Condie: Retia in L. ditropis
ABSTRACT
The Pacific salmon shark, Lamna ditropis (Hubbs & Follett, 1947), can maintain core
body temperatures 7 to 11°C above ambient waters—the highest body-temperature
elevation of any shark measured to date. Gross anatomical studies of three subjects
revealed that the mechanism for such endothermy is facilitated by counter-current
heat exchangers common in other lamnids and scombrids. In these fish, retia
mirabilia—"wonderful nets" of interwoven vessels and/or sinuses—act as metabolic
heat recyclers. L. ditropis possesses four retial systems, three of which serve the
viscera and muscle, much like the retia of its Atlantic cousin, the porbeagle shark,
Lamna nasus. The lateral cutaneous rete begins laterally in the white muscle and
serves the well-insulated internal red muscle. This tubular rete, composed of
bundles of arteries and veins, is small in area near the skin, but divides and widens
as it nears the internal red muscle. The viscera is warmed by the supra-hepatic rete,
a two-lobed tissue lying posterior to the heart and dorsal to the liver lobes. Minute
arteries repeatedly converge and coalesce in this rete in a honeycomb pattern, and
the entire arterial mass is contained in a large venous sinus. The sub-renal rete
consists of a small anterior retial lobe plus its lateral extension, which runs the
length of the kidney, conserving metabolic heat created there. We propose that the
combined efficiency of these retial systems allows L. ditropis to occupy its icy niche,
the most polar of any shark.
INTRODUCTION
Due to the extremely limited number of salmon sharks obtained for scientific
research, L. ditropis is as yet virtually uncharacterized. Knowledge of the salmon
shark has been mere supposition, and the consensus has been that its anatomy and
Condie: Retia in L. ditropis
endothermy are probably similar to that of other members of its family, Lamnidae.
Its closest relative and the only other member of its genus, the porbeagle shark,
Lamna nasus, is a North Atlantic species of similar external morphology and known
high levels of endothermy reaching 11°C above ambient (Carey & Teal, 1969). Yet
supposition seems inadequate in the realm of science.
A single published paper (Carey, 1985) exists describing the endothermic
characteristics of L. ditropis. No descriptive anatomical work was done, and only
two of the shark's four retial systems were even mentioned. Using the descriptive
work of Burne, Some Peculiarities of the Blood-Vascular System of the Porbeagle Shark
Lamna cornubica, published in 1923, as our guide to the general anatomy of a
lamnid shark, we set out to outline the circulation and retial systems of L. ditropis:
the orbital, lateral cutaneous, suprahepatic, and sub-renal retia mirabilia (figure 1).
We found a similar, yet even more impressive, system than that which Burne
described 70 years ago, as more detail could be seen with new tools and techniques.
MATERIALS AND METHODS
Collection
Two of the three Lamna ditropis specimens (LDI and LD2) were obtained from
local fishermen from Monterey Fish Company and stored whole at -20°C. The third
specimen, LD4, was found beached near Moss Landing, California, and stored at
-20°C. Careful morphometrics were taken, using straight rather than curved
measurements. Specimens were thawed individually for approximately 24 hours
before dissection.
Fork Lengtl
Total Mass

