~uologicalJuurnalofthe Linnean Socieh, (1996), 118: 151-164. With 5 figures
Evidence of brain-warming in the rnobulid rays,
Mobula tarapacana and Manta birostris
(Chondrichthyes: Elasrnobranchii: Batoidea:
Myliobatiforrnes)
R.L. ALEXANDER
Shark Research Centre, Diuision of LL$ Sciences, South African Museum, Cape Town, 8001,
South Africa; and Department of <oology, Universib of Cape Town, Rondebosch, 7700, South
Afica
Keceiued April 1995, accepted for publication Januay I996
The presence of cranial retia mirabilia in rays of the genus hfobula is well established. Although
previously regarded as consisting exclusively of arteries, the presence of veins has now been established
in gross dissections of the rete in the mobulid, Mania birustris. Histological examination of the retia in
Manta biro.rtris and Mobula tarapacana confirms the presence of veins. These firidirigs suggest the presencr
of a counter-current heat-exchanger that warms the brain.
01996 The Linncan Society of Lnndnn
ADDITIONAL KEY WORDS: - mobulids
~~
cranial retia
~
counter-current system.
CONTENlS
Introduction . .
Materiais . . .
Methods . . .
Results . . . .
Discussion . . .
Acknowledgements
References . . .
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152
153
160
I63
163
INTRODUCTION
The cranial retia mirabilia of the mobulid species Mobula thurstoni, M. japanica, M.
tarupacuna and Mobula sp. have been reported as comprising purely arterial networks
that surround the brain and fill the cranial cavity anterior to the telencephalon
(Schweitzer & Notarbartolo-di-Sciara, 1986). According to these authors, the retia
comprise a caudal component that surrounds and supplies the brain itself, and a
precerebral rete that extends anterior to the brain, passing antero-laterally towards
the olfactory bulbs. The only veins found by the authors were on the dorsal surface
of the arterial network, and were reported as being quite separate from the arteries
and thus not part of the retial system.
0024-4082/96/010151+14 $25.0010
151
01996 The Linnean Society of London
152
R. L. AI,F,XANDEK
During the course of a comprehensive study on the anatomy of mobulids, a wclldeveloped counter-current vascular network was found in the pectoral fin of Mobula
tarapacana, indicating that this species is warm bodied (Alexander, 1995). As a result
of this finding, a re-examination of the cranial retia rnirabilia of mobulids was
initiated. In this paper, results of the examination of the cranial retia of Ad. tarapacana
arid the closely related mobulid species, Manta birostris, are presented.
MAI‘ERIALS
Five specimens of Manta birostris were examined. The animals were caught in shark
ncts set by the Natal Sharks Board and included a 58 kg female, measuring 2.25 m
across the disc, caught at Salt Rock, north of Durban; a 52 kg female, with a disc
width of 2.47m, from St. Michael’s on Sea, on the south coast of Natal; a 74kg
female with a disc width of 2.35 m, from Ballito Ray, north of Durban; a 47 kg female
with a disc width of 2.14 111, exact location unknown; arid a 151 kg niale with a disc
width of2.6 ni from Brighton Beach. The nets are set approximately 400 m olTsliore,
in waters 10 to 14 m deep.
Three specimens of Mobula tarapacaiza were examined: an immature male specimen
weighing 17.7 kg and measuring 1.33 m across the disc; a maturc female weighing
345 kg and measuring 2.75 m across; and a mature female weighting 405 kg and
measuring 3.5 m across. ‘These rays had beached themselves at Pearly Beach, southwestern Cape Province; at First Beach, Clifton; and at Three Anchor Bay, Cape
‘170wn,South Mrica, respectively.
METHODS
Access to the cranial rete niirabilc of Manta birostrir specimens was gaincd by
removal of the skin and connective tissue that forms the roof over the anterior
fontanelle (Fig. 1). Overlying the vessels of the rete is a thin but strong sheet of
fibrous connective tissue that presumably supports the vascular nctwork within the
spacious cranial cavity.
