Current Knowledge of Hagfish Reproduction: Implications for Fisheries Management1
Powell, Mickie L
SYNOPSIS. This review briefly summarizes the latest findings on reproductive endocrinology of Atlantic hagfish (Myxine glutinosa) and implications for fisheries management. In response to a major decline or collapse of the fisheries (groundfish and anadromous species) industry in the Northeast, species that were once considered alternative or underutilized have and are being identified that may be suitable for commercial harvest, one such example is the hagfish. Hagfish in recent years have been sought after as valuable fish not only for their flesh, but also their skin. Currently, there are no regulations governing the harvesting of hagfish along the East Coast. There has been little to no information of the life history of hagfish including growth rate, age determination, reproductive biology, life span, and larval size at hatching. Thus, the level at which a sustainable fisheries for this species can be maintained is unknown. In some parts of the world, hagfish stocks are being depleted due to overfishing. In order for fisheries management to manage its hagfish stocks and develop a sustainable commercial hagfish fishery, critical information is needed to assist in determining the optimal use of this valuable resource.
Key elements of the reproductive system have not been elucidated in hagfish. However, there is new evidence from recent reproductive studies that Atlantic hagfish may have a seasonal reproductive cycle. These data include seasonal changes in gonadotropin-releasing hormone (GnRH), gonadal steroids, estradiol and progesterone, corresponding to gonadal reproductive stages along with the putative identity of a functional corpus luteum. This newly acquired data may provide important information to fisheries managers of the East Coast.
Modern vertebrates are classified into two major groups, the gnathostomes (jawed vertebrates) and the agnathans (jawless vertebrates). The agnathans are classified into two groups, myxinoids (hagfish) and petromyzonids (lamprey), while the gnathostomes constitute all the other living vertebrates including the bony and cartilaginous fishes and the tetrapods. Forey and Janvier (1994) had hypothesized from their phylogenetic and paleontological analysis that modern lamprey were more closely related to gnathostomes than they were to hagfish and considered paraphyletic. However, Janvier and his collaborators have recently reversed their position based on analysis of the complete mitochondria! DNA suggesting that lamprey and hagfish form a clade (Delarbre et al, 2002). These authors further suggested that due to unique anatomical and physiological characters of hagfish and lamprey these characteristics should be re-examined and the functional significance of these characters may be the “weighting criterion” in assisting to resolve the relationships of hagfish and lamprey to jawed vertebrates. Thus, information on the evolution of vertebrate brain/pituitary hormones and their genes in lamprey and hagfish can contribute to the ongoing phylogenetic analysis that may help in resolving the phylogenetic relationships between hagfish and lamprey to jawed vertebrates. Whatever their phylogenetic position, the hagfish are still considered the most primitive vertebrate known, living or extinct. This unique position of hagfish in vertebrate evolution may provide insight into the evolution of complex regulatory mechanisms in modern vertebrates (Fig. 1).
Hagfish are members of the family Myxinoidae, which is the only surviving family of the class Pteraspidomorphi. The species are divided into two primary groups: Eptatretus and Myxine. The genus Eptatretus has 37 species that are found in the Pacific Ocean. The genus Myxine has approximately 18 species that are found in the Atlantic Ocean (Martini, 1998). Hagfish are the product of a long evolutionary history and can be considered as primitive, specialized, and degenerative (Gorbman and Dickhoff, 1978). Hagfish are the oldest lineage of craniates and thus are considered important to evolutionary studies (Martini, 1998). The discovery of a fossilized hagfish, Myxinikela, that was found in sediments deposited roughly 330 million years ago, put the significance of the hagfish into new light (Bardack, 1991). The closest relatives to hagfish are considered the most primitive fossil fishes. Due to their lineage they are of interest to evolutionary biologists in regards to their anatomy, physiology and molecular evolution.
Life history of hagfish
Hagfish are found in marine environments all over the world. They are most commonly found at the bottom of the ocean but they can be found at a variety of depths. Water temperature is the primary factor that limits the habitat of the hagfish. Atlantic hagfish are found on both sides of the North Atlantic and in Arctic Seas. Atlantic hagfish live in deep water of 100-300 m on soft muddy bottoms, in which they burrow with just the tip of the head showing. Their distribution is varied and patchy, being confined to areas where the bottom is suitable. They are not found in waters warmer than 22°C, suggesting that this is near their thermal tolerance (Martini, 1998). The Japanese hagfish, Eptatretus burgeri is the only known species to migrate for breeding between shallow and deep water (Kobayashi et al., 1972).
