Cardiac function of the leopard shark,Triakis semifasciata

Summary The pressure difference between the cardinal sinus and the pericardium, and the transmural ventricular diastolic pressure at rest and during swimming in the leopard shark, Triakis semifasciata, was measured to characterize the mechanism of cardiac filling in chronically-instrumented fish and to evaluate cardiac responses to swimming. Echo-Doppler and radiographic imaging were also used to fully describe the cardiac cycle. Swimming induces an increase in preload as manifested by a large increment of cardinal sinus pressure (0.26/0.20 [systolic/diastolic] to 0.49/0.32 kPa) which always exceeds pericardial pressure. Increases in both mean ventricular diastolic transmural pressure (0.30–0.77 kPa) and cardinal sinus pressure during swimming suggest increased cardiac filling by vis a tergo as the mechanism for augmenting cardiac output. In contrast to mammals, the fluid-filled pericardial space of elasmobranchs is considerably larger and the pericardium itself does not move in concert with the heart throughout the cardiac cycle. Also, modest increases in heart rate drastically curtail the duration of diastole, which becomes much less than that of systole, a phenomenon not found in mammals. In the absence of tachycardia (<40 bpm), ventricular filling is characterized by a period of early rapid filling, and a late period of filling owing to atrial systole, separated by a period of diastasis. Ventricular filling in elasmobranchs is thus biphasic and is not solely dependent on atrial systole. Atrial diastole is characterized by three filling periods associated with atrial relaxation, ventricular ejection, and sinus venosus contraction. The estimated ventricular ejection fraction of Triakis (80%) exceeds that of the mammalian left ventricle.     

Age and growth estimates of the bull shark,Carcharhinus leucas,from the northern Gulf of Mexico

Synopsis Length at age and growth rates for 59 bull sharks, Carcharhinus leucas, collected from the northern Gulf of Mexico were estimated from the band patterns formed seasonally in the vertebral centra. The combined age at length data for both sexes were applied to a von Bertalanffy growth model producing parameter estimates of L = 285 cm TL, K = .076, t₀ = –3.0 yr. Lengths at age for males and females were similar except that males did not attain as great a length as females. Growth was apparently slow and varied among individuals, but in general, was estimated to be 15–20 cm yr^–1 for the first five years, 10 cm yr^–1 for years 6–10, 5–7 cm yr^–1 for years 11–16, and less than 4–5 cm yr^–1 thereafter. Males mature at 210–220 cm TL or 14–15 yr of age; females mature at>225 cm TL or 18+ yr of age. The largest male (245 cm TL) was 21.3 yr old; the largest female (268 cm TL) was 24.2 yr old.     

Age,growth and reproductive biology of the silky shark,Carcharhinus falciformis,and the scalloped hammerhead,Sphyrna lewini,from the northwestern Gulf of Mexico

Synopsis The silky shark, Carcharhinus falciformis, and scalloped hammerhead, Sphyrna lewini, represent >80% of the shark by-catch of the winter swordfish/tuna longline fishery of the northwestern Gulf of Mexico. This catch represents a potential supplemental fishery, yet little is known of the life histories of the two species. This report relates reproductive biology data to age and growth estimates for 135 C. falciformis and 78 S. lewini. Unlike other regional populations, C. falciformis in the Gulf of Mexico may have a seasonal 12 month gestation period. Males mature at 210–220 cm TL (6–7 yr); females at >225 cm TL (7–9 yr). Application of age at length data for combined sexes produced von Bertalanffy growth model parameter estimates of L_∞ = 291 cm TL, K = 0.153, t₀ = −2.2 yr. Adult male S. lewini outnumbered adult females in catches because of differences in the distributions of the sexually segregated population. Males mature at 180 cm TL (10 yr); females at 250 cm TL (15 yr). von Bertalanffy parameter estimates for combined sexes of this species were L_∞ = 329 cm TL, K = 0.073, t_o = −2.2 yr.     