Beginning with the most superficial blood vessels of the cxposed preccrchral rete,
arteries and veins were traced both anteriorly and posteriorly in order to determine
their origins and terminations. The walls of many of the veins are very fine, some
measuring less than 5 pm, arid their paths were easily lost in the network. A dilute
solution of water-soluble aniline blue was injected into the veins, which facilitated the
tracing of small vessels. In one individual, venous drainage of the head region was
investigated by contrast radiography, using barium sulphate as the contrast medium.
A niimbcr of transverse sections were made through the anterior half of sonic
specimens, in order to determine the relative positions of major drainage routes.
The brains and retia of the Mobula tarapacana specimens were removed and fixed
in a 10% formaldehyde solution. Since these were beached spec,imens, it is not
possible to determine the time of death. However, the brains of the two large females
were in good condition. The brain of the small male specimen, however, had started
to deteriorate so this individual was suitable for dissection only. The Manta birostris
specimens had been frozen for shipment to Cape l’own. Thc cranial cavities of these
rays wcrc injected with a 10% lbrmaldehydc solution prior to defrosting, which
helped to limit damage to the tissues.
For histological examination of the retia, blood vessel samples were taken from a
BRAIN WARMING IN MOBULID RAYS
153
F ic p r r 1. Dorsal view of cranial rete in .24anta birosh-ir, following removal of the skin and connective tissue
over the anterior fontanelle. Note the position of the rete relative to the mouth. Abbreviations: r = rete;
m = mouth.
number of positions along the length of the retia, and included samples from both the
caudal and precerebral retia. These sections were then processed and embedded in
wax, following standard histological procedure. Sections measuring between 2 pm
and 5 p m were cut on a rotary microtome and stained with haematoxylin and
eosin.
RESULTS
The pattern of arteries in Manta birostris is similar to that described for Mobula
species by Schweitzer & Notarbartolo-di-Sciara (1986), so the terminology used by
these authors for the major vessels of the cranial vasculature is applied to this
description.
The major arteries that supply the brain and give rise to the caudal and
precerebral retia lie ventral to the brain, on the floor of the cranial cavity. These
vessels, the profundae cerebri arteries, are branches of the internal carotid arteries,
which enter the lateral margin of the cranial cavity just behind the anterior margin
of the telencephalon (Fig. 2). Passing transversely across the floor of the cavity,
beneath the telencephalon, is the anterior communicating artery, which connects the
internal carotid arteries of the right and left sides. The profundae cerebri arteries pass
posteriorly towards the spinal cord, where they fuse on the median surface of the
medulla, to form the basilaris artery. This vessel continues on to the spinal cord as
the spinalis impar artery (Fig. 2).
Branches of the profundae cerebri arteries supply dorsal and ventral tissues of the
brain from the level of the telencephalon to the medulla. There are one or two
major, and numerous smaller, transverse connections between the profundae cerehri
artcries and their branches across the ventral surface of the brain. Blood supply to the
dorsal surface of the telencephalon is by way of branches that pass upwards from
R. L. ALEXANDER
I54
profundae cerebri arteries on the ventro-lateral surface of the brain. In addition, the
antero-dorsal aspect of the brain receives some branches of supply from the anterior
communicating artery, which also contributes to the precerebral rete.
The cerebellum is supplied by an elaborate network of vessels that branch from
the profundae cerebri arteries and form part of the caudal rete (Fig. 2). The branches
of this network are distributed primarily to the lateral and dorsal aspects of the
0
A
B
F i p r r 2. I)iaqaniatic representation of thr cranial rete mirabile in hfaantn hiros/nA, showing arteries (black)
m d vcins (white). A, ventral vitw showirig the vascular plexus on the surtace of thc brain, and the major
arteries that supply tlw t)rain. The veins represent both dorsal and ventral tributarirs as they pass
forwards Ihrough the arterial network. B, a dorsal view showing the position of the caudal rete around
the ccrebellum. Abbreviations: ac = antcrior cardinal sinus; c = rrrebellurn; ca = anterior conimunicating
artery; cr = caudal rrte; dc = dorsal collector- vein; ic = internal carotid artery; o = olfactory sinus;
p =plexus; pc = profiinda cerehri artery; prc = precerebral rete; si = spinalis inqiar artery; t = telencephaI o I i : v = cerebral veins.
