biology – pwoerpoint 2 slide
A deepwater fish with ‘lightsabers’ –
dorsal spine-associated luminescence in
a counterilluminating lanternshark
Julien M. Claes1
, Mason N. Dean2
, Dan-Eric Nilsson3
, Nathan S. Hart4 & Je´roˆme Mallefet1
1
Laboratoire de Biologie Marine, Earth and Life Institute, Universite´ catholique de Louvain, 1348 Louvain-la-Neuve, Belgium, 2
Department of Biomaterials, Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany, 3
Lund Vision Group, Lund
University, 22362 Lund, Sweden, 4
The School of Animal Biology and The Oceans Institute, The University of Western Australia,
Crawley, WA 6009, Australia.
We report the discovery of light organs (photophores) adjacent to the dorsal defensive spines of a small
deep-sea lanternshark (Etmopterus spinax). Using a visual modeling based on in vivo luminescence
recordings we show that this unusual light display would be detectable by the shark’s potential predators
from several meters away. We also demonstrate that the luminescence from the spine-associated
photophores (SAPs) can be seen through the mineralized spines, which are partially translucent. These
results suggest that the SAPs function, either by mimicking the spines’ shape or by shining through them, as
a unique visual deterrent for predators. This conspicuous dorsal warning display is a surprising complement
to the ventral luminous camouflage (counterillumination) of the shark.
I
n the permanent darkness of the deep-sea a surprising diversity of organisms, from bacteria to fish, have
evolved the capability to produce a visible light1,2. This bioluminescence comes from a chemical reaction1–3 that
occurs either in the exterior environment or within cells; these photogenic cells (photocytes) are often
embedded in organs called photophores that can reach a high degree of sophistication, including reflectors, light
guides, wavelength-specific filters and optical lenses4–6. Numerous adaptive benefits have been suggested for
bioluminescent glows and flashes (light emission , 2 s), all falling in the three functional categories essential to an
organism’s survival: predator evasion, food acquisition and reproduction1–3,7.
In the mesopelagic zone (200–1000 m), many animals produce a continuous ventral glow to match downwelling
sunlight and conceal their silhouette from organisms swimming beneath them, a process called counterillumination2,8,9.
Conversely, dorsal glows are rarely observed in this environment, probably because they are
highly detectable against the darker background of deeper waters when viewed from above. As a consequence,
such conspicuous displays are usually of a predatory nature, produced by ‘anatomical fishing lures’ that attract
prey from a distance2,7. Probably because of the stark difference in the apparent roles of dorsal and ventral
luminescence, dorsal and ventral photogenic organs are rarely observed to glow concomitantly in the same
mesopelagic organism10. Here, however, we report a particular exception where the dorsal glows of a deepwater
shark might actually complement the counterilluminating luminescence produced by its ventral photophores.
The velvet belly lanternshark, Etmopterus spinax, is a diminutive shark (# 60 cm in total length) that spends
most of its time at mesopelagic depths11. Like other members of the Etmopteridae family, E. spinax harbours
defensive mineralized spines in front of its dorsal fins and has a ventral surface covered by thousands of tiny (,
200 mm in diameter) photophores11,12. Stimulated by melatonin or prolactin, these organs produce a long lasting
blue-green luminescence13. This self-produced light, the chemistry of which remains unknown, provides this
lanternshark with a counterilluminating camouflage14 and perhaps aids in intraspecific communication15,16.
When investigating this species’ bioluminescence, we were surprised to observe glows from the frontal edges of
the dorsal fins, forming conspicuous arcs immediately behind the spines (Fig. 1). This dorsal luminescent display
seems in direct conflict with the camouflage role of the ventral photophores. Here we document the structural
basis of this intriguing display and investigate its functional significance. We examined the histology and light
emission of the dorsal photophores and determined the 3D structure and spectral transmittance of their associated
spines. Using these structural and in vivo luminescence data, ocular measurements from local fishes and
marine mammals and a recent theory for visual detection in pelagic habitats17, we then determined the detection
SUBJECT AREAS:
BEHAVIOURAL ECOLOGY
ICHTHYOLOGY
THEORETICAL ECOLOGY
ANIMAL BEHAVIOUR
Received
9 January 2013
Accepted
4 February 2013
Published
21 February 2013
Correspondence and
requests for materials
should be addressed to
J.M.C. (julien.m.
claes@uclouvain.be)
SCIENTIFIC REPORTS | 3 : 1308 | DOI: 10.1038/srep01308 1
range of the luminescence produced by the spine-associated photophores
(SAPs) in the shark’s environment, characterizing theirin situ
performance and suggesting their ecological role.