Collection Method
LDI
129 cm
34.9 kg
gill net
45.2 kg
LD2
144 cm
gill net
92 cm
12.1 kg
LD4
beached
Condie: Retia in L. ditropis
Silicon Injection
Segments of Intramedic polyethylene catheter tubing (ID 0.76, OD 1.2 mm)
and Alpha Wire Corporation PVC tubing, size codes PV 105 17 and 105 18,
previously cut at differing lengths, were epoxied together using Duro Master Mend
Epoxy, Extra Strength Quick Set. A Cole-Parmer three-way stop-cock was epoxied to
the catheter tubing of largest diameter. Once catheters were readied (generally the
epoxy was allowed to dry for several hours), small "windows" were skinned from the
well-iced specimen using stainless steel surgical blades (Feather, no. 10 and Schein,
no. 22) to expose vessels of interest. A small incision was made in the vessel, the
catheter-syringe hook-up was inserted into the vessel through the incision, and the
vessel was rinsed with sea water to clear it of blood and test it for knicks and leaks
before injecting with a silicon. Microfil^-brand injection compound, a radiopaque
silicon rubber made by FlowTech, Inc. of Carver, Massachusetts, was prepared in
desired volumes by mixing MV-color: MV-dilutent: MV-curing agent in ratios of 5: 4:
0.45. Red-dyed silicon denoted arterial vessels, and blue, venous.
Äfter rinsing the vessels with sea water, a catheter was fed several inches down
the vessel past the point of incision, and surgical suture was tied in a loose knot (or
a hemostat was held with light pressure) to seal off the vessel incision to prevent
backflow. As someone stood ready with extra hemostats and gauze to clip off any
leaky vessels in the injection vicinity, another began to apply hand pressure to the
Microfil-filled syringe or to pump the attached sphygmomanometer up to
physiological pressures of between 30 and 80 mmHg (i.e., between 4.00 and 10.67
kPa). Injection proceeded until back-pressure was too great for the injector to force
any more compound from the syringe, or until the entire prepared volume of
Microfil was injected.
Condie: Retia in L. ditropis
Injections generally required two or more people, and accidents such as
bursting of the balloon of the sphygmomanometer or severing of the catheter tubing
at sites where it had been epoxied did occur, releasing injection compounds at high
pressures. Thus the need for safety goggles and protective clothing was
demonstrated.
Gel-time for Microfil, timed from the moment that curing agent is added to
the mixture, is 20 minutes (i.e., the compound had set to the point of being
uninjectable through the syringe in approximately 20 minutes). The compound was
generally allowed 2 hours to set up completely after being injected into a vessel
before further dissection was undertaken.
Microfil-injected vessels and retia could be sliced easily with scalpels and/or
knives, and injected tissues could be frozen or fixed in 10% phosphate-buffered
formalin without altering the injectant.
Alternative Injection Compounds
Although Microfil-MV was the injection compound of choice, two other
compounds were tested.
Bateson's 17
Bateson’s 17, a methylmethacrylate corrosion casting compound, was used to
inject both lobes of the suprahepatic rete in LD1. A mixture of Bateson's monomer:
catalyst B: promoter C: methylmethacrylate in the ratio 50: 10: 1: 50, plus a small
amount of either blue or red pigment, was injected, following the injection
procedure outlined above. However, Bateson's 17 produces more dangerous fumes
and should be restricted to well-ventilated areas. Working time for Bateson's was
approximately 30 minutes; set time approximately 2 hours.
Casts created with Bateson's were excised from the specimen and placed in 6%
potassium hydroxide (KÖH) to corrode the tissue surrounding the injected vessels
Condie: Retia in L. ditropis
and tissue was allowed to macerate for several days. KOH solution was replaced
with fresh approximately once a day. Casts were eventually stored in 50% ethanol.
CP-101
A new cast-corrosion product made by Flow Tech, Inc. was tested in the
ventral right lobe of the suprahepatic rete in LD2. CP-101 compound was mixed
with stannous octoate and ethyl silicate in ratios 20: 1: 1. However, we found that
when corrosion was attempted in 10% KOH solution (reduced from the
recommended 20-35% solution), the solution began to macerate the compound itself
after 2 days, while the cast was still not cleared of tissue. The cast disintegrated and
was unusable.
Retia Injected
Retia were injected in the following approximate order:
A. Orbital Rete
Eight to 15 ml, scaled to size of specimen, were injected into the
pseudobranchial artery, which extends laterally over hyomandibular cartilage,
between this cartilage and the jaw bone within the head.
B. Lateral Muscle Rete (Arterial)
Thirty to 70 ml were injected into the lateral cutaneous artery as anteriorly as
possible, ideally making the injection incision in the artery just as it arises
from the third gill arch, just above the level of the dorsal-most portion of the
third gill slit (figures 2 and 3). Complete injections filled the arterial portion
of the muscle rete, which derives from the posterior side of the artery and
courses through the white muscle (figure 4). Bundles of retial vessels
alternate with white muscle fibers, forming bands (figure 5).
C. Lateral Muscle Rete (Venous)
Condie: Retia in L. ditropis
Twenty to 55 ml were injected into one of the two lateral cutaneous veins
(above and below the lateral cutaneous artery), filling both vessels (figure 3).
Complete fills of these veins gave a good venous injection of the muscle rete
below them, descending all the way to the base of the red muscle against the
visceral cavity wall (figures 6, 7).
D. Sub-renal Rete (Arterial)
Injecting 20 to 40 ml into one of the two lower lateral cutaneous arteries
would generally fill both vessels with injectant, as they converge (figure 3).