BRAIN WARMING IN MOBULID RAYS
155
cerebellum, but there are also some branches to the deep surface of the cerebellum
and the optic tectum. Its contribution to the blood supply of the telencephalon is
minimal.
The precerebral rete, which does not supply brain tissue, arises from the
profundae cerebri arteries at the level of the telencephalon, near the junction of this
lobe with the cerebellum. Several arteries, initially straight, become highly
convoluted as they pass anterior to the brain. This mass of convoluted vessels fills the
cranial cavity. These arteries extend antero-laterally, gradually diminishing in size,
towards the olfactory capsules, where they terminate. Posterior to the olfactory
filaments is a venous sinus, the boundaries of which are formed by the cartilaginous
walls of the olfactory capsule. It appears that the blood within the precerebral rete
drains to this venous sinus (Fig. 2).
Dissection of the cranial rete of Manta birostrk revealed the presence of numerous
veins. O n both right and left sides there is a large collector vein that lies superficially
on the dorsal surface of the precerebral rete (Fig. 3A). This vein is formed by fusion
of a number of tributaries passing forwards from both superficial and deep aspects of
the rete. The tributaries, when traced posteriorly through the rete, were found to
comprise branches from both dorsal and ventral aspects of the brain. Further
dissection revealed a dense, diffuse plexus of both veins and arteries on the ventral
surface of the brain (Figs 2, 3D).
The dorsal tributaries arise from the network on the ventral aspect of the brain as
a series of small vessels that join to form larger veins that pass forwards to the dorsal
collector vein. As they pass over the lateral and dorsal aspects of the brain, the veins
run parallel to the branches of the profundae cerebri arteries that give rise to the rete;
these veins are so closely associated with the arteries that they had to be dissected
away from the walls of the arteries in order to trace their routes.
The ventral tributaries were traced forwards from the plexus, across the surface of
the brain, to the floor of the cranial cavity where a diffuse network of veins was found
(Fig. 3B, C). As these veins cross the floor, they pass directly over the anterior
communicating artery (Fig. 3B). From their origin on the surface of the brain to their
termination in the dorsal collector vein, these ventral veins run through the
precerebral arterial network and are closely associated with it.
In order to confirm these observations, aniline blue solution was injected into the
dorsal veins of another individual. The injected veins were traced to the plexus on
the ventral aspect of the brain, and were observed to gradually diminish in diameter
(Fig. 3D). At the point at which the injected solution stopped, the veins appear to
connect with small uninjected vessels, presumed here to be small arteries. Due to the
diffiuse nature and small size of the vessels in this plexus, it was difficult to isolate and
follow many of the vessels. However, in all cases where retial vessels determined to
be veins were injected slowly with small amounts of dye, only veins were filled. The
plexus on the ventral surface of the brain is interpreted as including the site of the
capillary bed.
From its anterior position on the dorsal surface of the rete, the large collector vein
was traced antero-laterally to a foramen posterior to the olfactory capsule. Following
aniline blue injection into the collector vein, transverse sections through the head of
the specimen showed that the venous blood from the brain drains into the anterior
cardinal sinus.
In addition to the veins that course through the rete to join the dorsal collector
vein, there are other small cerebral veins that pass directly through the walls of the
1.56
R. 1.. AT.E>CZNL)EK
chondrocranium to reach the anterior cardinal sinus (Fig. 2). The anterior cardinal
sinus terminates in the sinus venoms of the heart.
