Abstract
The jumbo squid, Dosidicus gigas, exhibits chromatophore-effected flashing
behavior at frequencies of about 3 Hz. In order to characterize this behavior
quantitatively and to make it possible to speculate as to its function, videos of D. gigas in
its natural environment were taken and analyzed. There is great variability in the
consistency of flashing rates among individuals: some stay relatively stable while others
fluctuate widely in short spans of time. Videos of two squid encountering one another
suggested that squid adjust to their conspecifics' flashing patterns in complex ways. A
greater corpus of data will be necessary before specific hypotheses about the function of
D. gigas’ flashing behavior can be formulated.
Introduction
Relatively little is known about Dosidicus gigas, commonly called the Humboldt squid,
red devil or jumbo squid. Three intraspecific groups have recently been distinguished by their
difference in adult size, with the largest squid reaching mantle lengths of 2 1 m and weights of
50 kg (Nesis, 1983). Adult males in the small-sized group reach 260 mm, and the medium-sized
adult males reach 420 mm. Dosidicus gigas is fished extensively off Baja California and the
California coast (37-40 deg. N) as well as the west coast of Central and South America (45-47
deg. S) (Nesis, 1970). It is epipelagic to several hundred meters, and rises to the surface both
night and day to feed. Its diet consists of fish (most commonly epipelagic lanternfish) and squid
(especially D. gigas), while it is preyed on by sperm whales, billfishes, tunas and humans
(Norman, 2000).
The life cycle of D. gigas is normally one year, and female fecundity is the highest
known among cephalopods (Nigmatullin et al., 2001). In Peruvian populations, spawning takes
place off the coast of Peru throughout the year, with a mean between October and January and a
secondary peak between July and August (Tafur et al., 2001), and planktonic larvae are carried
south to southern Peru and Chile (Fernández and Vásquez, 1995).
Beyond this broadly drawn life history, little work has been published on the behavior of
Dosidicus gigas, and what has been published is anecdotal. In particular, its flashing behavior
has never been quantified or described in any detail.
In Dosidicus as in other cephalopods, flashing is effected by millions of chromatophores,
neuromuscular organs in the dermis that are innervated by the brain. Chromatophores are tiny
cytoelastic sacs filled with pigment granules and encircled by excitatory radial muscle fibers
Introduction
Relatively little is known about Dosidicus gigas, commonly called the Humboldt squid,
red devil or jumbo squid. Three intraspecific groups have recently been distinguished by their
difference in adult size, with the largest squid reaching mantle lengths of 2 1 m and weights of
50 kg (Nesis, 1983). Adult males in the small-sized group reach 260 mm, and the medium-sized
adult males reach 420 mm. Dosidicus gigas is fished extensively off Baja California and the
California coast (37-40 deg. N) as well as the west coast of Central and South America (45-47
deg. S) (Nesis, 1970). It is epipelagic to several hundred meters, and rises to the surface both
night and day to feed. Its diet consists of fish (most commonly epipelagic lanternfish) and squid
(especially D. gigas), while it is preyed on by sperm whales, billfishes, tunas and humans
(Norman, 2000).
The life cycle of D. gigas is normally one year, and female fecundity is the highest
known among cephalopods (Nigmatullin et al., 2001). In Peruvian populations, spawning takes
place off the coast of Peru throughout the year, with a mean between October and January and a
secondary peak between July and August (Tafur et al., 2001), and planktonic larvae are carried
south to southern Peru and Chile (Fernández and Vásquez, 1995).
Beyond this broadly drawn life history, little work has been published on the behavior of
Dosidicus gigas, and what has been published is anecdotal. In particular, its flashing behavior
has never been quantified or described in any detail.
In Dosidicus as in other cephalopods, flashing is effected by millions of chromatophores,
neuromuscular organs in the dermis that are innervated by the brain. Chromatophores are tiny
cytoelastic sacs filled with pigment granules and encircled by excitatory radial muscle fibers
(Hanlon and Messenger, 1988). When the fibers are stimulated, they contract and the
chromatophore expands. The pigment granules create what looks like a tiny spot on the dermis,
a state we will refer to as ON. In D. gigas, when a group of chromatophores are simultaneously
switched ON, the entire area appears reddish-brown because of the red-brown chromatophore
pigment. To colorblind conspecifics (and to us, in blue-shifted light), this state appears simply as
dark. Somewhat counterintuitively, then, the bright flashes occur when the chromatophores are
in the OFF state.
Öther animals exhibit similar flashing behavior, both within the class Cephalopoda and
outside it, notably in the class Insecta. Within Cephalopoda, the binary nature of D. gigas
flashing, as well as its speed distinguishes it from the flashing of other species. Sepia species,
for example, often use their ornate patterning as camouflage and countershading (Messenger,
1988). The best-studied group of flashing insects is the fireflies (genus Pteroptyx and Photinus)
(Buck, 1988). Buck reviews many of the competing theories of the function of firefly flashing,
all of which relate flashing behavior to courtship and mating.
We used image analysis software to examine video clips of D. gigas swimming freely
and on fishing lures. Pairs of individuals appear to synchronize briefly and sporadically with one
another, and to maintain a more constant rhythm when they are isolated.