Atlantic hagfish are considered an important species in the Gulf of Maine for the following reasons as summarized by Martini et al. (1997): 1) hagfish play a significant role in the benthic ecosystem throughout the Gulf of Maine; 2) hagfish have both important direct and indirect effects on commercial fisheries in the Gulf of Maine, and 3) Atlantic hagfish are targeted by American and Canadian fishermen to meet the South Korean demand for “eelskin” used to manufacture leather goods.
In response to a major decline or collapse of the fisheries (groundlish and anadromous species) industry in the Northeast United States, other species that were once considered alternative or underutilized species have and are being identified that may be suitable for commercial harvest. One such example is the hagfish. Hagfish have often been considered to be pests by many fishermen; however in recent years hagfish have been sought after as valuable fish not only for their flesh, but also their skin (Kato, 1990). Since the 1960s there has been an extraordinary increase in the use of tanned hagfish skin for leather products. It is a lucrative business because the skin can be easily removed in a long strip substantially decreasing the processing time (Kato, 1990). During the past 30 years, the number of hagfish harvested from Korean and Japanese waters has increased dramatically (Gorbman et al., 1990). In Japan the fisheries in the Nigata area and Sado Strait have declined to such an extent that they no longer support a viable commercial fishery. The sale of eelskin leather goods, all produced from hagfish skin, brought South Korea revenues of approximately US $100 million (Gorbman et al, 1990). It has been suggested the demand for hagfish skin has greatly depleted the hagfish populations (Martini, 1998). To meet the demand for hagfish skins Japanese companies moved to the northeast Pacific coast in the 1980s and as catch numbers declined there they moved to the East Coast of the US in 1992 (Gorbman et al., 1990; Nardi, 1993). The landings for hagfish off the coast of Maine and Massachusetts have ranged from 1 to 12 million lbs each year during 1996 to 2002. These numbers may increase as more fishermen look for nontraditional species to exploit due to stricter federal regulations on groundfishing. The landings for hagfish in the Gloucester area were 1.5 million to 2 million lbs. worth over $1 million dollars in 1995 and 3 million lbs. worth over 1.5 million dollars in 1996 (Roland Barnaby, UNH Sea Grant Extension, personal communication). In 1998, the landings for hagfish in Massachusetts and Maine were 1,261,403 lbs valued at $326,704 and 1,929,874 lbs valued at $582,558, respectively (Gerry Gaipo, Fishery Information section, DOC/NOAA/NMFS). In 1999, the landings for hagfish in Massachusetts and Maine were 2,065,033 lbs valued at $595,278 and 2,907,644 lbs valued at $755,988, respectively. In 2000, the landings were approximately 12 million lbs worth over $3,000,000 (Gerry Gaipo, Fishery Information section, DOC/ NOAA/NMFS).
Currently, there are no regulations governing the harvesting of hagfish along the East Coast. Discard rates of hagfish from the fishery reach up to 50 to 60% (NOAA/NMFS) (Martini, 1998). Since there has been little or no information about the life history of hagfish including growth rate, age determination, reproductive biology, life span, and larval size at hatching, the level at which a sustainable fisheries for this species can be maintained is unknown. In order for fisheries management to manage its hagfish stocks and develop a sustainable commercial hagfish fishery, an information base is needed for optimum use of the hagfish resource (Barss, 1993). To address these issues, the New England Fishery Council has begun the process of developing regulations in 2003 although it will likely take a few years to develop a fisheries management plan.
Because of their inaccessibility, hagfish reproduction and early development have escaped observation. The reproductive patterns of most species of hagfish are unknown. To date, no one has been able to successfully reproduce any hagfish species in captivity. There is limited information on reproduction in one hagfish species in Japan. The Japanese hagfish, E. burgeri, is the only known species of hagfish that has a regular annual reproductive cycle and undergoes an annual migration (Ichikawa et al., 2000; Kobayashi et al, 1972; Nozaki et al., 2000). A study by Martini et al. (1997) suggested that Atlantic hagfish have limited reproductive potential based on the small number of eggs produced (less than 30 per female), about 25% of the animals examined did not have visible gonadal tissue and the small number of males (less than 6% of the population), gravid females (less than 1%) and postovulatory females (less than 5%).
As reported in two papers (Ichikawa et al., 2000; Nozaki et al, 2000), the seasonal migration and seasonal development of gonads were studied in E. burgeri near the Misaki Marine Biological Station of the University of Tokyo during the period from October 1970 to October 1975. In these studies, the reasons for the seasonal migration of E. burgeri did not appear to be simply related to reproduction but too many other factors such as food supply and water temperature (Ichikawa et al., 2000). There are other reports on seasonal movements of two other species of hagfish (E. deani and E. stoutii), however, the reasons for these migrations are not known (Martini, 1998). In terms of the gonads, there was no difference in the annual growth curves of developing eggs or testicular development in hagfish sampled in shallow versus deep water; however, the sizes of the developing eggs were the smallest in October and the largest in September (Nozaki et al, 2000).