Histological staining of sections of the cranial retia of Manta birostrzs (Fig. 4) arid
iVl06ula tarapacana (Fig. 5) indicates that the structure and arrangement of vessels is
similar in both species, and that arteries and veins are present in both the c,audal and
precerebral divisions of the retia. Based on standard histological criteria, it was
concluded that vessels with thick walls and a relatively narrow lumen are arteries,
whereas those with thin walls and a relatively large lumen are veins.
In Manta hirostris, arteries from the caudal (Fig. 4A) and precerebral retia (Fig. 4B,
C:) range in diameter from 70 pin to 400 pm. In both the vessels comprising the retia
and the vessels on the surface of the lobes of the brain, similar ranges of vessel
diamcters were observed for all regions. Anteriorly, among the convoluted vessels
BRAIN WARMING IN MOBULID RAYS
157
filling the cavity, are some large arteries with diameters of between 600pm and
1 mm.
In Mobula tarapacana, as in Manta birostris, some large arteries with diameters up to
1 mm were found in the precerebral rete. With the exception of these large arteries,
vessel diameters ranged from 80pm to 500pm in all regions sampled (Fig. 5).
The walls of the arteries are relatively thick, with an average thickness of 60 pm in
Manta birostris, and 40 pm in Mobula tarapacana. There is a lot of variation in arterial
b'iwre 3. A, superficial dorsal veins (white arrows) draining into the large dorsal collector vein (blark
arrow). B, view ofthe ventral tributarics (arrows) on the floor of the cranial cavity, following reflection of
the rete. 'l'he brain is clcarly visible. The arrow-head indicates the anterior communicating artery,
beneath the veins. C, ventral and dorsal tIibutaries passing through the arterial network to drain to a
large vein (black arrow) that passes through the network to become the dorsal collector vein. I), vciiis
(small white arrows) arising frorn the plrxus, on the ventral aspect of the brain. The veins surround the
profuridae cerebri arteries. 'l'he dark coloration of the veins is due to aniline blue. Thc, position of thr
optic nerve is showsn. Abhreviations: b = brain; o = optic nerve; p = plexus; pc = profunda cerebri
arten,.
K.L. ALEXANDER
I58
Figure I. Histological sections of cranial rete ofManla birustris. A, caudal retc:; H, section from the latenil
surfart, of tbrcbrain; C, section frcm prcccrebral rete. Abbrcviations: a = artery; v = vein. Scale
Im = 50um.
BRAIN WARMING IN MOBULID RAYS
Figure 5. Histological sections of cranial rete of Mobula tarupacana. A, dorsal surrace of forebrain; B, C,
precerebral rete. Abbreviations: a = artery; v = vein. Scale bar = 100 pm in A and 50 pni in B and
C.
159
.
1ti0
R. L. ALEXANDER
wall thickness, however, with those of the largest arteries measuring as much as
150 pm and those of the small arterioles in the precerebral rete measuring as little as
1 0 pm.
The veins in both genera are sinus-like in appearance, in that they are large,
irregularly shaped, and very thin-walled. Due to their irregular shape, the diameters
of the veins were riot measured. The walls of most of the veins, however, measure
approximately 5 pm, with a maximum diameter of 15 pm for the largest veins.
Arteries arid veins are typically found to occur in close association with one another,
with closely apposed walls (Figs 4, 5). An abundance of collagen occurs throughout
the vascular network, between the vessels, and presumably acts as supporting
tissuc.
DISCUSSION
‘I‘he cranial rete mirabile of Manta birostris is very similar to that previously
described for species of the genus Mobula (Schweitzer & Notarbartolo-di-Sciara,
1986), with all of the major arteries reported in Mobula species present in hfanta
hirostris. The anastomoses between the arteries, which appear to be a characteristic
feature of clasmobranch cerebral circulation (Daniel, 1934), result in the formation
of a vascular triangle. However, a much more complex and extensive system of veins
is present in both Manta birostrir and Mobula tarapacana than previously described for
the genus Mobulu by Schweitzer & Notarbartolo-di-Sciara (1986), who only reported
supcrficial veins that they did not associate with the rete.
