Biology of Endangered Species Fishes, Turtle and Bivalves

Harshavardhan D. Joshi^#, Deepti Manjari Patel^# , Milind B. Katare^#, Kishor K. T. ^#, Karankumar K. Ramteke^* and Kapil Sukhdhane^*

^#KAFSU, College of Fisheries, Manglore, Karnataka, India

^*Central Institute of Fisheries Education, Seven Bunglows, Versova, Andheri (w.),

Mumbai-400061, Maharashtra, India

The Whale Shark

The Whale Shark is the world's largest fish, and one of only three filter-feeding shark species, along with the Basking Shark (Cetorhinusmaximus), and the Megamouth Shark (Megachasmapelagios). The Whale Shark is easily recognizable due to its broad, flattened head, its large mouth, and its pattern of light spots and stripes on a dark background. The ventral surface is typically whitish (Norman, 2002).

The only member of the family Rhincodontidae, the largest Whale Shark found to date measured 20 metres and weighed 34 tonnes (Chen et al. 1997, in Chen & Phipps, 2002). Despite its immense size, the Whale Shark is harmless to humans. It has few defences, although its ability to reach a very large size combined with a tough (thick) skin present on the dorsal surface can be used for protection (Norman, 1999). It is generally encountered singly, though occasionally in aggregations of up to hundreds of sharks (Compagno, 1984; Norman, 1999). The Whale Shark has typical "K" selected life history characteristics, including slow growth, late maturation and extended longevity (Colman, 1997a).

Lifespan

Data on the lifespan of the Whale Shark is limited. Taylor (1994) indicates that based on the late age of sexual maturity in Whale Sharks (estimated at around 30 years of age), the Whale Shark may be one of the longest living animals in the world, with an estimated lifespan of over 100 years.

There have been two reviews of Whale Shark growth rates ;ie Wintner (2000) and Uchida (2000). In both data has come from studies of captive individuals held in aquaria, as well as from the vertebrae of stranded Whale Sharks from South Africa. Wintner (2000) studied the growth rate of two animals held in aquaria and found that growth rates of Whale Sharks kept in captivity were 1.1 -1.3 times higher than would be expected from an animal in the wild, possibly due to the reliable availability of food in an aquarium. Uchida (2000) notes life span in aquaria range from three to 2056 days, with mean growth rate in the latter specimen at 29.5cm per year. Norman (2004) reports on another juvenile Whale Shark with a growth rate of 46cm per year over the 630 days while held in captivity.

It is likely that due to their very small size at birth (~ 0.5m) (Joung et al., 1996) and hence lack of defense to predation in the early stages of life, Whale Sharks grow very fast initially and then the rate of growth would decline (Norman, 1999). Some evidence of this rapid growth has been collected from a newborn Whale Shark that grew 143cm over 143 days while in captivity in a Taiwanese aquarium (Chang et al., 1997).

Diet

The Whale Shark is primarily a suction filter feeder (Compagno, 1984). It feeds on a wide variety of planktonic and nektonic prey, including small crustaceans, small schooling fishes such as sardines, anchovies and mackerel, and occasionally on small tuna and squid (Compagno, 1984; Last and Stevens, 1994). Whale Sharks at Ningaloo Reef have been observed actively feeding on swarms of the tropical krill Pseudeuphausialatifrons (Taylor, 1994; Norman, 1999). On a separate occasion, a Whale Shark was seen sucking the surface slick of coral spawn into its mouth while orientated at 450 to the surface (Norman, 1999). At Christmas Island, Indian Ocean the Whale Sharks have been observed feeding on localized concentrations of mysids (Anisomysisspinata) and crab magalopa (G. natalis) (Norman, 1999).