These vessels were found by skinning below the lateral cutaneous artery, at
approximately the line of countershading, where dark gray turns to ventral
ecru. The lower lateral cutaneous arteries arise and diverge into two vessels at
the level of mid-scapula, just behind the gills. A complete fill of these arteries
ensures a fill of the kidney rete's arterial vessels.
E. Sub-renal Rete (Venous)
An average of 20 ml was injected into the lower lateral cutaneous veins, which
lie below each of the lower lateral cutaneous arteries, when these veins weren't
filled by the injection of the upper lateral cutaneous veins. The injection of
these vessels is mandatory if venous injection of the kidney rete is desired.
More injectant would have been needed for a full venous fill of the kidney
rete.
Suprahepatic Rete (Arterial)
The pericardial arteries, found in the dorsal wall of the pericardial cavity, just
under the heart, were injected, 50 to 70 ml in each in order to fill the minute
arterial system of each lobe of the suprahepatic rete (figures 8, 9). An incision
was made at the ventral side of the pectoral girdle, down the ventral midline
of the shark (about level with the pectoral fins), and the coracoid bar cartilage
was severed to reveal the pericardial cavity. Generally, it was necessary to
F.
Condie: Retia in L. ditropis
excise the atrium and ventricle just anterior to the muscular conus arteriosus
in order to get enough access to the pericardials for injection.
G.
Suprahepatic Rete (Venous)
Venous feed of the suprahepatic rete arises from the hepatic sinus, which lies
within the posterior pericardial cavity and fills the venous sinus bathing the
arterial retia vessels. Via this sinus, the venous portion of the hepatic rete
may be filled. Approximately 90 ml of injectant may be necessary, as
multiple sinuses are found in this area, and all are inter-connected (figure 8).
Caution must be taken not to pierce the liver lobes: if pierced, squalene oil
will leak at a rapid rate.
H. Red Muscle Vein
The red muscle vein—found by steaking at approximately 38% of body length,
or just behind insertion of dorsal fin, and searching the region of the red
muscle lying closest to the visceral wall—was injected with between 20 and 40
ml of blue silicon. This produced a satisfactory venous fill of the cranium and
orbital retia.
After we were confident that our injections had had sufficient time to set, we
began dissection. Careful skinning revealed both of the lateral vessel systems,
including the lateral cutaneous artery and the two veins surrounding it, and the
lower lateral cutaneous arteries and the vein below each (figure 3). This allowed us
to confirm that the retia had been successfully injected before vessels feeding them
were severed in further dissection. Using calipers, internal vessel diameters were
taken of all lateral vessels (table 1).
Steaking of the lateral muscle tissue, from posterior to anterior, followed. We
took careful note of distance from the caudal fork so that accurate red-muscle¬
placement calculations could be made. Development of the lateral muscle retia
Condie: Retia in L. ditropis
could be seen in steak progressions, and muscle rete areas were measured (table 2).
Steaks were labeled and either frozen or fixed in 10% phosphate-buffered formalin.
Excision of the visceral organs was necessary once the viscera had been exposed
in steaking. The stomach was emptied, and the heart, liver, gall bladder, pylorus,
pancreas, spleen, spiral valve, and rectal gland were weighed individually using an
Ohaus balance (table 3). The suprahepatic rete was usually excised last, both lobes
at once.
The kidney could be found by first removing the claspers (in the case of the
male) and anal fins, and then finding where the lower lateral cutaneous arteries (at
the level of the cloaca) converged into the cloacal arteries and turned sharply
toward the ventral midline (figure 10). Just under the ureter, these cloacal arteries
enter the posterior lobe of the sub-renal rete (figure 11 ), which comprises the
posterior-most point of the kidney on the ventral side. The entire kidney was
generally excised for weighing and study of the rete (f igure 12).
After weights were taken, a razor blade was used to make retial sections of the
suprahepatic retial tissue (figure 9), and of the posterior lobe of the sub-renal rete
(figure 13) and the muscle rete (figure 4), to be viewed under the dissecting scope.
Optical micrometry measurements of vessel diameters were taken on a Leica MZ8
stereomicroscope, with a 6x Ernst Leitz Wetzlar ocular micrometer, under variable
magnification.
RESULTS
Muscle Retia
The lateral cutaneous rete consists of bundles of vessels, both venous and
arterial, intermingled, which extend throughout the white muscle toward the red
muscle which lies adjacent to the visceral cavity. Each bundle consists of
Condie: Retia in L. ditropis
approximately 8 to 10 vessels of each type, with the venous vessels generally
encircling the arterial.
The trapezoidal area of the muscle rete (measured in cross-sections of muscle
tissue) is a result of vessels being closely packed near the surface, but branching and
spacing as they extend through the white muscle. Increased retial area occurs with
increases in the total length of the shark, with the greatest mean retial area (12.136
+ 3.59 cm2) being found in the largest shark, LD2, followed by 10.775 + 3.93 cm2 of
muscle rete per cross section in LD1, our next smallest specimen, and 6.77 + 0.23
cm2 in the yearling LD4 (table 2). Area increases are the result of increased
branching of vessels as they diverge from the densest portion of the rete nearest the
skin and extend toward the internalized red muscle. Venous vessels extend from the
visceral side of the red muscle all the way to the lateral cutaneous veins (figure 7,
with internal vessel diameters of these lateral veins ranging from 3.0 to 5.5 mm in
