Staaf et at: Distribution of ommastrephid paralarvae in the eastern tropical Pacific 
81 
Water column data were collected with conductivity- 
temperature-depth (CTD) profilers 1 h before sunrise 
and 1 h after sunset on each survey day and with ex- 
pendable bathythermographs (XBTs) during daylight 
hours at intervals of ~55 km. Samples of surface water 
were collected in bottles during the CTD casts and in 
buckets concurrent with XBT casts at depths from 1 
to 3 m. Precruise calibration factors (fluorometer cali- 
bration factor, F, and acid ratio of pure chlorophyll, x) 
were used to calculate chlorophyll-a and phaeophytin 
values from digital fluorometer readings of these sur- 
face water samples. Sea-surface temperature (SST) and 
salinity (SSS) were measured continuously (around the 
clock) with a thermosalinograph while the ship was un- 
derway. Details of the complete data set are available 
in NOAA data reports (Philbrick et al., 2001, a-c; Am- 
brose et al., 2002, a and b; Watson et al., 2002; Jackson 
et al., 2004; 2008). 
Sample processing 
Cephalopods were removed manually from 654 bongo 
(1998, 2000, 2003, 2006) and 784 manta (1998, 1999, 
2003, 2006) samples. Bongo samples with >25 mL of 
plankton were fractioned to -50% of the original sam- 
ple volume before they were sorted. The absolute count 
from each tow was divided by the volume of water 
filtered during that tow, as computed from flowmeter 
readings, to give paralarvae densities per cubic meter 
(following techniques described in Kramer et al., 1972). 
Adult and paralarva! specimens were identified by 
morphological characteristics (Wormuth et al., 1992). 
Adults were identified to species by the presence of 
a fused funnel-locking cartilage in purpleback squid 
and the absence of the fused structure in jumbo squid. 
Ommastrephid paralarvae are known as rhynchoteu- 
thions; their distinctive form is recognized easily by 
the presence of a proboscis. For individuals missing 
the proboscis or in which the proboscis already had 
separated into tentacles, identification was based on 
the characteristic inverted-T funnel-locking cartilage 
of this family. When proboscis suckers were visible, 
they were checked to separate individuals of the gen- 
era Hyaloteuthis, Eucleoteuthis, and Ommastrephes 
(enlarged, lateral suckers on proboscis) from individu- 
als of the genera Dosidicus and Sthenoteuthis (equal- 
size suckers on proboscis). Hyaloteuthis, Eucleoteuthis 
and Ommastrephes are relatively rare in the ETP (all 
the molecularly identified ommastrephids in this study 
were Sthenoteuthis or Dosidicus', see also Yatsu 3 ), and 
3 Yatsu, A. 1999. Morphological and distribution of rhyncho- 
teuthion paralarvae of two ommastrephid squids, Dosidicus 
gigas and Sthenoteuthis oualaniensis, collected from eastern 
tropical Pacific Ocean during 1997-preliminary report. In 
Report of the Kaiyo Maru cruise for study on the resources of 
two ommastrephid squids, Dosidicus gigas and Ommastrephes 
bartramii, in the Pacific Ocean, during September 11- 
December 24, 1997 (A. Yatsu, and C. Yamashiro, eds.), p. 
193-206. Fisheries Agency of Japan, Tokyo. 
only 9 specimens were tentatively identified as Eucleo- 
teuthis and 2 specimens were tentatively identified as 
Ommastrephes by proboscis suckers and photophores 
(6 others were excluded from Dosidicus or Sthenoteuthis 
but were too small to be assignable to the other 3 gen- 
era). Therefore, any specimens damaged such that the 
terminal suckers were not preserved were assigned to 
the SD complex. The presence of paralarvae from other 
cephalopod families was recorded, but these specimens 
were not identified to genus or species, or counted. 
Morphological techniques for reliable differentiation 
between paralarvae of jumbo squid and purpleback 
squid are not available. Wormuth et al. (1992) and 
Yatsu 3 used proboscis length and photophores as dis- 
tinguishing characters, but the muscular proboscis can 
extend and retract (Staaf et al., 2008), and reactions to 
fixatives have not been quantified. Additionally, there 
may be variability in ontogenetic timing of photophore 
formation (Gilly et al., 2006). Ramos-Castillejos et al. 
(2010) suggested several distinguishing indices that 
used morphometric ratios; however, samples in this 
study were prepared in different fixatives (ethanol 
for jumbo squid and formalin for purpleback squid) 
that can distort or shrink specimen proportions. 
The efficacy of indices for diagnoses of individual speci- 
mens of unknown species also were not tested. There- 
fore, we attempted no species-level identification of SD- 
complex specimens that were preserved in formalin. 
Molecular identification of SD-complex ommas- 
trephids from ethanol-preserved samples followed pro- 
tocols described in Gilly et al. (2006). Two frozen bongo 
samples were also sent to Hopkins Marine Station for 
sorting and molecular identification. The frozen sam- 
ples were selected on the basis of a high abundance 
of ommastrephid paralarvae in the matching codend, 
and they were sorted primarily to test whether it is 
possible to reliably identify paralarvae from a frozen 
plankton sample. 
Mantle lengths (ML) of ommastrephid paralarvae 
from 1998 manta and bongo tows were measured with 
an ocular micrometer. For tows with 10 or fewer om- 
mastrephids, all individuals were measured. For tows 
with more than 10 ommastrephids, 10 individuals were 
selected for measurement. Selection was arbitrary and 
aimed to be representative; e.g., the largest (or small- 
est) specimens were not always included. 
Data analysis and modeling 
We constructed a data set of ommastrephid paralarval 
abundance and 5 in situ oceanographic variables: SST, 
SSS, mixed-layer depth (MLD), temperature at thermo- 
cline (TT), and surface concentration of chlorophyll-a 
(CHL). MLD is defined as the depth at which tempera- 
ture is 0.5°C less than SST (Fiedler, 2010). TT is tem- 
perature at the depth of the thermocline as determined 
by the “maximum slope by difference” method (Fiedler, 
2010). MLD, TT, and CHL values were collected from 
the station nearest the net tow; these data were used 