Materials and Methods
All of the videos used in analysis were filmed off Santa Rosita, BCS in October 2001.
They were made by Bob Cranston, an independent filmmaker, using SCUBA at depths of 2 to 50
0
meters. The videos were filmed with a Sony broadcast HDTV camera and custom housing, and
directly copied onto Hi-8 master videotapes. Selected sequences from these tapes were
converted to .avi files using Adobe Premier and analyzed using ImagePro Plus. In ImagePro
Plus, we used the AOI tool and the Intensity histogram to determine the grayscale value of
selected areas of the image. At all times the original sampling rate of the video camera (30 Hz)
was conserved.
After culling data from the .avi clips, we employed Microsoft Excel 5.0 and IgorPro to
generate the graphs and figures. Excel was used to create the original spreadsheets, which were
imported to Igor to create the flashing graphs.
Further observations on chromatophore density and arrangement in D. gigas were made
using the Hi-8 tapes.
Results
The flashing behavior Dosidicus gigas is unique among the cephalopods. Instead of a
large array of patterns, Dosidicus has two states, which we have called ON and OFF. ON
indicates chromatophore expansion; the squid appears dark red. ÖFF indicates chromatophore
contraction, in which the squid has a whitish appearance.
When not flashing, the squids remain in either the ON or the ÖFF state; there is no clear
"resting state“ despite the fact that the ON state requires ongoing excitation. Moreover, squid in
the video documentation (free-swimming, at least several meters from the photographer) never
exhibited partial activation: each flash seemed to be an all-or-none phenomenon.
The minimum number of flashes in a train is just one. In abnormal conditions (usually
while it was being tagged out of water), the squid sometimes flashed only its fins or arms. The
maximal rate of flashing observed was 6 flashes/sec, though most of the squid in the clips
examined flashed at a rate between 3/sec and 4/sec.
A typical flashing cycle is shown in Figure 1, a series of twelve frames spanning 333 ms.
To determine more quantitatively the flashing pattern of an individual squid, we examined
several clips of single squid that remained relatively still and stayed close to the camera for at
least five seconds. We then took intensity measurements on three or four areas of the squid’s
body and plotted these as a function of time (see Figure 2).
Along the squid body, there is no completely consistent pattern in peak ON states, but the
chromatophores on the head and arms are generally activated first. They are followed within 100
ms by mantle and fin chromatophores. However, the mantle and fins do not follow a strict
anterior-posterior waveform. Sometimes the fins are fully activated before the entire mantle
reaches its peak brightness.
Figures 3 through 5 examine the behavior of a single squid. These clips were chosen
with the criterion that there be only one individual pictured for the duration of the clip. Of
course, we cannot be sure that there were not squid nearby but outside the camera’s scope, but
given the data this seemed the most reasonable method. Figures 3a, 4a, and 5a plot the light
intensity of the squid mantle against time, while figures 3b, 4b and 5b plot the peak ON signals
against time.
These three sets of figures show the variability of D. gigas flashing behavior. The squid
analyzed in Figure 3 shows a relatively stable rate of flashing, between 11 and 12 frames per
cycle (or about 2.9 Hz). The following figure, Figure 4, exemplifies an intermittent flashing
pattern, in which bursts of flashing are punctuated by periods of weaker flashing or a cessation of
chromatophore activation. Finally, the graphs in Figure 5 are of a squid with a rapidly
fluctuating flashing rate (between 7 and 15 frames per cycle, or 4.7 and 2.2 Hz).
The markers at the very top of each figures 3a, 4a and 5a, are placed at each peak ON
frame to indicate the regularity and relative frequency of flashing periods.
We were also interested in answering the question of whether squids alter their behavior
on the basis of other flashing squid in their proximity. Photic waves were plotted against time
for multiple squid and their position relative to one another was noted. Graphs of this data for
five groups of squid are included in Figures 6-9.
In Figures 6-9, the first graph is a simple plot of grayscale intensity against time.
Higher grayscale values correspond to a lighter appearance, which occurs when the
chromatophores are in the ÖFF state.
Figures 6b, 7b, 8b, and 9b are plots of the number of frames between peak ON states,
which correspond to the darkest frame in the squid’s cycle, or low grayscale values. They
present changes in the individual organisms’ flashing periodicity.
Figure 6 is taken from a clip of two squid swimming parallel to one another. For the first
two seconds they are nearly perfectly synchronized, and then within 1.5 periods they switch,
becoming out of phase with respect to one another. Figure 7 is from a clip of two squid
swimming parallel to one another. Remarkably, their flashing frequency shifts in similar ways
so that they remain synchronized for eight cycles. Here, as in figure 8, it is difficult to tell
whether and to what degree the two individuals adjust to the flashing behavior of their
conspecifics.
In the clip for Figure 8, one squid is swimming underneath the second, who is on a
fishing lure. Drawings along the top of Figure 8 indicate the relative positions of the two squid
during the course of the flashing sequence. The dashes below these drawings mark the peak ON
periods of the two individuals. The two are flashing synchronously for the first three seconds.