During the past three years we have examined steroid concentrations from in vitro incubations of gonads, and have correlated brain concentrations of gonadotropin-releasing hormone (GnRH) with development and maturation of gonadal tissues in Atlantic hagfish captured monthly from the Atlantic Ocean. Our data show an annual cycle of brain GnRH as well as estradiol and progesterone production from the gonads in medium and large hagfish (Fig. 2a, b) (Powell et al, 2004). The peaks in estradiol and progesterone production are preceded by an increase in brain concentrations of GnRH in both the medium and large size classes of hagfish. The increase in in vitro estradiol production is also correlated with an increase in the number of maturing eggs in female hagfish. We have identified “brown bodies” in the ovaries and have shown that these brown bodies have both biochemical and morphological properties consistent with a functional corpus luteum. These data from our studies suggest a seasonal reproductive cycle in the Atlantic hagfish.
To further examine the development of reproductive stages, we recently performed microscopic examination of histological tissues taken from three areas (anterior, middle, and posterior) of each of 20 hagfish samples from three different size classes which include small (20-35 cm), medium (35-45 cm) and large (50-60+ cm) over an 18 month time period (Powell et al, 2004). Ten distinct gonad stages were identified (one indeterminate, six females, two males and one hermaphrodite) as described by Gorbman (1990) (Fig. 3). Females were the dominant sex identified in all size classes. Histological examination of the gonad tissues revealed a direct correlation between size and gonad stage in female hagfish. Male hagfish were rarely observed and occurred in the medium size class. Mature males were most abundant in the early spring (March thru May) and although mature females were present year round the number of females with large eggs decreased in June. As shown in the following figure (Fig. 4a, b) these data do indicate that Atlantic hagfish may be seasonal in their reproduction. Based on these histological studies examining small hagfish, medium size hagfish and large hagfish, annual changes in gonadal development were shown (Powell et al., 2004). A higher incident of the largest eggs (stage 7) occurred March through May in the large class size of hagfish. It has been documented that at least two species of hagfish spawn throughout the year, for Myxine glutinosa females and those nearing ripeness, have been recorded during all seasons of the year (Bigelow and Schroeder, 1948). Our histological data and the measured seasonal changes of GnRH, estradiol and progesterone led us to a different conclusion than Bigelow and Schroeder and we hypothesize that Atlantic hagfish may spawn in one season such as April to May. However, further studies will be needed to determine the spawning period of Atlantic hagfish.
Hagfish were once considered functional hermaphrodites and the frequency of hermaphrodites in most hagfish populations is not known. Hermaphrodites were rarely observed in a study on the Japanese hagfish (Ichikawa et al., 2000). In general, little information is available on sex ratios and sex determination in hagfish. Based on our recent studies, about 40 to 50% of hagfish medium size sampled during 2001-2002 had gonads with distinct male testicular tissue in the posterior region (Fig. 4a, b) and ovarian tissue in the anterior region (Powell et al., 2004). From part of this same study, 58% of all hagfish studied (n = 1,080) from all size classes contained only female gonad tissue, 41% were hermaphrodites and 0.05% were males with no ovarian tissue present. Wc propose that a certain percentage of Atlantic hagfish may indeed be functional hermaphrodites since a small percent of the identified hermaphroditic adult hagfish were found with large oval eggs and mature sperm (Powell et al., 2004). Even though this is a high percentage of hermaphroditic hagfish, it is not yet known whether these Atlantic hagfish are functional hermaphrodites-i.e., releasing mature eggs and sperm simultaneously for self-fertilization. The question on whether Atlantic hagfish are hermaphroditic and the possible reproductive significance of this group remains unknown. We hypothesize that Atlantic hagfish have the capability of being functional hermaphrodites as well as functioning separately as males and females. We speculate that this may be a strategy that has helped the hagfish to survive over millions of years in sometimes extreme ocean conditions such as a nutrient limited environment.