our three specimens (table 1). The mean vessel diameters of the retia associated
with these lateral veins range from 0.10 mm to 0.12 mm in diameter, + 0.032 (table
4). The diameters of muscle-rete vessels tend to increase with increasing body mass
(figure 14).
Suprahepatic Retia
Ratios of suprahepatic retia to mass of viscera appear consistent with
measurements found by Carey et al., 1985, for L. ditropis . His values, 0.0279 and
0.0536, lie within close range of our calculated ratios, 0.031, 0.033, and 0.06 (table
3). The mass of the suprahepatic rete in L. ditropis appears to be a linear function
of visceral mass (figure 15), fitting its line of regression with an r2 closeness of fit
value of 0.954. This linear relation echoes Carey et al., 1985, data for mako shark
Isurus oxyrinchus, another member of the lamnid family.
Condie: Retia in L. ditropis
L. ditropis heart to body-mass ratios appear higher than those found for I.
oxyrinchus in Carey et al., 1985. L. ditropis ratios ranged from 0.32% to 0.42%,
whereas mako values clustered between 0.14% and 0.20% (figure 16). L. ditropis
heart to total-mass ratios follow a linear regression with a closeness of r2 - 0.998
(figure 17)
In accordance with Carey et al., 1985, findings for the mako, visceral mass and
suprahepatic rete mass in the salmon shark may increase exponentially with
increases in total body mass, with closeness of fit values of 0.995 and 0.852,
respectively (figures 18 and 19). But L. ditropis values cluster well above would-be
lines of regression for Carey's mako data, indicating a much greater relative size of
the suprahepatic retial tissue with respect to both total mass and visceral mass in
the salmon shark. Only in viscera to total-mass ratios do the mako and salmon
shark cluster together, indicating that visceral masses in the salmon shark are
consistent with those of other members of its family (figure 16).
The vessels of the arterial portion of this rete are among the smallest of any
retial vessels in the body, as shown in figure 20. Mean diameters and their standard
errors (figure 21) ranged from 0.1117 + 0.008 to 0.0773 +0.005 mm, for our largest
and smallest sharks, respectively.
Arterial vessels of the suprahepatic rete are structurally distinct from those of
the lateral muscle rete. As shown in figure 9, vessels divide, coalesce, and redivide
multiple times, creating a honeycomb-like appearance. Venous blood is supplied in
a large sinus encompassing each of the lobes of the rete.
Sub-renal Retia
The arterial vessels of the kidney rete also divide and converge, only to divide
again, in a fashion similar to that of the suprahepatic rete (figure 13). Unlike the
suprahepatic, the venous supply to the kidney rete is not a simple sinus feeding
Condie: Retia in L. ditropis
another sinus. Rather than a simple steady bathing of warm blood from a large
open sinus, the sub-renal rete instead receives blood directly from lower lateral
cutaneous veins which enter the posterior lobe of the rete and surround the arterial
vessels. As figure 13 elucidates, the venous blood is not confined by vessels as in
the lateral muscle retia; it also appears that the kidney rete can contain a
significantly smaller volume of venous blood than the spacious sinus of the
suprahepatic rete. However, as only a partial venous injection of one shark was
achieved, more research is needed to fully distinguish the venous character of the
sub-renal retia. As shown in figure 20, the honeycombing vessels of the sub-renal
rete are the largest of any of the retial vessels by a substantial margin, with mean
diameters which are sometimes almost double those of other retia in the same
specimen. Arterial vessel diameters of the sub-renal rete appear to be dependant on
total body mass , but are fairly consistent within each rete (figure 22), with
standard errors of 0.011 and 0.007 mm for LD2 and LD4, respectively
DISCUSSION
Only three salmon sharks have ever been studied alive or freshly caught. In
1983, Smith and Rhodes caught three while hand-trolling for salmon in Alaska.
According to the unspecific “deep body temperature" measurements they acquired,
L. ditropis can attain body temperatures 8 to 11°C above surface water
temperatures. However, none of the three salmon sharks caught was hooked in
surface waters, but instead at depths ranging from 12 to 40 meters, where waters are
even colder. This implies an even greater differential between core body
temperature and even colder water temperatures at depth. Therefore, the salmon
shark must be the most endothermic of the lamnid sharks, followed closely only by
its Atlantic cousin L. nasus. Visceral temperatures, however, have never been
documented for L. ditropis. Nor has descriptive anatomical work produced
12
Condie: Retia in L. ditropis
documentation to support the suppositions of similarity between counter-current
heat exchange mechanisms in L. ditropis and those of other lamnids. Lack of such
data seems negligent in light of the fact that salmon sharks have been identified as
occupying the coldest of shark habitats, 2° C or colder, and thus probably possess the
most efficient of retial systems in all of nature.
Relative heart size is larger in Lamna species than in other lamnid sharks.
clustering well above heart ratios in I. oxyrinchus (figure 16). Both L. nasus and L.
ditropis possess heart: body ratios of between 0.263% and 0.398% in data collected
by Carey et al., 1985, and reaching a maximum of 0.42% in our data (table 3). If
the assumption that heart size is related to cardiac output, demand for oxygen, and
thus heat production, is made (Carey et al., 1985), then anatomical data supports L.
ditropis' physical physiology being best suited of all lamnids for the highest levels of
endothermy—the highest degree to which the shark may be independent of water
temperature
In 1923, anatomist R. H. Burne published a detailed anatomy paper on L.