Age and growth of the bonnethead shark, Sphyrna tiburo, from northwest Florida, with comments on clinal variation

Age and growth rates of the bonnethead shark, Sphyrna tiburo, from northwest Florida were estimated from vertebrae collected between October 1992 and October 1995. The von Bertalanffy growth equation was fit to male and female vertebral age data. Initial growth was rapid (≈ 200 mm TL) for both sexes from age 0–1. At age 2 growth slowed for males but continued for females. Similar to many species of sharks, females grew slower than males (K = 0.28 and K = 0.69, respectively) but attained a larger maximum size (L_∞=1226 and L_∞=897). Maximum age was estimated in males and females to be 8+ and 12+ years, respectively. Growth of young-of-year sharks was 21 to 30 mm TL per month determined by three different methods. A comparison of age and growth estimates from populations at more southerly latitudes suggest that clinal variation in total length may be evident among bonnethead sharks in the Gulf of Mexico with females reaching larger sizes in northern areas as compared to south Florida. This revised version was published online in July 2006 with corrections to the Cover Date.     

Age and growth of the Australian sharpnose shark,Rhizoprionodon taylori,from north Queensland,Australia

Synopsis Age and growth were studied inRhizoprionodon taylori using specimens caught in Cleveland Bay, North Queensland, Australia. Von Bertalanffy growth parameters were estimated using three different techniques: vertebral ageing, back calculation and length frequency. Vertebrae from 138 specimens were sectioned and narrow circuli counted to estimate age. Marginal increment analysis verified that circuli were produced annually in late summer, probably as a result of stress during the mating season. The oldest female was 7 and male 6 years old. Von Bertalanffy growth parameters estimated from vertebral ageing data for males were t_O = 0.410 yr, K = 1.337, L = 652.2 mm, and for females t_O = 0.455 yr, K = 1.013 and L = 732.5 mm. Growth parameters determined by length frequency and back calculation techniques concurred with those from vertebral ageing. Growth of the 0+ age class was very rapid, averaging 140% of the size at birth in the first year. Males and females matured after only one year, the lowest age at maturity reported in the family Carcharhinidae. Annual growth increments decreased rapidly after maturity, and little growth occurred after three years.     

The abundance of the blue shark,Prionace glauca,in the western English Channel

Synopsis Since 1952, a sport fishery for the blue shark,Prionace glauca, has existed off the south coast of Cornwall in England. Annual catches from this fishery have ranged from < 200 to>6000 sharks. The fishery was based on a previously unexploited stock in the 1950s. The abundance of the species in the English Channel declined in the early 1960s and again in the mid-1970s. The declining abundance was investigated in relation to sea surface temperature (SST), prey abundance, and fishing pressure. Short-term fluctuations in SST were found to be responsible for changes in the distribution of the population, but not for changes in abundance. The abundance of prey species in the Channel was observed to be inversely related to the abundance of blue sharks. The reduced abundance of blue sharks lowered the level of effective predation on the prey populations, allowing their abundance to increase. It was concluded that the nature of the fishing practice off the Cornish coast was responsible for a significant part of the decrease in shark abundance. By killing large numbers of sub-adult females, the reproductive capacity of the population was lowered. Continued fishing pressure prevented the population from recovering; and as of 1987, the abundance of the species in the Channel is still declining. Various conservation measures have been proposed.     

Geographic variation in reproduction between two populations of the bonnethead shark,Sphyrna tiburo

Synopsis A study of two populations of the bonnethead shark,Sphyrna tiburo was conducted in Florida Bay and Tampa Bay, Florida from September 1982 to December 1986. The maintenance of sharks in captivity at the Marine Science and Conservation Center in the Florida Keys, and the collection of sharks from widely separated geographical areas allowed the examination of latitudinal variation in reproduction. Several reproductive parameters were found to differ: (1) size at maturation, (2) age at maturation, (3) time of fertilization, (4) rate of embryonic development, (5) size at birth, (6) the energetic investment in producing offspring, (7) gestation period, and (8) the incidence of infertility. Average litter size and maximum age of females was not different between the two populations. These contrasting life history parameters are not easily explained. Food limitation and seasonal differences between the two areas are considered as factors controlling reproduction in these populations. It is noteworthy that the average size of adult females in Tampa Bay is significantly greater than that of Florida Keys sharks. This size difference may be important in explaining the observed differences in reproduction.     