Results
Structural basis. SAPs form two to four vertical rows on both sides of
the dorsal fins, at a distance of about 1–3 mm behind the finspines
(Fig. 2a). These organs appear structurally similar to ventral
photophores18, each organ comprising a pigmented sheath covered
by one or several lens cells, and enclosing a cluster of photogenic cells
(photocytes) containing fluorescent vesicles (Fig. 2b,c). Considering
the relative orientation of lenses and photocytes, SAPs emit most of
their light laterally, forming sickle-shaped glows on the sides of the
leading edges of the dorsal fins, mimicking the curve of the spines
anterior to them. Given this intimate association of photophores and
spines (Fig. 2d) and that dorsal fins have been shown to rotate during
swimming in some shark species19, we examined the potential
interaction of SAP luminescence and spines by measuring the
spectral transmittance of these structures. The spines could
hypothetically transmit about 10% of the SAP luminescence if the
dorsal fins were to rotate laterally during swimming (Fig. 2e).
Functional significance. Ventral photophore and SAP luminescences
are long-lasting and their intensities are significantly correlated
(Fig. 3a), suggesting a common hormonal control for these
photogenic structures. This information, combined with local
environmental data and the visual theory developed by Nilsson
et al.17 allowed us to calculate the depth at which the ventral
photophores provide an optimal counterilluminating camouflage for
the brightest of our specimens (Fig. 3b). This theoretical depth should
reflect the preferred depth of the shark in its natural habitat. Using
Nilsson et al. ’s theory again as well as the spine transmittance data and
ocular anatomy of local fishes and marine mammals, we determined
that, at theoretical counterillumination depth, SAPs can emit a close
range luminescent signal, both around and through the spines, that
would be detectable by most of the shark’s potential predators—but not
its main prey—from several meters away (Fig. 3c).
Discussion
In the mesopelagic zone, where most animals—including E. spinax—
have converged on camouflage strategies20, the dorsal long-lasting
luminescence produced by the shark’s SAPs appears to be a paradoxical
and risky display. As a consequence, a strong adaptive benefit
of these intriguing photogenic structures might be expected.
We explored the functional significance of SAP luminescence
using an integrative method involving structural and performance
data and a recent theoretical visual modeling17. This novel approach
appears particularly useful to test performance hypotheses involving
deep-sea animals like E. spinax, which live in a largely inaccessible
environment and demonstrate a physiological fragility that considerably
restricts their use in behavioral experiments. The validity of
our modeling was confirmed by the similarity between the theoretical
counterillumination depth (164–254 m deep, depending on
surface light conditions), which suggests the occurrence depth of
the shark, and the actual capture depth of our specimens (170–
230 m). At this theoretical counterillumination depth, the particular
window of detection distances of SAP luminescence would not be
effective in prey attraction: our calculations predict that the main
prey of the shark (the pearlside fish Maurolicus muelleri)
21 can only
Figure 1 | Spine-associated bioluminescent display. (a) Location of
mineralized finspines in an adult Etmopterus spinax specimen. (b) Closeup
of first dorsal spine with SAPs in natural light. (c) Close-up of second
dorsal spine with spontaneously glowing SAPs in darkness. (d)
Spontaneous luminescence from another specimen of this species showing
the glowing dorsal SAPs (white circles), counterilluminating ventral
photophores14 (CVPs) and lateral photophore areas believed to be
involved in intraspecific communication15,16: ca, caudal; ic, infracaudal; la,
lateral; pe, pectoral. Scale bars, 5 cm.