nasus, then referred to as L. cornubica. In it, he describes four retia mirabilia
(appearance and location only, no mention of function) that he had identified in
the porbeagle. Salmon sharks possess all four of the retia Burne outlined for their
Atlantic relative. Scientific literature, however, has failed to anatomically
characterize most of these retia, and has completely passed over one of these four
the sub-renal rete of the kidney, whose function is still unknown. Presumably,
however, this rete utilizes the metabolic heat produced in the kidney by the
hydrolysis of ATP in active reabsorption and secretion of ions to warm the
oxygenated arterial blood coming into the lobe of the rete from the lateral cutaneous
arteries. This counter-current exchange could speed the clearing of nitrogenous
wastes, excess ions, and salts via the excretory system to ready this scavenger¬
Condie: Retia in L. ditropis
predator for its next meal. Such a hypothesis parallels the reasons given for an
endothermic fish warming its viscera to increase digestive enzyme efficiency and
clear meals from its digestive tract at a rapid rate (Fudge, 1996; Carey, 1981), and
extends such rapid systemic clearing to the excretory as well as the digestive systems
The suprahepatic rete of L. ditropis, conceivably the largest retial tissue in
nature, has never been described. This rete, embodying up to 0.638 % of the shark's
total body mass, nearly doubles comparative percentages of retia of other lamnids
(in others, such as C. carcharias and L. nasus, the suprahepatic rete averages
between 0.20 to 0.30% of total body mass (Carey, 1985)). If assumptions of relative
size of retial tissue can be extended to indicate efficiency levels, L. ditropis should be
physiologically equipped to attain the highest levels of independence from
surrounding water temperatures of any warm shark or fish.
The functional advantage of the arterial "honeycomb" of the suprahepatic
and sub-renal retia is still a mystery. Both honeycomb retia, though, appear to be
immersed in a low-pressure sinus or sinus-like sac of tissue, thus receiving a constant
bathing of cold arterial blood in a warm venous pocket for maximal heat transfer. It
is also unclear from previous lamnid research whether such a honeycomb nature of
retia is a characteristic of the entire family Lamnidae, or if it is species-specific to L.
ditropis.
Carey (1983) mentioned the existence of a venous suprahepatic bypass vessel
in the porbeagle, running beneath the lobes of the rete. The salmon shark also
possesses such a vessel. It is unclear whether this vessel is truly functional, since it
seems unlikely that the diameter of this venous bypass could withstand the volume
of blood which the rete continually encounters. The arterial vessels he suggests as
hypothetical bypasses are even more unlikely because of their small diameters— too
little blood could circumvent the rete for the vessels to be considered a bypass
system. The amount of blood re-routed is probably no more than is necessary to
14
Condie: Retia in L. ditropis
serve the ultimate sites of these vessels with oxygenated blood, which would be a
constant function of the vessels and thus not a regulatory, efficiency-altering system
with respect to the rete. However, more research would be necessary to come to solid
conclusions as to the effectiveness and exact mechanism of these bypass systems.
The lateral muscle retia in the salmon shark appears to be similar in structure
and size to that of the porbeagle. However, anatomical measurements indicate that
the suprahepatic rete is larger and more defined in L. ditropis than in other
lamnids, and measurements and characterization of the kidney rete are as yet
singular, lacking anything to which to compare it. As anatomical analysis can only
infer the answers to physiological temperature-regulation questions, more data will
need to be collected from live sharks before definite conclusions can be made about
the salmon shark's degree of endothermy, and therefore about regional endothermy's
upper limits in nature. Implications from post-mortem physiology place L. ditropis
in the highest position of endothermy of any shark.
CONCLUSIONS
Lamna ditropis may well have the most well-developed and efficient retial
system in all of the animal kingdom. It can claim the largest rete yet found in
nature, the massive two-lobed suprahepatic rete, which is also structurally distinct.
with minute arterial vessels which branch but then recombine with incredible
consistency. Its lateral muscle retia occupy a similar cross-sectional area within the
muscle, as well as a similar structure of parallel bundles of tubular, longitudinally-
directed vessels, to lateral cutaneous retia described in the porbeagle. And the newly
defined sub-renal rete, rediscovered after 70 years of oversight, appears to be truly
unique, with its lobe and lateral extension down the entire ventral length of the
kidney consisting of a single layer of arterial vessels beneath a protective connective
tissue sheath. The functionally unknown anastomosis and reconvergence of the
Condie: Retia in L. ditropi.
arteries in the latter two retia present a need for future mechanical and efficiency
experiments with such model vessel systems.
Condie: Retia in L. ditropis
LITERATURE CITED
Burne, R. H. (1923). Some Peculiarities of the Blood-Vascular System of the
Porbeagle Shark (Lamna cornubica). Philos. Trans. R. Soc. London 212B:
209-257
F. G. and Teal, J. M. (1969). Mako and porbeagle: warm-bodied sharks.
Carey
Comparative Biochemistry and Physiology, 28, 199-204.
Carey, F. G., J. M. Teal, and J. W. Kanwisher (1981). The Visceral Temperatures of
Mackerel Sharks (Lamnidae). Physiol. Zool 54 (3), 334-344.
F. G., J. G. Casey, J. L. Pratt, D. Urquhart, and J. E. McCosker (1985).
Carey
Temperature, Heat Production and Heat Exchange in Lamnid Sharks
Memoirs of the Southern California Academy of Sciences,