Three faecal samples analysed by Norman (1999) revealed exoskeletal remains of calanoid and harpacticoidcopepds, larval decapods and the scales of small fishes. A further two Whale Shark faecal samples from Ningaloo Reef contained eyes, legs, and fragments of exoskeleton from crustacean prey, namely P. latifrons, suggesting that Whale Sharks aggregating seasonally off Ningaloo Reef feed predominantly on this tropical krill (Wilson &Newbound, 2001). Whale Sharks have been observed feeding passively by swimming forward with mouth agape, and feeding actively by opening their mouths and sucking in prey. Whale Sharks are also reported as hanging vertically in the water while feeding (Colman, 1997a).

Reproduction

The Whale Shark is a livebearer with an ovoviviparous mode of development (Joung et al., 1996). It may be the most fecund of all live bearing sharks. The only pregnant female found to date measured 10.6m, weighed 16t, and contained 307 embryos. The embryos measured between 42 and 63 cm in length (Joung et al., 1996).

There is scant information on the age at which Whale Sharks become reproductive, although Norman (1999) presents evidence (collected during an intensive study of this species at NMP between 1995-97) that the length at maturity of male Whale Sharks is approximately 8.6m TL. Alternatively, examination of two female Whale Sharks (TL = 7.9 and 8.6m) by researchers in India revealed immature ovaries in each specimen (Pai et al., 1983; SatyanarayanaRao, 1986 in Colman, 1997). Taylor (1994) indicates that the Whale Shark may not reach sexual maturity until 30 years of age. There is currently limited evidence with which to accurately determine age at maturity or the maximum age for this species (Wintner, 2000).

Information on the frequency at which Whale Sharks are able to reproduce is not available. Likewise, it is not known where Whale Shark mating takes place, although it is considered likely in the waters surrounding Taiwan, Philippines and India where sightings of juvenile Whale Sharks have been recorded (Norman, 2004).

The Green Sea Turtle:

The Green sea turtle (Cheloniamydas) or green turtle is a large sea turtle of the familyCheloniidae. It is the only species in the genus Chelonia. Its range extends throughout tropical and subtropical seas around the world, with two distinct populations in the Atlantic andPacific Oceans. Their common name derives from the usually green fat found beneath their carapace (upper shell).

The green sea turtle is a sea turtle, possessing a dorsoventrally flattened body covered by a large, teardrop-shaped carapace and a pair of large, paddle-like flippers. It is usually lightly colored, although parts of the carapace can be almost black in the Eastern Pacific. Unlike other members of its family such as the hawksbill sea turtle and loggerhead sea turtle, Cheloniamydas is mostly herbivorous. The adults commonly inhabit shallow lagoons, feeding mostly on various species of seagrass.

Cheloniamydas is listed as endangered by the IUCN and CITES and is protected from exploitation in most countries. It is illegal to collect, harm or kill them. In addition, many countries have laws and ordinances to protect nesting areas. However, turtles are still in danger because of several human practices. In some countries, turtles and their eggs are hunted for food. Pollution indirectly harms turtles at both population and individual scales. Many turtles die, caught in fishing nets. Also, real estate development often causes habitat loss by eliminating nesting beaches.

As one of the first sea turtle species studied, much of what is known of sea turtle ecology comes from studies of green turtles. The ecology of Cheloniamydas changes drastically with each stage of its life history. Newly emerged hatchlings are carnivorous, pelagic organisms part of the open ocean mini-nekton. In contrast, immature juveniles and adults are commonly found in seagrass meadows closer inshore as herbivorous grazers.

Habitat

Green sea turtles move across three habitat types depending on their life stage. They lay eggs on beaches. Mature turtles spend most of their time in shallow, coastal waters with lush seagrass beds. Adults frequent inshore bays, lagoons and shoals with lush seagrass meadows. Entire generations often migrate between one pair of feeding and nesting areas.

Turtles spend most of their first five years in convergence zones within the open ocean.These young turtles are rarely seen as they swim in deep, pelagic waters.