The shark nursery of Bulls Bay,South Carolina,with a review of the shark nurseries of the southeastern coast of the United States

Synopsis Shark nurseries, or nursery areas, are geographically discrete parts of a species range where the gravid females of most species of coastal sharks deliver their young or deposit their eggs, and where their young spend their first weeks, months, or years. These areas are usually located in shallow, energy rich coastal areas where the young find abundant food and have little predation by larger sharks. Nurseries are characterized by the presence of both gravid females and free swimming neonates. Neonates are young bearing fresh, unhealed umbilical scars in the case of placental species, or those at or near the birth size in aplacental species. Bulls Bay, South Carolina, is a nursery for the blacknose, spinner, finetooth, blacktip, sandbar, dusky, Atlantic sharpnose, scalloped hammerhead, and smooth dogfish sharks. The lemon shark has its nursery in shallow waters of south Florida and the Bahamas. The bull shark has its nursery in the lagoons of the east coast of central Florida.     

Validated age,growth and maturity of the bonnethead Sphyrna tiburo in the western North Atlantic Ocean

The age, growth and maturity of bonnetheads Sphyrna tiburo inhabiting the estuarine and coastal waters of the western North Atlantic Ocean (WNA) from Onslow Bay, North Carolina, south to West Palm Beach, Florida, were examined. Vertebrae were collected and aged from 329 females and 217 males ranging in size from 262 to 1043 mm and 245 to 825 mm fork length, L_F, respectively. Sex‐specific von Bertalanffy growth curves were fitted to length‐at‐age data. Female von Bertalanffy parameters were L_∞ = 1036 mm L_F, k = 0·18, t₀ = ?1·64 and L₀ = 272 mm L_F. Males reached a smaller theoretical asymptotic length and had a higher growth coefficient (L_∞ = 782 mm L_F, k = 0·29, t₀ = ?1·43 and L₀ = 266 mm L_F). Maximum observed age was 17·9 years for females and 16·0 years for males. Annual deposition of growth increments was verified by marginal increment analysis and validated for age classes 2·5+ to 10·5+ years through recapture of 13 oxytetracycline‐injected specimens at liberty in the wild for 1–4 years. Length (L_F50) and age (A₅₀) at 50% maturity were 819 mm and 6·7 years for females, and 618 mm and 3·9 years for males. Both female and male S. tiburo in the WNA had a significantly higher maximum observed age, L_F50, A₅₀ and L_∞, and a significantly lower k and estimated L₀ than evident in the Gulf of Mexico (GOM). These significant differences in life‐history parameters, as well as evidence from tagging and genetic studies, suggest that S. tiburo in the WNA and GOM should be considered separate stocks.     

Effects of body size, age and maturity stage on diet in a large shark: ecological and applied implications

Ontogenetic diet shifts are a widespread phenomenon among vertebrates, although their relationships with life history traits are poorly known. We analyzed the relative importance of body size, age and maturity stage as determinants of the diet of a marine top predator, the copper shark, Carcharhinus brachyurus, by examining stomach contents using a multiple-hypothesis modeling approach. Copper sharks shifted their diet as size and age increased and as they became sexually mature, incorporated larger prey as they grew, and had a discrete shift in diet with body size, with only individuals larger than ≈200 cm total length able to prey on chondrichthyans. Body size was the most important trait explaining the consumption of chondrichthyans, while age determined the consumption of pelagic teleosts. Pelagic teleosts were consumed mostly by medium-aged sharks, a result, probably, of a risk-reducing feeding strategy at young ages coupled with either a senescence-related decline in performance or a change in sensory capabilities as sharks age. Copper sharks of all sizes were able to cut prey in pieces, implying that gape limitation (i.e., the impossibility of eating prey larger than a predator’s mouth) did not play a role in producing the diet shift. Our results suggest that, contrary to the current practice of setting minimum but not maximum size limits in catches, any plan to conserve or restore the ecological function of sharks, through their predatory control of large prey, should aim to maintain the largest individuals.     

Contributions of growth, stasis, and reproduction to fitness in brooding and broadcast spawning marine bivalves

Reproductive modes in marine invertebrates can be generally grouped into two types: those brooding larvae and those broadcast-spawning gametes into the water. We asked if these different life-history strategies differ based on how contribution to fitness is partitioned between growth, stasis, and reproduction. To investigate this question, we used published demographic data on ten diverse species of marine bivalves. We parameterized simple matrix-population models and calculated the sums of elasticities to growth, stasis, and reproduction parameters and plotted the results on triangular axes. We also assessed whether contribution patterns were correlated with reproductive mode and tropical, temperate, or polar environments. We found that some of the broadcast spawners fell in the region of the plot with high elasticities for stasis and that some of the brooders fell in the region of the plot with higher growth and reproduction elasticities than stasis ones. However, instead of a sharp dichotomy, we found a continuum in contributions of stasis parameters with long-lived brooders and short-lived broadcast spawners in the same region of the plot. There was no clear pattern of reproductive mode associated with any particular environment, but we think these preliminary results are intriguing and that further work on comparative demography of marine invertebrates is warranted. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Demography and movement patterns of leopard sharks (Triakis semifasciata) aggregating near the head of a submarine canyon along the open coast of southern California, USA