9, 92-108
Fudge
Douglas Steven (1996). Anatomical and Biochemical Adaptation to
Visceral Endothermy in Bluefin Tuna (Thunnus thynnus). Masters thesis,
University of Guelph.
Smith, R. L. and Rhodes, D. (1983) . Body Temperature of the Salmon Shark,
Lamna ditropis. J. mar. biol. Ass U. K., 63, 243-244.
Condie: Retia in L. ditropis
TABLES
Table 1: Vessel diameters of all lateral vessels. Measurements given are of internal
vessel diameter unless indicated by (ext) following a measurement, in which case the
measurement taken was and external diameter. Capillaries between refers to tiny
arterioles which connect the lateral cutaneous artery with the upper of the lower
lateral cutaneous arteries— these branches split off the main vessel at approximately
every 5 mm, running down the white muscle just under the skin. These vessels run
in a parallel, diagonal fashion and connect the two lateral vessel systems, indicating
that the two share common blood and/or mixing occurs. Diameters may be biased
by how great the volume of injectant used to fill the vessel (i.e., some vessels more
stretched than others?)
Note that the lateral cutaneous artery tends to be larger in diameter than the either
of the lower lateral cutaneous arteries. Also, lower of the veins surrounding the
lateral cutaneous artery tends to be the larger, whereas vein lying between the two
lower lateral cutaneous arteries was consistently the larger of the lower lateral
cutaneous veins.
Table 2: Mean trapezoidal areas of tubular muscle retia. Note that area of rete is
consistent with increasing total size of the specimen.
Table 3: Viscera masses, followed by relative mass as a percent (visceral mass/total
body mass x 100). Total mass of viscera used to estimate ratio between mass of
suprahepatic rete and the organs it serves.
Ratios will be crude estimates, as retia
were injected with silicon injection compound before being weighed each time. Thus
suprahepatic masses, as well as that of kidneys, will be altered. Note that the
suprahepatic rete of LD4 is an uninjected mass, and thus reflects blood rather than
silicon within the vessels, and may be inconsistently high. Stomach contents were
emptied from LDI before weighing; however, no stomach contents were found in LD2
or LD4. Heart not included in viscera totals. Note that right/left of lobes is from a
dorsal view of the specimen, (i.e., left lobe of liver if looking from dorsal view). Also
note that some loss of oils from liver occurred during dissection, and thus masses of
liver lobes may be inaccurately lower than their true values. Pancreas was never
identified, and thus never weighed, in LDI or LD4, nor was rectal gland in LDI and
Condie: Retia in L. ditropis
LD2. Heart of LDI weighed only in its entirety, atrium and ventricle together. Aorta
mass in LD2 reflects organ weight while still blood-filled.
Note that ratio of suprahepatic rete mass to viscera served is consistent in LDI and
LD2, in both of which rete was injected before weighing. Ratio seems to increase
when rete was uninjected in LD4, possibly due to blood remaining within sinus of
the rete, which was rinsed or displaced in injected retia.
Table 4: Mean vessel diameters, + their standard deviation from the mean, for
each of the three retia in each of three specimens. Sub-renal vessels from LDI were
never measured due to injection compound difficulties.
Condie: Retia in L. ditropis
FIGURE LEGENDS
Figure 1: Rough locations of the four retia found in Lamna ditropis. Red
represents arterial vasculature.
Figure 2: The origins of the left lateral cutaneous artery, arising from within the
third inner gill arch and following the curve of the fourth outer gill arch before
extending laterally down the length of the fish and giving rise to the lateral muscle
rete. Photo of LD4.
Figure 3: LD4, double-injected with blue venous and red arterial. The upper of the
two vessel systems consists of the lateral cutaneous artery, which can be seen
extending from above the gills, and two lateral cutaneous veins, one above and one
below this artery. The more ventral system consists of two lower lateral cutaneous
arteries and a corresponding vein below each of them. The division of the arteries
into two from a single common vessel can be seen. This anastomosis occurs as the
artery rounds the cartilage of the pectoral girdle (white and shiny in the photo).
Figure 4: The minute, finely-branching extending from the lateral cutaneous
artery and veins of LD4. Rete vessels are at their most compact state, as they are
near the skin and the rete has not begun to widen in cross-sectional area. The
parallel nature of the vessels of this rete. Photo taken with sample submerged in
water, on stereomicroscope under 160 x magnification.
Figure 5: Cross-section of LD4. The venous vessels of both lateral muscle retia are
injected, showing the increase in retial area as in approaches and enters the
internal red muscle.
Figure 6: An up-close of the same rete. The bundled nature of the retial vessels is
evident, with fibers of white muscle between.
Figure 7: A cross-section of the muscle rete of LDI, prepared from a sample
preserved in 10% phosphate-buffered formalin and sectioned in paraffin. The
alternation of retial vessels bundles and white muscle fibers is clear. The pattern of
artery-vein meshing is seen. Arteries are thick-walled and filled with silicon. Veins
are unrinsed and thus show residues of blood.
Condie: Retia in L. ditropis
Figure 8: Both lobes of the suprahepatic rete of LD2, excised from the viscera and
esophagus. The enlarged pericardial arteries which feed the arterial portion of this
rete are evident in the anterior portion of the photo.
Figure 9: The suprahepatic rete of LD2 under 140 x magnification. Note the
honeycomb pattern of the arteries.
Figure 10: Convergence of the lower lateral cutaneous arteries into one cloacal
artery, which feeds the posterior lobe of the sub-renal rete. Once converged, the