The close morphological association between the arteries and veins in the retia, as
shown by dissection and histological staining, suggests that they form a countercurrent system for heat exchange. The arrangement of the vessels would increase the
contact surface area between the two types of vessels and would facilitate heat
transfer, as would the extensive coiling of the arteries in the precerebral rete.
‘I’he diameters of the arteries in the retia fall within the ranges observed in other
vertebrates, including other elasmobranchs (Block & Carey, 1985). ‘The observation
that the diameters of vessels closer to the brain in mobulids have smaller diameters
than more distal vessels is consistent with observations in lamnid sharks (Block 8r
Carey, 1985). The wide range of vessel diameters observed for each of the regions
sectioned (from 70 pm to 500 pm) results from the close proximity of large parent
arteries and their branches within the retia, and by the convolutions of the arteries
throughout the network.
l‘he hypothesis that the vascular network in the brains of Manta birostris and Mobula
tarapacana functions as a counter-current heat-exchanger is based purely on
ariatoniical observations. Clearly, temperature measurements of live mobulids are
necessary in order to confirm this. However, there is a cuunter-current heatexchanger in the pectoral fin of M. tarapacana, with associated increased levels of
internalized red muscle (Alexander, 1995). This species also has retia elsewhere in
thc lmdy, around the oesophagus and vertebral column, and the reproductive organs
(Alexander, unpublished results).
One of the major advantages of endothermy in fishes is independence from the
thermal environment (Carey el a/., 1971; -Carey, 1982; Block & Finncrty, 1994).
Block ef al. (1993: 213), in an analysis of the Scombroidei, suggested that ‘the
minimum required endothermic capacity’ Tor the expansion of thermal niches is
warming of the brain. Although there are species of fishes in which the brain is warm
BRAIN WARMING IN MOBULID RAYS
161
and the body is cold (Carey, 1982; Block, 1986), there are no records of species in
which the body is warm and the brain cold. The presence of a heat-exchanger
around the brain of Mobula tarapacana would thus be consistent with the presence of
retia in the fins and body of this species.
All counter-current heat-exchange systems require a heat source. In warm-bodied
fish, heat is derived from the metabolic activity of red muscle, or from specially
modified muscle tissue. This heat is then retained within the body by means of
arteries and veins arranged in close association for counter-current exchange (Carey
& Teal, 1969; Carey, 1982; Bone & Chubb, 1983; Block & Carey, 1985; Block,
1986; Wolf et al., 1988).
The source of heat for the cranial rete mirabile of rnobulid rays has not yet been
established. However, the eye muscles of all Mobula species examined are large and
reddish in colour. Histochemical results for some of these species (e.g. Mobula kuhliz)
indicate that their eye muscles contain large numbers of red fibres (Alexander,
unpublished results). Red eye muscles are a potential source of cranial heat in
endothermic fishes (Wolf et al., 1988; Block & Finnerty, 1994). Although the eye
muscles of Mobula tarapacana and Manta birostris have yet to be examined
histochemically, they are large and may function as a heat source.
Retention of heat and maintenance of elevated temperatures is difficult for fish.
Heat is lost both at the surface of the body by conduction, and also by way of
convection as blood is transported around the body and across the gills (Stevens &
Sutterlin, 1976; Graham, 1983). In experiments on carp, Crawshaw (1976) found
that the brain lost heat more rapidly than other better-insulated parts of the body,
such as the dorsal musculature and intestine. The brain is a metabolically active
organ and probably contributes to warming of the carotid rete in lamnid sharks
(Block & Carey, 1985; Wolf et al., 1988). However, the brain is generally small
relative to body size, and the amount of heat it generates is unlikely to be significant
(Carey, 1982; Wolf et al., 1988).