The demography, spatial distribution, and movement patterns of leopard sharks (Triakis semifasciata) aggregating near the head of a submarine canyon in La Jolla, California, USA, were investigated to resolve the causal explanations for this and similar shark aggregations. All sharks sampled from the aggregation site (n?=?140) were sexually mature and 97.1 % were female. Aerial photographs taken during tethered balloon surveys revealed high densities of milling sharks of up to 5470 sharks ha^?1. Eight sharks were each tagged with a continuous acoustic transmitter and manually tracked without interruption for up to 48 h. Sharks exhibited strong site-fidelity and were generally confined to a divergence (shadow) zone of low wave energy, which results from wave refraction over the steep bathymetric contours of the submarine canyon. Within this divergence zone, the movements of sharks were strongly localized over the seismically active Rose Canyon Fault. Tracked sharks spent most of their time in shallow water (≤2 m for 71.0 % and ≤10 m for 95.9 % of time), with some dispersing to deeper (max: 53.9 m) and cooler (min: 12.7 °C) water after sunset, subsequently returning by sunrise. These findings suggest multiple functions of this aggregation and that the mechanism controlling its formation, maintenance, and dissolution is complex and rooted in the sharks’ variable response to numerous confounding environmental factors.     

The effect of biogeographic and phylogeographic barriers on gene flow in the brown smoothhound shark,Mustelus henlei,in the northeastern Pacific

We assessed the effects of the prominent biogeographic (Point Conception and the Peninsula of Baja California) and phylogeographic barriers (Los Angeles Region) of the northeastern Pacific on the population connectivity of the brown smoothhound shark, Mustelus henlei (Triakidae). Data from the mitochondrial control region and six nuclear microsatellite loci revealed significant population structure among three populations: northern (San Francisco), central (Santa Barbara, Santa Catalina, Punta Lobos, and San Felipe), and southern (Costa Rica). Patterns of long‐term and contemporary migration were incongruent, with long‐term migration being asymmetric and occurring in a north to south direction and a lack of significant contemporary migration observed between localities with the exception of Punta Lobos that contributed migrants to all localities within the central population. Our findings indicate that Point Conception may be restricting gene flow between the northern and central populations whereas barriers to gene flow within the central population would seem to be ineffective; additionally, a contemporary expansion of tropical M. henlei into subtropical and temperate waters may have been observed.     

Ontogeny of the umbilical cord and placenta in the Atlantic sharpnose shark,Rhizoprionodon terraenovae

Synopsis The Atlantic sharpnose shark is a viviparous anamniote that develops an epitheliochorial yolksac placenta. Initially, contents of the yolksac nourish the embryos. Yolk is partially digested in the yolk syncytial-endoderm complex and subsequently transferred to the vitelline circulation. Yolk is also transported by ciliary activity of the yolk stalk ductus to the fetal gut for digestion. When embryos are 4.0cm in length, vascular ridges, termed appendiculae, develop on the yolk stalk. As yolk stores are depleted, the yolksac differentiates into the fetal portion of the placenta and the uterus abutting the yolksac differentiates into the maternal portion of the placenta. The yolk stalk differentiates into an elongate umbilical cord. The uterine epithelium produces secretions that are positive by the periodic acid-Schiff and alcian blue methods and metachromatic when stained with toluidine blue. Uterine capillaries are continuous and the surface epithelium is active both in secretion and transport of nutrients. When the embryos are 7–10cm in length, appendiculae are elongate, branched and populated by separate microvillar and granulated cells. Appendiculae may function as a paraplacental nutrient absorptive organ and be involved in the regulation of osmolarity of periembryonic fluids. The fetal placenta has two functional regions: a proximal portion that is presumed to function as a steroid producing organ and a distal portion that effects nutrient and metabolic exchange between the mother and fetus. Characteristics of the fetal placenta include endocytotic activity, crystalline-like cytoplasmic bodies and fenestrated capillaries. Fetal and maternal components of the placenta are separated by the egg envelope.     

