Staaf et al Distribution of ommastrephid paralarvae in the eastern tropical Pacific 
85 
Figure 4 
Geographic distribution and abundance of (A) genetically identified paralarvae and (B) morphologically 
identified adult ommastrephids caught in the eastern tropical Pacific during surveys conducted in 2006. 
The numbers at each station (small dot) represent the total number of individuals of purpleback squid 
( Sthenoteuthis oualaniensis) (outlined in black) and jumbo squid ( Dosidicus gigas) (solid black) captured 
at that station. At stations where numbers do not appear, no squid were caught. 
ETP (Vecchione, 1999). The consistency of this result 
seems surprising, because ommastrephid egg masses 
are thought to occur near the pycnocline, typically 
tens of meters deep, and not at the surface (O’Dor and 
Balch, 1985). The only reported observation of an in 
situ egg mass of jumbo squid was in the Gulf of Cali- 
fornia at a depth of 16 m near the pycnocline (Staaf et 
al., 2008). Presumably, this characteristic is common to 
purpleback squid, but we are unaware of descriptions 
of natural egg masses for this species. 
Not only are egg masses of jumbo squid found at 
depth, but paralarvae are negatively buoyant. Paralar- 
vae in the laboratory can swim to the surface but sink 
as soon as they stop swimming (Staaf et al., 2008); this 
negative buoyancy indicates that surface tension is in- 
sufficient for passive retention. We can only assume 
that purpleback squid paralarvae share this trait, and 
that tissue density of wild paralaravae is similar to 
laboratory-reared animals. 
A preferred surface habitat, in which maintenance 
of position requires significant energy expenditure, 
strongly indicates that some benefit is derived from 
this behavior; the benefit may be access to increased 
food quantity or to food of higher nutritional value 
(Yamamoto et al., 2007). Nothing is known of the diet 
of jumbo squid paralarvae, but amphipods, copepods, 
and crab zoeae have been found in the digestive tracts 
of purpleback squid paralarvae (Vecchione, 1991); 
these and other zooplankton, as well as phytoplank- 
ton, also have been found in paralarvae of another 
ommastrephid, lllex argentinus (Vidal and Haimovici, 
1998). Furthermore, a case has been made for the use 
of dissolved and particulate organic material by om- 
mastrephid paralarvae (O’Dor et al., 1985). At certain 
times and in certain regions, oceanic surface waters 
may have high concentrations of these foods. The depth 
of the chlorophyll-# maximum in the ETP ranges from 
60 to 90 m in open-ocean regions to near the surface 
in coastal boundary regions (Pennington et al., 2006). 
It would be valuable to examine the vertical distri- 
bution of paralarvae with systematic oblique or hori- 
zontal tows at a series of discrete depths through the 
upper 100-200 m of the water column at a variety of 
times in a given area. This approach would give a more 
accurate picture of habitat use and of any association 
with the subsurface chlorophyll-a maximum or acoustic 