cloacal artery turns toward the ventral midline of the shark, away from the lateral
course the lower lateral cutaneous arteries.
Figure 11: The posterior lobe of the sub-renal rete of LD2. The cloacal arteries,
seen extending from the rete on the right side of the photo, feed the tiny arteries of
this rete. The meshwork of minute vessels in this compact lobe are evident.
Figure 12: The entire kidney of LD2, from a ventral view. The cloacal arteries feed
the conical posterior lobe of the rete, with vessels from this lobe extending down the
length of the kidney (exposed from under the cylindrical bands of connective tissue
lining the ventral side of the kidney which normally protect these retial vessels).
Figure 13: Cross-section of the double-injected sub-renal rete of LD4 under 180 x
magnification. Bubble are an artifact of the water in which the rete was submerged
for photos. Note the honeycomb pattern of orange arterial vasculature and the
sinusoidal blue "fill" representing venous blood, unconfined by venous vessels.
Figure 14: Mean arterial vessel diameters of muscle retia, as calculated via optical
micrometry, with error bars representing standard error. Trend of vessel diameter
increase is consistent with increases in the total mass of the specimen. N values
indicate sample size of vessels measured. Exact values as follows: 0.1105 + 0.0055
(LD1), 0.1218 + 0.0072 (LD2), 0.1018 + 0.0064 (LD4).
Figure 15: Demonstrates the approximate (r2-0.954) linear relationship between
the mass of the suprahepatic rete and the mass of the viscera (liver and digestive
organs), both in kilograms.
Condie: Retia in L. ditropis
Figure 16: Plots L. ditropis data (open dots in red) on plots for I. oxyrinchus
(Carey et al., 1985), demonstrating the clustering of data for L. ditropis consistently
above would-be lines of regression for the Carey’s mako data with the exception of
viscera: body weight, where L. ditropis clusters with I. oxyrinchus, indicating a
similar relative visceral size in makos and salmon sharks. L. ditropis data is further
analyzed in figures 15, 17, 18, and 19.
Figure 17: Shows the linear relation (r2 = 0.998) of heart mass to total body mass
in L. ditropis.
Figures 18: Demonstrates the exponential (r2= 0.995) increase in visceral mass
with body mass, both in kilograms.
Figure 19: The mass of the suprahepatic rete may increase exponentially with
increases in total body mass (r2=0.852)
Figure 20: A comparison of vessel diameters among three retia, clustered according
to specimen. Note that largest vessel diameters are found in the kidney retia,
whereas vessel diameters of muscle and suprahepatic retia appear similar within the
same shark. Error bars indicate standard error, calculated via sample size (n) values
enumerated in figures 14, 21, and 22.
Figure 21: Mean vessel diameters of the suprahepatic retia, compared over three
specimens. Error bars indicate standard error from the mean vessel diameter,
namely, 0.1117 + 0.0055, 0.1682 + 0.0072, and 0.0773 + 0.0064 mm. N values
listed indicate sample size of vessel diameters measured for the calculation of
standard error.
Figure 22: Mean sub-renal retia vessel diameters, compared in two specimens.
Note that no measurements were possible in LD1, as our injection compound failed
to set up. Error bars denote standard error, calculated via the sample size n of
vessels measured using optical micrometry shown above each bar. Exact values are
0.31427 + 0.0115 (LD2) and 0.1554 + 0.0072 (LD3).
Condie: Retia in L. ditropis
ACKNOWLEDGEMENTS
Excessive thanks are in order for my advisor, Barbara Block, for her countless hours of
assistance, inspiration, and energy—never was she too busy to come see the wonders
we’d found over at the TRCC, and for remembering her spring quarter 97 shark
mommies when further research opportunities present themselves in Alaska, etc.;
Chuck Farwell, whose supportive words of praise kept us thinking we were doing
important work, as well as for his incredible trust in loaning us his camera for the
entire quarter, despite his fears of our covering it in blood and gore; to Tom
Williams, for his injection expertise and assistance and for his graciousness in
allowing us to use his x-ray equipment and contaminate his veterinary clinic with
shark crania multiple times; to Henry Mollet, elasmobranch expert, who served as
our link to the elasmobranch internet network, as well as teaching us the art of
proper morphometrics and sharing with us his carefully-collected compilation on
the natural history of the lamnid shark; to my research partner, Victor Tubbesing
for his patience, dedication, and words of warning when my scalpel seemed to be
cutting too deep; to Ellen Freund for her humor and interest in our project, and for
making us feel welcome as a part of the Block lab and for going through the trauma
of proof-reading this paper; to Heidi Dewar, for her consistently thought-provoking
physiological questions and willingness to explain basic fish anatomy to the
icthyologically-impaired, and for her flattering interest in our progress; to Doug
Fudge, for his tolerance with the smells and mess we created in his TRCC domain.
and his prompt ordering of supplies we needed "by tomorrow," as well as his
interesting questions he tried to pass off as answers to our questions in such a
teacher-like fashion; to Dave Marcinek, for obtaining the sharks for us so that our
project was possible at all, and for trusting us with the sharks before he had
Condie: Retia in L. ditropis
collected his data from them; to Loretta Arcangeli and Carol Reeb for their patient
sacrifice of valuable lab time to the failed DNA portion of our project; to Jens
Franck, for selflessly getting off this computer so that I could write this before Barb
leaves for New Hampshire; and to our beloved salmon shark, for being such good
sports and amazing creatures.
a
L
a
L