Figure 2 | Structural basis and spatial organization of spine-associated
bioluminescent display. (a) Frontal edge of the second dorsal fin in
transversal section (TS; 7 mm) showing the position of SAP rows (red
arrows). ce, ceratotrichia; pl, pigmented layer. (b) Close-up of a SAP (TS;
7 mm) showing the position of photogenic cells (photocytes; white
arrows). le, lens; ps, pigmented sheath. (c) Same SAP (TS; 7 mm) showing
the autofluorescence of the photocyte vesicles (red arrows) under U.V.
application. (d) Micro-CT of first and second dorsal finspines. Top inserts
are transversal sections situated close to the tip and the base. Note how the
spine cross-section in 2d have been rotated 90u anticlockwise relative to the
cross-section at the top of the figure. Scale bars correspond to 500 mm in
(a), (d) and to 150 mm in (b), (c). (e) Spectral transmittance of the spine
(grey line) with a superimposed Gaussian spectrum (dashed line) of shark
luminescence14. The spine can hypothetically transmit , 10% of SAP
luminescence frontally.
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SCIENTIFIC REPORTS | 3 : 1308 | DOI: 10.1038/srep01308 2
detect SAPs from short distance away (, 1.5 m); conversely potential
predators can detect them at several meters. This strongly suggests
that dorsal bioluminescence in the lanternshark serves as a
‘‘predators-eyes-only’’ signal and therefore somehow functions as
an aposematic (warning) beacon to deter approaching predators
(or conspecifics), albeit without jeopardizing prey capture.
The exact message of this aposematic signal remains unclear and
intriguing. Given the particular spatial organisation and close association
of SAPs with the defensive dorsal finspines, and the fact that
the finspines are capable of transmitting a portion of SAP luminescence
(probably thanks to a reduced enameloid covering compared
to the spines of other non-bioluminescent squalid sharks12), we propose
that SAPs evolved as a means of highlighting the spines themselves.
Dorsal mineralized finspines, sometimes linked to venomous
glands, are found in chimaeras and a number of sharks9,22,23. While
stingray barbs can be easily whipped into defensive position in the
event of a threat, the dorsal anchoring of the finspines largely restricts
their use to risky post-capture defensive events as a means of wounding
the soft tissue in the mouth of a biting predator.
SAP ‘‘advertisement’’ of these spines—by luminescent strips mimicking
spine shape and/or shining through the spines when the fins
rotate—would be particularly advantageous, since it would allow the
shark to signal to potential predators from a distance the danger of
the stinging appendages, maybe preventing attacks before they happen.
The high density of E. spinax individuals present in Norwegian
fjords24 limit the time between encounters with this species, perhaps
facilitating a predator’s avoidance learning through a succession of
unsuccessful predation events.
Bioluminescent aposematism has been experimentally demonstrated
in some terrestrial invertebrates25,26, and suggested for a number
of marine animals including cnidarians27, molluscs28, annelids29,
crustaceans30, echinoderms31 and teleost fish32; this, however, is the
first suggestion of this behaviour in a cartilaginous fish.
Transmission of bioluminescent signals by mineralized structures,
such as the spines of the velvet belly lanternshark, are rare in nature,
having only been observed in several organisms including opisthoproctid
fishes33 and a marine snail (Hinea brasiliana)
27. Further
examination of E. spinax’s spines may provide insights into evolutionary
co-opting of structural biomaterials for optic functions.
Although SAPs appear structurally and physiologically similar to
the other photophores harbored by the velvet belly lanternshark,
their dorsal glow represents a unique bioluminescent display. This
‘abnormal’ luminescence might also be found in other lanternsharks
(Etmopteridae), which harbor similar dorsal finspines and ventral
photophores11. Even though the function of these organs remains
hypothetical, they clearly demonstrate that warning luminescence
and counterillumination, two apparently contradictory strategies,
could surprisingly operate in a single organism at the same time.
Methods
Sharks. Adult sharks were collected using longlines in the Raunefjord (Norway)
between December 2009 and October 2012, transferred to Espeland Marine Station
and housed in tanks placed in a dark cold room. Our protocol, including fish sacrifice,
was in accordance with national guidelines for experimental fish care (fish handling
approval #1664 was given by the National Committees for Research Ethics of
Norway).