Predators

Only human beings and the larger sharks feed on C. mydas adults. Specifically, tiger sharks (Galeocerdocuvier) hunt adults in Hawaiianwaters. Juveniles and new hatchlings have significantly more predators, including crabs, small marine mammals and shorebirds. In Turkey, their eggs are vulnerable to predation by red foxes and golden jackals.

Life cycle

C. mydas hatchling

Green sea turtles migrate long distances between feeding sites and nesting sites. Some C. mydasswim more than 2,600 kilometres (1,616 mi) to reach their spawning grounds. Mature turtles often return to the exact beach from which they hatched. Females usually mate every two to four years. Males on the other hand, visit the breeding areas every year, attempting to mate. Mating seasons vary between populations. For most Cheloniamydas in the Caribbean, mating season is from June to September. The French Guiana nesting subpopulation nests from March to June. In the tropics, green turtles nest throughout the year, although some subpopulations prefer particular times of the year. In Pakistan, Indian Ocean C. mydas nest year-round but prefer the months of July to December.

Green sea turtles mating is similar to other marine turtles. Female turtles control the process. A few populations practice polyandry, although this does not seem to benefit hatchlings. After mating in the water, the female moves above the beach's high tide line where she digs a hole with her hind flippers and deposits her eggs. Litter size depends on the age of the female and species, but C. mydas clutches range between 100 to 200. She then covers the nest with sand and returns to the sea.

At around 45 to 75 days, the eggs hatch during the night and the hatchlings instinctively head directly into the water. This is the most dangerous time in a turtle's life. As they walk, predators such as gulls and crabs grab them. A significant percentage never make it to the ocean. Little is known of the initial life history of newly hatched sea turtles. Juveniles spend three to five years in the open ocean before they settle as still-immature juveniles into their permanent shallow-water lifestyle.It is speculated that they take twenty to fifty years to reach sexual maturity. Individuals live up to eighty years in the wild.

Each year on Ascension Island in the South Atlantic, C. mydas create 6-15,000 nests. They are among the largest green turtles in the world, many more than a meter in length and weighing up to 300 kilograms (661 lb).

Breathing and sleep

Sea turtles spend almost all their lives submerged but must breathe air for the oxygen needed to meet the demands of vigorous activity. With a single explosive exhalation and rapid inhalation, sea turtles can quickly replace the air in their lungs. The lungs permit a rapid exchange of oxygen and prevent gases from being trapped during deep dives. Sea turtle blood can deliver oxygen efficiently to body tissues even at the pressures encountered during diving. During routine activity green and loggerhead turtles dive for about 4 to 5 minutes and surface to breathe for 1 to 3 seconds.

Turtles can rest or sleep underwater for several hours at a time but submergence time is much shorter while diving for food or to escape predators. Breath-holding ability is affected by activity and stress, which is why turtles quickly drown in shrimp trawlers and other fishing gear.

Giant Clam

The giant clam, Tridacnagigas (known as pā'ua in Cook Islands Māori), is the largest livingbivalve mollusc. T. gigas is one of the most endangered clam species. It was mentioned as early as 1825 in scientific reports. One of a number of large clam species native to the shallow coral reefs of the South Pacific and Indian oceans, they can weigh more than 200 kilograms (441 lb) measure as much as 120 centimeters (47.2 in) across, and have an average lifespan in the wild of 100 years or more. They are also found off the shores of the Philippines, where they are called taklobo. T. gigas lives in flat coral sand or broken coral and can be found at depth of as much as 20 m (66 ft).^[4] Its range covers the Indo-Pacific, but populations are diminishing quickly and the giant clam has become extinct in many areas where it was once common. T. maxima has the largest geographical distribution among giant clam species; it can be found in high- or low-islands, lagoons, or fringing reefs. Its rapid growth rate is likely due to its ability to cultivate plants in its body tissue.

Although larval clams are planktonic, they become sessile in adulthood. The creature's mantletissues act as a habitat for the symbiotic single-celled dinoflagellate algae (zooxanthellae) from which it gets nutrition. By day, the clam opens its shell and extends its mantle tissue so that the algae receive the sunlight they need to photosynthesize.

