Grover: Feeding habits of pelagic summer flounder larvae 
255 
Summer flounder 
Plaice 1953a 
Plaice 1953b 
- 3 - 
Plaice 1964 
— E3— 
Plaice 1978 
Onset of time block 
Figure 5 
Incidence of feeding of pelagic larval summer flounder, P. dentatus, in the NW 
Atlantic Ocean and of pelagic larval plaice, P. platessa, in the Southern Bight 
of the North Sea and the English Channel, as a function of time of day. Sources 
of plaice data are as follows: 1953a = 48-h station, Shelbourne, 1953; 1953b = 
other stations (Shelbourne, 1953); 1964 = Ryland, 1964; 1978 = Last, 1978. 
copepod nauplii were the most impor- 
tant prey in the diet of all premeta- 
morphic stages of Japanese flounder 
larvae. However, Ikewaki and Tanaka 
(1993) later reported that olive floun- 
der diet, in Wakasa Bay, was domi- 
nated by copepod nauplii only for first- 
feeding larvae and by appendic- 
ularians, Oikopleura spp., for later 
stages through early-metamorphic- 
phase larvae. Minami (1982) re- 
ported that appendicularians, cope- 
pods, and mysids were the dominant 
prey of early-, mid-, and late-meta- 
morphic larvae, respectively. The siz- 
able contribution of copepod nauplii 
to the diet of p reflexion (PF) summer 
flounder larvae resembled the con- 
tribution observed in the diet of first- 
feeding (Ikewaki and Tanaka, 1993) 
and early stage olive flounder larvae 
(Minami, 1982). However, appen- 
dicularians, the dominant prey item 
for early-metamorphic larvae (Min- 
ami, 1982), and for all larval stages 
beyond first-feeding in olive flounder 
(Ikewaki and Tanaka, 1993), occurred only in the diet 
of premetamorphic and metamorphic (PM+M) sum- 
mer flounder larvae. 
Pelagic summer flounder larvae that were collected 
in the NW Atlantic Ocean displayed a diurnal feed- 
ing pattern similar to that reported for several other 
flatfish larvae (e.g. plaic e,Pleuronectes platessa : Shel- 
bourne, 1953; Ryland, 1964; Last, 1978; dab, 
Limanda limanda : Last, 1978; English sole, Pleuro- 
nectes vetulus, and butter sol e, Pleuronectes isolepis ; 
Gadomski and Boehlert, 1984). Both incidence of 
feeding and gut-fullness data (Figs. 3 and 4) appear 
to confirm the visual nature of larval summer floun- 
der feeding in oceanic collections. This is not sur- 
prising, because marine fish larvae are mostly vi- 
sual feeders (Hunter, 1981; Blaxter, 1986; Huse, 
1994). However, the optimal illumination level for 
feeding varies with species (Huse, 1994). For ex- 
ample, Atlantic cod, Gadus morhua, larvae feed pref- 
erentially at very low light levels, and turbot, Scoph- 
thalmus maximus, larvae feed preferentially at high 
levels of illumination, whereas plaice larvae feed over 
a wide range of illumination levels (Huse, 1994). 
Within the flatfishes, additional relationships be- 
tween illumination and larval feeding have been dem- 
onstrated. For example, sole, Solea solea, larvae can 
feed in the dark from the early posthatching stage 
(Blaxter, 1969). Oceanic collections of yellowtail floun- 
der, Pleuronectes ferrugineus, larvae have shown that 
the highest incidence of feeding occurred between 
1600 and 0100 (Smith et ah, 1978). The near absence 
of feeding between 0700 and 1300 suggests that on- 
set of feeding in yellowtail flounder larvae is trig- 
gered by something other than, or in addition to, il- 
lumination (Smith et al., 1978). 
Data from oceanic collections in the present study 
suggest that the relationship between illumination 
and feeding of summer flounder larvae is much like 
that observed in one of the two flatfish species stud- 
ied by Huse ( 1994). Laboratory observations are re- 
quired to ascertain whether summer flounder lar- 
vae feed preferentially at high illumination levels or 
over a wide illumination range. However, an exami- 
nation of incidence of feeding of pelagic plaice larvae 
(Shelbourne, 1953; Ryland, 1964; Last, 1978) in rela- 
tion to comparable summer flounder data (Fig. 5) re- 
veals similar patterns of feeding periodicity. From this, 
a similarity in optimal illumination levels for feeding 
for summer flounder and plaice larvae is suggested. To 
wit, pelagic summer flounder larvae would be expected 
to feed well over a wide range of illumination levels. 
If the ecological analogy between plaice larvae and 
summer flounder larvae extends through metamor- 
phosis, then a dramatic increase in light sensitivity 
(and a lower threshold light intensity for feeding) at 
metamorphosis (Blaxter, 1968) would be predicted 
for summer flounder. Limited field evidence supports 
this. A comparison of the incidence of feeding of oce- 