The problem of cranial heat loss may be compounded in elasmobranchs, in which
the brain is generally not well protected. Whereas swordfishes and billfishes have
large amounts of insulating fat surrounding the brain (Carey, 1982; Block, 199l), the
brain of a warm-bodied lamnid shark sits in a poorly insulated chondrocranium with
a very thin roof (Wolf et al., 1988). In addition, the palate on the ventral aspect of the
cranium is continuously exposed to cold water taken in through the mouth and
passed over the gills (Block & Carey, 1985).
Heat loss from a poorly insulated cranium may be an even greater problem for
mobulids. The large anterior fontanelle is covered only by a thin layer of skin and
connective tissue measuring between 3 and 5 mm in thickness. Furthermore, these
rays are dorso-ventrally flattened and some have extremely wide mouths (Fig. I).
Mobulid rays feed by taking large volumes of water containing plankton into the
mouth. Thus, a large surface area of the floor of the cranium is exposed to cold
incoming water. Conductive heat loss from the brain of mobulids is thus likely to be
considerable.
However, it has been suggested that mobulid rays, with their dark dorsal surfaces,
may be at risk of over-heating the brain when they engage in basking behaviour
(Schweitzer & Notarbartolo-di-Sciara, 1986). According to these authors, the cold
arterial blood in the rcte mirabile could act to reduce intracranial temperature.
However, the presence of veins in the network suggests that warming, rather than
cooling, is the function of the rete in Manta birostris and iMobula tarapacana.
I62
R. L. ALEXANDER
Furthermore, most of the retial arteries are situated either anterior to, or on the
ventro-lateral aspects of the brain. In Manta birostris, the dorsum of the brain is
relatively exposed (Figs. 2, 3B). Since the dorsal surface is the area most at risk from
ovcr-exposure to radiant heat, it seems likely that if the function of the rete was to
cool the brain, the precerebral and caudal retia should be concentrated on the
dorsum of the brain.
Unlike the dorsal surface of the brain, the ventral aspect is not directly exposed to
the sun during basking. It is, however, equally affected by water flow, since water
passes through the mouth during feeding. The ventral position of much of the rete
may be an adaptation designed to buffer the brain from incoming cold water.
Basking behaviour in mobulids may be an adaptation for raising body
temperature. Basking is a known form of behavioural thermoregulation utilized by
many animals, and is an adaptive strategy that builds up and maintains a heat store,
thus raising metabolic rate and permitting prolonged activity time (Cossins & Bowler,
1987).
Behavioural thermoregulation has been observed in a number of fishes.
Ectothermic blue sharks are thought to be able to increase the time spent at depth
hunting for prey, through regular forays to the surface where they take up heat from
warmer waters. They are apparently able to retain this heat in the swimming muscle
and thus remain at depth for longer (Carey & Sharold, 1990). Swordfish, which have
specialized brain-warming tissues (Carey, 1982), typically spend the day at depth,
coming to the surface at night. Some swordfish, however, have been observed at the
surface during the day. It is suggested that these fish bask in warm surface waters in
order to raise their body temperature following long periods in deep, cold waters
and, like the blue shark, utilize the energy gained to prolong the time spent at depth
(Block & Finnerty, 1994). Warm-bodied tunas are also known to use behaviour as a
means of adjusting body temperature, both to enhance or retard heat gain and loss
(Holland et al., 1992).
'l'he presence of a retial system to warm the brain in mobulids would thus not be
inconsistent with basking behaviour. In scombroids and mobulids, basking would
enable them to absorb energy for periods when higher levels of activity are required,
while brain-warming mechanisms may function to maintain a constant thermal
environment for the brain while undertaking vertical migrations.