Young T. gigas are difficult to distinguish from other species of Tridacnidae. Adult T. gigas are the only giant clams unable to close their shells completely. Even when closed, part of the mantle is visible, unlike the very similar T. derasa. However, this can only be recognized with increasing age and growth. Small gaps always remain between shells through which retracted brownish-yellow mantle can be seen.

T. gigas has four or five vertical folds in its shell; this is the main characteristic that separates it from the very similar shell of T. derasa, which has six or seven vertical folds. As with massive deposition of coral matrices composed of calcium carbonate, the bivalves containing zooxanthellae have a tendency to grow massive calcium carbonate shells. The mantle's edges are packed with symbiotic zooxanthellae that presumably utilize carbon dioxide,phosphates, and nitrates supplied by the clam.

Feeding

 Algae provide giant clams with a supplementary source of nutrition.^[9] These plants consist of unicellular algae, whose metabolic products add to the clam's filter food.^[4] As a result, they are able to grow as large as100 cm length even in nutrient-poor coral-reef waters.^[9] The clams cultivate algae in a special circulatory system which enables them to keep a substantially higher number of symbionts per unit of volume.

In small clams — 10 milligrams (0.010 g) dry tissue weight—filter feeding provides about 65% of total carbon needed for respiration and growth; large clams (10 g) acquire only 34% of carbon from this source. A single species of zooxenthellae may be symbionts of both giant clams and nearby reef—building (hermatypic) corals.

Reproduction

T. gigas reproduce sexually, and are hermaphrodites (producing both eggs and sperm). Self-fertilization is not possible but, this characteristic does allow them to reproduce with any other member of the species. This reduces the burden of finding a compatible mate, while simultaneously doubling the number of offspring produced by the process. As with all other forms of sexual reproduction, hermaphroditism ensures that new gene combinations are passed to further generations.

Since giant clams cannot move themselves, they adopt broadcast spawning. They release sperm and eggs into the water. A transmitter substance called Spawning Induced Substance (SIS) helps synchronize the release of sperm and eggs to ensure fertilization. The substance is released through a syphonal outlet. Other clams can detect SIS immediately. Incoming water passes chemoreceptors situated close to the inccurent syphon, which transmit the information directly to the cerebral ganglia, a simple form of brain.

Detection of SIS stimulates the giant clam to swell its mantle in the central region and to contract its adductor muscle. Each clam then fills its water chambers and closes the incurrent syphon. The shell contracts vigorously with the adductor's help, so the excurrent chamber's contents flows through the excurrent syphon. After a few contractions containing only water, eggs and sperm appear in the excurrent chamber and then pass through the excurrent syphon into the water. Female eggs have a diameter of 100 micrometres (0.0039 in). Egg release initiates the reproductive process. An adult T. gigas can release more than 500 million eggs at a time.

Spawning contractions occurred every 2—3 minutes, with intense spawning ranging from thirty minutes to two and a half hours. Braley also hypothesized that clams that do not respond to the spawning of neighbor clams may be reproductively inactive.

Development

The fertilized egg floats in the sea for about 12 hours until eventually a larva (trocophore) hatches. It then starts to produce a chalk shell. Two days after fertilization it measures 160 micrometers (0.0063 in). Soon it develops a "foot", which is used to move on the ground; it can also swim to search for appropriate habitat.

At roughly one week of age, the clam settles on the ground, although it changes location frequently within the first few weeks. The larva does not yet have symbiotic algae, so it depends completely on plankton. Free floating zooxanthellae are also captured while filtering food. Eventually the front adductor muscle disappears and the rear muscle moves into the clam's center. Many small clams die at this stage. The clam is considered a juvenile when it reaches a length of 20 cm . It is difficult to observe the growth rate of T. gigas in the wild, but laboratory-reared giant clams have been observed to grow 12|cm a year.