to about 150 or 200 mm. After that length is 

 reached it increases again. In a/ispersi/s and 

 alh/fnifta there is an increase in relative depth up 

 to about 100 or 125 mm. and then it decreases as 

 in the preceding three species, in fish up to about 

 200 mm. After that length is reached the depth 

 increases in these two species also; but surpris- 

 ingly, in albigufta the relative depth undergoes 

 another change and the largest specimens become 

 slender in comparison. In dentatus the alternate 

 changes are apparently about the same as in 

 (uhperxus except that they occur at a somewhat 

 larger size. In caltfornicuf; there is apparently a 

 gradual decrease in relative depth from the small- 

 est to the largest specimens. 



These alternate changes in relative depth in 

 most species may be due to changes in the rate of 

 growth in length. That is, in the smaller fish 

 there possibly is a marked acceleration in the 

 tempo of growth in length which increases at a 

 greater rate than the depth, the particidai- length 

 at which this takes place differing with the species. 



980,335°— 52 3 



With increase in size the accelerated growth in 

 length slows up and the relative depth increases. 



The head in young fish, those under 50 mm., is 

 notably long in all species of Paralichthys. Its 

 rate of gi-owth soon slows down very markedly, 

 and in somewhat larger specimens, about 50 to 70 

 mm., it becomes almost abruptly and pali)ably 

 shorter in comparison with the standard length. 

 It then continues to decrease slowly in relative 

 length as the fish gi-ows. In two species, Icfho- 

 stif/ma and dentatu.i, it appears to increase again 

 in the largest specimens; but those measured are 

 not in sufficient number to be certain of this, and 

 the increase, if any. is moderate. Changes in the 

 relative length of the maxillary with size, in gen- 

 eral, follow that of the head length. The inter- 

 orbital gradually increases in width as the fish 

 grows. 



In striking contrast to the species of Para- 

 lichthy.<i, the head in //. stomafa (table 7) increases 

 gradually in relative length from the small to the 

 large specimens. The depth in that species 

 changes little with size. 



It is evident that, with size, changes in the form 

 of any part often differ markedly with the species. 

 This is very important to bear in mind in connec- 

 tion with the use of measurements in distinguish- 

 ing species. A difference between two species 

 based on proportional measurements may hold at 

 a given size, but not at other sizes. A striking 

 ca.se showing that a difference between two species 

 may be reversed at a certain size is furnished by 

 lefhostlgma and albigutta. Small and medium- 

 sized specimens of Jethostigmu are more slender 

 on the average than large specimens; but in «/- 

 hiyutta the body evidently becomes more slender 

 in lai-ge fish. As a result we have the interesting 

 condition when comparing albigutta with letho- 

 st/gm-a. that the smaller specimens are relatively 

 deeper in the former; but the proportions are re- 

 reversed in large specimens, the former species 

 being relatively more slender (table 8). The 

 divergence between the two species with respect to 

 dei^th is greater for the larger specimens. 



It is to be noted that the length of the maxillary 

 relative to the body length generally decreases in 

 the lai-ger specimens of Paralivhtln/s. However, if 

 the kMigth of the maxillary is judged by the rela- 

 tion of its posterior extremity to the position of 

 the eye, it seemingly increases with size, since its 