The presence of retia in fishes appears to be related to vertical migrations and
activity levels, since retia have been recorded in active predators such as tunas,
swordfishes, billfishes and lamnid sharks (Burne, 1923; Carey & Teal, 1969; Carey,
1982; Bone & Chubb, 1983; Block & Carey, 1985; Block, 1986; Wolf et al., 1988;
Holland et al., 1992). Some of these species undertake extensive vertical migrations
in the pursuit of food and, as a result, they encounter wide ranges in water
temperature.
Although not predatory animals as in the examples cited above, mobulids are
thought to be highly active and migratory, ranging through tropical, subtropical and
temperate waters (Coles, 1913; Bigelow & Schroeder, 1953; Stehmann, 198 1). They
are apparently fast swimmers attaining high speeds, and have been observed to leap
clear of the water (Gill, 1908; Coles, 1910, 1916; Nichols & Murphy, 1944; Bigelow
& Schroeder, 1953; Notarbartolo-di-Sciara, 1988; Notarbartolo-di-Sciara & Hillyer,
1989). Mobulid rays are primarily filter feeders on planktonic animals such as
euphausiids and copepods (Bigelow & Schroeder, 1953; Cadenat, 1960; Notarbartolo-di-Sciara, 1987))which are known to undertake regular, and even daily, vertical
BRAIN WARMING IN MOBULID RAYS
163
migrations (Pillar & Hutchings, 1989). In species such as Mobula tarapacana and Manta
birostris, a high level of activity would be necessary to gather enough food to meet the
requirements of their large body mass.
However, although retia in fishes are adaptations for higher body temperatures,
over-heating of the body and brain can be a problem. In some species, vascular
shunts that serve to bypass retia have been identified (Carey et al., 1981), and various
mechanisms that could alter the efficiency of the retia, and so regulate temperatures
have been postulated (Linthicum & Carey, 1972; Holland et al., 1992).
Mechanisms that could regulate temperatures have not been positively identified
in mobulids. However, blood flow to the brain is through both the internal carotid
arteries and the spinalis impar artery (Schweitzer & Notarbartolo-di-Sciara, 1986).
Unlike the internal carotid and profundae cerebri arteries, there is no retial network
associated with the spinalis impar artery. During periods of heat stress, blood flow to
the brain through the internal carotid arteries may be reduced and flow through the
spinalis impar artery could be increased. In addition, drainage of warm venous blood
that bypasses the rete by emptying directly through the walls of the chondrocranium
into the anterior cardinal sinus, may contribute to reducing temperatures.
There are numerous aspects of mobulid morphology and physiology that require
further investigation. In addition to obvious questions of possible endothermy in
these rays, and the extent to which the anatomical features that are suggestive of
endothermy vary between different species, there is still much to be learned about
their basic anatomy. Studies on these rays are continuing in an attempt to clarify
some of these issues.
ACKNOWLEDGEMENTS
I am indebted to the Natal Sharks Board, and particularly Mr Sheldon Dudley
and Mr Geremy Cliff, for their co-operation in this project. I thank the Captains and
crew of the Border, Barrier and Sezela (Unicorn Shipping) for transporting frozen
specimens from Durban to Cape Town. Captains Doug Young and Hank Wester
were especially helpful.
Special thanks to the South African Museum, and to the Departments of
Anatomical PatholoLgy(Neuropathology Section), and Anatomy and Cell Biology,
University of Cape Town, for support. I am especially grateful to Ms Anne Smith for
her histological expertise and advice. Mrs Edwina Stellenboom assisted in the
preparation of histological material; Mrs Michelle van der Menve and Mr Peter
Richards took the photographs of the specimens, and the slides, respectively; Mr
Cedric Hunter drew Figure 2, and Dr S. Kidson provided histological advice.
Ms A.E. Louw is thanked for reading early drafts of the manuscript and for
editorial advice; thanks also to Dr L. Compagno and Prof. J. Jarvis for reading the
manuscript, and to two anonymous reviewers for their helpful comments. Dr
Compagno also provided much needed financial and logistical support.
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