After a transition period they begin flashing exactly out of phase, and then after another
transition they move back into phase, and then out of phase. Figure 8b includes a plot of the
relative timing of each squid’s peak ON state (black line and markers). During the first segment,
when the two are synchronized, the difference in peak ON states remains close to zero. During
the transition segment the difference jumps rapidly to around half of a period (180 ms). The
final “synchronized" segment shows the difference in peak ON states grow larger and then
approach zero at the very end.
Figure 9 is drawn from a clip of two squid intermittently near and far from each other.
As the two squid approach, one is flashing steadily and the other begins to flash. They are not
perfectly synchronized, but note the crossing lines plotting periodicity, in Figure 9b. This
suggests that the two individuals are adjusting to one another’s tempo. The second squid
remains quite steady in its flashing frequency as they swim apart, while the first one ceases to
flash. As they approach one another again, the first squid resumes flashing and the second squid
begins to modulate its frequency, possibly to synchronize with the first.
We estimated the number of chromatophores on the skin of an adult Dosidicus gigas to
be 3.9 x 10°. This estimate was taken from a video of a recently sacrificed, medium-sized adult
with a skin surface area of about 0.5 m’ including fins, head and arms. Using video shots of
squid calibrated with ruler that was placed on its skin, we found that there were approximately
70 chromatophores on an area equivalent to 9 mm’, or 7.8/mm’. An independent estimate based
on the same video put chromatophore density at 10/mm’ (Packard, personal communication),
yielding a total of 5 x 10° chromatophores.
Whereas many cephalopods have several types of chromatophores that differ in
pigmentation, D. gigas appears to have just one pigment (reddish brown). Although D. gigas has
only one type of chromatophore pigment, two types of chromatophores were distinguished in this
video on the basis of size rather than pigment. The large and small chromatophores appear to be
grouped into separately controlled chromatomotor fields.
The same video segment captured distinct groups of chromatophores that expanded
synchronously upon being lightly touched. These patches varied in size between 10 and about
60 chromatophores, with a mean size of 23 chromatophores. These groups of chromatophores
probably correspond to what Messenger calls chromatomotor fields (Messenger 2001), an
amalgamation of Packard’s terms, motor field’ and ’chromatophore field’ (Packard 1974).
Packard induced local expansion with a stimulating electrode, but his chromatophore fields share
with the morphological units in the video the quality of having irregular edges that are
complementary to the edges of adjacent fields.
The touch-induced excitation also seemed to spread somewhat spontaneously to
surrounding fields, with flickering lasting up to ten seconds after the initial stimulus. This
suggests that chromatomotor field activation may be spread by muscular contraction after it has
been initiated by excitation from the CNS.
Discussion
The graphs of between-peak-ON intervals show that there can be great variability when
two squid are flashing in clear view of one another (see Figure 8b), or there can be relative
stability (see Figure 7b). The same is true when one squid is flashing on its own (compare
Figures 3b and 5b). However, more data must be gathered in order to refine this hypothesis.
The study of this informationally rich behavior would be greatly aided by documentation
at a higher sampling rate. Specifically, this would help disambiguate variability in periodicity of
flashing. Another goal would be to obtain longer videotaped segments of small groups (2-4) of
squids swimming or hovering near each other. This is a challenge because the animals are free-
swimming; observation in captivity is one possible way of eliminating that problem. Such data
sampling would also allow the study of variability in flashing frequency within one individual
over longer periods of time, as well as giving us insight into possible correlations between
signaling behavior and the individual’s sex. This might give us clues about the adaptive function
of Dosidicus flashing behavior.
As noted earlier, all the theories put forth to explain firefly flashing behavior are built on
the assumption that it promotes mating in some way that makes it adaptive, either for the
individual or the group. The reasons behind this assumption illuminate several differences
between firefly and Dosidicus flashing. In fireflies, there are clear differences in the flashed
signals of the two sexes, and those signals are stereotyped. For a given species, there will be an
unchanging flash frequency or patter of flashing. In the videos of Dosidicus used in this study
included some squid that never flashed, some that flashed for a few seconds and then stopped,
and others that flashed continuously. However, no sex-correlated differences in flashing could
be determined because there is little sexual dimorphism in D. gigas, and none that is
distinguishable at the distances from which the video was taken. Another important difference
between the two types of flashing is that the majority of firefly flashing behaviors involve
synchronization of many individuals into either single pulses or wavelike flashes of light.
Dosidicus gigas does not appear to have such rigid behaviors, perhaps due to the fact that they
are usually in motion rather than resting stationery in trees, as the fireflies are.
In short, the flashing patterns of D. gigas are complex, involving intermittent
synchronizations between pairs as well as many intermittent individual patterns. These
observations are certainly open to revision in light of a larger corpus of data, but at present they
suggest that D. gigas uses its binary chromatophore-based signaling in multiple ways and
probably for multiple purposes. These could include intraspecific signaling relating to
navigation, aggression and mating, as well as defensive patterning directed at potential predators.
Acknowledgements
Many thanks to Dr. Gilly for your instruction, patience and contagious fascination with
Cephalopoda. Thanks also to S. H. Thompson for the use of your computer.
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