U
80
FIGURE 2
FIGURE 3
FIGURE 4
FIGURE 5
FIGURE 6
FIGURE 7
FIGURE 8
FIGURE 9
FIGURE 10
FIGURE 11
FIGURE 12
FIGURE 13
0.15-
0.05
0-

FIGURE 14
Comparative Arterial Diameters, Muscle Retia
n=20
n=20
.
n=12
I



Body Mass (kg)
FIGURE 15
Suprahepatic Rete vs. Visceral Mass
0.25
y = 0.027x + 0.033 r2 = 0.954
024

-----------
0.15+
H
0.1+
005
tataakata-
Visceral Mass (kg)
-0.25
0.2
-0.15
os
FIGURE 16
Isurus oxyrinchus vs. Lamna ditropis: Visceral and Heart Ratios
40 —
30
200
20
100
10





O 22 40 60 80 100 120 140 160 180 200
20 40 60 80 100 120 140 160 180 200
B00v Wele (4g)
200
150

00
100
50


0 20 40 60 80 100 120 140 160 180 200
30—
20
40
BO0Y WEIGHT (kg)
VISCERA WEHT (g)
Carey et al., 1985
FIGURE 17
Heart vs. Total Mass
0.15
0.15
y = 0.003x + 0.016 r2 =0.998
0.125 +—
--.---..-.
0.125
0.1+
0.1
0.075+
—--+0.075

—+ 0.05
total mass (kg)
FIGURE 18
Visceral vs. Total Mass
—
y =0.706 * 100.021x 12 - 0.995
.--------------
.

4
A
3...........-------

----------------

Total Mass (kg)
FIGURE 19
Suprahepatic Rete vs. Total Mass
0.25
0.25
y =0.051 * 100.012x r2 -0.852
0.2+
P
0.2
3
0.15+
------
-%0.15
0.1-
0.1

0.05 +
—0.05
Total Mass (kg)
LDI
FIGURE 20
A Comparison of Retial Vessel Diameters
LD2
LD4
Specimen
muscle
E liver
kidney
FIGURE 21
Comparative Arterial Vessel Diameters of Suprahepati
0.2
n=31
0.15-
n=13
0.1 -
D=13
0.05-


Specimen
0.4
0.3
0.1 -
0-
FIGURE 22
Arterial Vessel Diameters, Sub-Renal Retia
n=50
n=48


Specimen