The adult ovary is comprised of germinative tissue that is restricted to the free edge of the membranous gonad (Sower and Gorbman, 1999). Female hagfish produce between 20 and 30 yolky, shelled eggs at a time. Numerous small oocytes move upward within the membrane as they grow. As the development proceeds, the eggs change shape from round to oblong, and finally to a spindle shape (Sower and Gorbman, 1999). Females do not have oviducts so the mature eggs are released into the abdominal cavity (Gorbman, 1997). The eggs vary in size from 14 to 25 mm, with the size depending mostly upon the species (Patzner, 1998). The eggs usually have hooked filaments at either end that keep the eggs connected together. This enables them to be ejected in a long chain. A current theory is that the eggs are ovulated through the cloacal pore and are expelled under pressure by muscular contractions (Gorbman, 1997). Empty follicles that appear as brown nodes remain in the ovary after the egg has been discharged. Researchers for over 100 years have observed and described this structure as a “brown body” in the gonad of the Atlantic hagfish. Although hypothesized to be a corpus luteum to date no study has associated any of the functional characteristics of a functioning corpus luteum with this structure. Based on our current hagfish studies, we hypothesize that these brown nodes or yellow nodes are indeed a type of corpus luteum.
The corpus luteum is a transient endocrine gland that forms in the ovary of mammals and some nonmammals from the follicular layers after ovulation. Although the role of the corpus luteum is well defined in most mammals its role in non-mammalian vertebrates is unclear. To date, the question of luteal-like structures in lampreys and hagfish is unresolved (Chieffi and Baccari, 1998). In non-mammalian vertebrates, the corpus luteum secretes mainly progesterone involved in the retention of eggs and regulation of hepatic vitellogenin synthesis (Chieffi and Baccari, 1998). The presence of a luteal like structure in cyclostomes has been observed but its function is still unclear. These structures form through a process of follicular atresia accompanied by hypertrophy of the follicular cells. Yellow or brown bodies resembling corpora lutea also form in the gonads of hagfish from degenerating ovulated and atretic follicles (Walvig, 1963). Little is known about the steroids associated with these structures in hagfish. Progesterone has been identified in the gonads of the hagfish species E. burgeri (Hirose et al., 1975) but the steroid production of the yellow bodies has not been examined. During a one-year study, we have documented several corpora lutea-like structures in 30 percent of the hagfish examined (unpublished data, M.L.P.). Based on histological examination these lutea-like structures appeared to be follicular tissues reformed from absorbed eggs and postovulatory follicles. We also examined progesterone production in these structures. The corpus lutealike structures and normal developing ovary tissue were incubated in vitro with pregnenolone. The results from these studies clearly demonstrated high levels of progesterone synthesis by these lutea-like structures compared to ovary tissue alone. Progesterone concentrations measured from normal female ovaries (n = 13) averaged 143 pg/mg wet tissue compared to the corpora lutea-like structures which averaged 1,000 pg/ mg wet tissue. Importantly, it is likely that progesterone is a major steroid in hagfish that is involved in various critical aspects of developing eggs. These are the first studies in hagfish to document progesterone production from corpora lutea-like structures. Further studies will be necessary to determine the structure and function of these corpus lutea-like structures.
Little is known about hagfish embryology or juvenile hagfish due to the remarkably low numbers of hagfish embryos available for study. No fertilized hagfish embryos of Eptatretus have been collected for study since 1905 (Gorbman, 1997). Developing eggs in their natural environment have never been observed (Gorbman, 1997). In addition, there is little known about how the hagfish feed, grow, or sexually mature (Martini, 1998). Due to these and other factors, it is imperative that further research be conducted to understand the basic reproduction and ecology of hagfish.
Only three fertilized eggs of the species Myxine have ever been found and documented. These three were all badly damaged (Holmgren, 1946). They were collected during a twenty-four year program with local fishermen along the west coast of Sweden. Two of the embryos were used for a description of general structure (Holmgren, 1946) despite being poorly preserved. The third Myxine embryo was used for a pituitary development study (Gorbman, 1997). Approximately 150 fertilized eggs with developing embryos, of the genus Eptatretus, were collected over 100 years ago in Monterey, California. It has been the only successful recovery of hagfish embryos and since then none have been discovered despite many attempts. The majority of the Eptatretus embryos were doomed to faulty sectioning and preparation or superficial staining and mounting (Gorbman, 1997). A modern discovery of fertilized hagfish embryos would most likely yield better results. Advancements in technology would allow for the preservation of samples and the planning of well-designed experiments to study their embryology and development.