Photographs of spontaneous luminescence were taken in complete darkness with a
Canon 7D camera (whole shark, sensitivity 6400 ISO, objective 20 mm; dorsal fins,
12800 ISO, 60 mm; both with an aperture of 2.8 and an exposure time of 30 s). For
visibility purpose, a post adjustment of brightness and contrast was applied to the
entire pictures using Adobe PhotoshopH. Other pictures were taken with the same
camera in natural light.
Luminescence recording. A Berthold FB12 luminometer coupled to an optical fibre
was used to record in vivo intensity of dorsal and ventral continuous luminescence
from several live specimens according to Claes et al.14. Values were corrected for fiber
absorption and angular losses. Ventral photophore spacing/density and mean
intensity of specimens was estimated according to Claes and Mallefet15, by counting
the photophores present in standard (0.25 cm2
) ventral skin patches under a
binocular microscope.
Spine structure. Dorsal finspines were removed from dead specimens wrapped in
plastic wrap with a small amount of fluid for hydration, and packed tightly with paper
towels into plastic tubes for micro-CT scanning (1174 scanner, SkyScanH, Kontich,
Belgium). The X-ray source was set at 50kVp (peak kilovoltage) and 800 mA; 220
projections were acquired over an angular range of 360degrees. Samples were scanned
with an isotropic voxel size of 16.8 mm, an integration time of 4500ms, and a 0.25mm
aluminum filter to decrease beam-hardening effects. Scans were reconstructed using
commercial software (NReconH SkyScanH software, version 1.6.1.2) then visualized
in 3d and segmented virtually in multiple planes using Amira software (Mercury
Computer Systems) to examine spine structure and morphology.
Histology. Skin tissues containing SAPs from dorsal fins of freshly dead specimens
were excised and fixed in a 3.5% formaldehyde solution for two weeks. Tissues were
progressively dehydrated (50, 70, 2 3 90%, 1 hour each), placed 1 h in 100% butanol
and left overnight in 100% butanol at 60uC. Tissues were then submerged in paraffin
wax for different periods (12 h, 1 h and 3 h) at melting temperature (58uC). Sections
were performed with a metal knife microtome, stained using Masson’s trichrome, and
photographed under a light/fluorescence microscope (Leitz Diaplan, Leitz, Wetzlar,
Germany).
Spine transmission. The second dorsal finspines of freshly dead specimens were
removed and their anterior-posterior spectral transmittance was measured near the
tip, at 75% of the exterior spine length, using an Ocean Optics S2000
spectroradiometer. Broadband white light from an Ocean Optics PX-2 pulsed xenon
lamp was delivered to the caudal surface of the spine via an optical fibre terminated
with a quartz lens. Light transmitted anteriorly by the spine was collected via a second
quartz lens coupled to an optical fibre connected to the spectroradiometer.
Visual modeling. The visibility ranges of both ventral and dorsal glows were
calculated according to the theory developed by Nilsson et al.17. This range depends
on the intensity of down-welling daylight and thus on depth in the sea. Etmopterus
spinax was assumed to occur at ‘counterillumination depth’ where its silhouette,
Figure 3 | In vivo recordings and visual modeling of Etmopterus spinax luminescence. (a) Correlation between SAP (from the second dorsal fin) and
counterilluminating ventral photophore (CVP) luminescence intensity (I). Mq s21
, megaquanta (106 photons) per second. (b) Detection of CVP
luminescence as a function of the shark’s occurrence depth for heavy overcast and clear skies (sun at 45u). Counterillumination occurs when detection 5
0. (c) Detection distance of SAP luminescence at counterillumination depth as a function of the pupil diameter of the observer. This distance is calculated
(1) around or (2) through the spine, assuming 10% transmission. Dots represent pupil diameter of the shark (grey), its main prey (the pearlside fish
Maurolicus muelleri; white) and its potential predators (piscivorous fishes and marine mammals; black).
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SCIENTIFIC REPORTS | 3 : 1308 | DOI: 10.1038/srep01308 3