Age and growth of the blacknose shark, <Emphasis Type="Italic">Carcharhinus acronotus</Emphasis>, in the western North Atlantic Ocean with comments on regional variation in growth rates

We examined the age and growth of the blacknose shark, Carcharhinus acronotus, in the western North Atlantic Ocean by obtaining direct age estimates using vertebral centra. We verified annual deposition of growth increments with marginal increment analysis and validated it by analyzing vertebrae marked with oxytetracycline from a female blacknose shark held in captivity. Von Bertalanffy growth parameters indicated that female blacknose sharks have a lower growth constant (k), a larger theortical maximum size (L), and are longer lived than males. We compared these growth parameters for blacknose sharks in the western North Atlantic Ocean to growth parameters for blacknose sharks collected in the eastern Gulf of Mexico to test for differences between regions. Females in the western North Atlantic Ocean have a significantly lower L, lower k, and a higher theoretical longevity than females in the Gulf of Mexico. Males in the western North Atlantic Ocean have a higher L<>, lower k, and higher theoretical longevity than males in the Gulf of Mexico. The significant differences between these life history parameters for blacknose sharks suggest that, when possible, future management initiatives concerning blacknose sharks should consider managing the populations in the western North Atlantic and the Gulf of Mexico as separate stocks.     

Reproductive output,costs of reproduction,and ecology of the smooth snake,Coronella austriaca,in the eastern Italian Alps

A 5-year mark-recapture study of smooth snakes (Coronella austriaca) in the Carnic Alps (1100 m above sea level) of north-eastern Italy provided extensive information on the biology and life-history of these small viviparous snakes. Offspring were relatively large (mean=15 cm total length, 2.9 g) when they were born in late summer, and females grew to maturity (44 cm, 50 g) in approximately 4 years. Larger neonates retained their size advantage for at least 12 months, but did not have a higher probability of survival. Although sexual size dimorphism (at birth and at mean adult body sizes) was minor, the sexes differed significantly in several respects. Females grew faster than males during juvenile life, and adult females diverged in dietary habits from the rest of the population. Whereas juveniles (of both sexes) and adult males fed primarily on lizards, larger females shifted to feeding less frequently, but taking larger prey (mammals and snakes). Reproductive output increased strongly with maternal body size: larger females reproduced more frequently, produced larger litters of larger neonates, had higher relative clutch masses (RCMs), and had a lower proportion of stillborn off-spring. Most females produced a litter every 2nd or 3rd year. We did not detect significant year-to-year variation in reproductive traits over the 5 years of our study. Females were consistent from one litter to the next in several traits (e.g., litter sizes, offspring sizes and shapes, proportions of stillborn neonates, RCMs), but this consistency was due to differences in body size among females rather than to size-independent maternal effects. Overall litter sex ratios averaged 50/50, but sex ratios tended to be more male-biased in litters that were unusually large relative to maternal body size, and in litters containing a high proportion of stillborn offspring. Costs of reproduction appear to be high in this population, in terms of both energy allocation and risk. Reproduction reduced growth rates, and females that recovered condition more quickly in the year after reproduction were able to reproduce again after a briefer delay. Mortality was highest in reproducing females with high RCMs, and in females that were very emaciated after parturition. The marked increase in reproductive output with increasing maternal body size in C. austriaca may reflect a reduction in costs as females grow larger, and the dietary shift to larger prey may enhance the rate that females can accumulate energy for reproduction.     

Optimal age and size at maturity in annuals and perennials with determinate growth

Summary A model predicting optimal age and size at maturity is presented, exploring the conflict between growth and energy allocation to reproduction. According to the model, the factors promoting delayed maturity and large adult body size are as follows: (1) high rate of somatic growth, (2) high percentage increase in reproductive rate with body size increase, (3) long life expectancy at maturity for annuals or large number of expected productive days (when either growth or reproduction is possible) for perennials with growth ceasing at maturity, (4) life expectancy increasing with body size. All these factors are combined in the mathematical formula predicting optimal age and size at maturity, which allows for quantitative predictions. The optimal schedule of growth and reproduction may be achieved by natural selection, developmental plasticity, or when one species replaces another. Sexual size dimorphism is also discussed, resulting from different optimal age at maturity for either sex.