There have been successful retrievals of unfertilized eggs of the Pacific hagfish (Fernholm, 1974). Mature males and females, of E. burgeri, were kept in cages in the sea for about two months by Fernholm (1974) during what is believed to be their normal spawning period, in October. Nine mature females ovulated 200 eggs while 178 were deposited naturally. In a concurrent experiment conducted in aquaria only one of 17 hagfish deposited eggs (Fernholm, 1974). Fertilized egg deposition by hagfish in captivity is very rare; however, unfertilized egg release of E. stouti is common. Sower (unpublished) as well as others, have maintained this same species of hagfish for periods of up to 12 months and have observed the release of unfertilized eggs. Nansen (1887) kept the hagfish M. glutinosa in cages in the sea for up to six months that did not produce any eggs. While there are no documented answers as to how hagfish reproduce, considerable data have led to the following: reproduction takes place at a depth in excess of 30 fathoms 50 meters and the eggs are fertilized externally and anchor themselves by their hooks not far from where they were extruded.
Gonadotropin-releasing hormone (GnRH) is the major hypothalamic neurohormone involved in mediating reproductive activity in all vertebrates (Sower, 2003) (Fig. 5). GnRH is a peptide composed of ten amino acids and released from the hypothalamus in response to appropriate internal and external cues, and then travels to the anterior pituitary where it stimulates the release of gonadotropins. The gonadotropins, luteinizing hormone and follicle-stimulating hormone (GTH, LH, FSH) enter the bloodstream and act on the gonads to stimulate steroidogenesis and induce the maturation of eggs or sperm (Sower, 2003).
Evidence supporting the presence of GTH in the hagfish is not conclusive. Matty et al. (1976) identified only limited abnormalities in the testes and ovaries of 150 hypophysectomized hagfish during a 7 month study. Gametogenesis appeared to be unaffected by hypophysectomy suggesting that the hagfish gonad was independent of hypophysial gonadotropic control. However, Patzner and Ichikawa (1977) observed a decrease in the number of follicles containing spermatocytes and only a few follicles containing spermatides in hypophysectomized hagfish when compared to sham operated hagfish. These results suggested that the development of the hagfish gonad was under hypophysial gonadotropic control.
Lampreys are the most basal vertebrates next to the hagfish and serve as a demonstrative model of two distinct GnRHs (lamprey GnRH-I and -III) that control the hypothalamic-pituitary-gonadal axis. There is substantial data showing that the structure and function of the hypothalamic GnRHs in vertebrates is highly conserved throughout vertebrate evolution (Gorbman and Sower, 2003). However, in hagfish there seems to be a lack of, or poor regulation of reproduction by hypothalamic-pituitary peptides (Sower and Gorbman, 1999). Inadequate experimental support has confounded the presence and possible function of sex steroid hormones and hypothalamic hormones such as GnRH. In fact, GnRH has not been isolated in hagfish. Although, in two studies using anti sera to lamprey GnRH-III from Dr. Sower’s laboratory, as well as other GnRH antisera in Dr. Northcutt’s laboratory, immunoreactive GnRH was detected in the brain of the Atlantic hagfish and Pacific hagfish (Sower et al., 1995; Braun et al., 1995; Oshima et al, 2000). These data suggested the presence of a lamprey GnRH-III-like molecule in the hagfish brain and that hagfish may have reproductive control mechanisms that are similar to other vertebrates. Based on this evidence that GnRH is indeed present in hagfish, we measured GnRH in the hagfish sampled seasonally in our study of Powell et al, 2004 (Kavanaugh et al, 2005). We clearly showed seasonal changes of GnRH correlating with the proposed seasonal cycle of steroid hormones and gonadal development (Kavanaugh et al., 2005). As shown in Figure 2a, b, there was an increase in brain GnRH concentrations that occurred prior to the increased concentration of estradiol and progesterone in the gonad tissue of the hagfish. In addition, we performed a preliminary study and injected 48 micrograms of microencapSLilated lamprey GnRH-III into hagfish with an appropriate control group in February in order to test if GnRH had an effect on gametogenesis. Hagfish were sampled after one month for comparison to a control non-injected group (n = 6). Subsequent histological analysis showed that lamprey GnRH-III appeared to stimulate reproductive development in female hagfish compared to controls. These current data with data from earlier studies suggest an active neuroendocrine axis in the reproductive cycle of the hagfish. However, to fully determine the hypothalamic-pituitary-gonadal axis in hagfish will require the identification of the native GnRHs and other reproductive hormones to fully assess the extent of the neuroendocrine control.
Our studies examining reproductive cycles in hagfish are a good start in our understanding of the growth and reproduction of Atlantic hagfish. These data along with future extensive studies on hagfish reproduction are needed in our understanding to prevent the exploitation of the Atlantic hagfish off the New England Coastline so that the fishery and the resource can be managed for the long term.
Hagfish represent a valuable scientific and economic resource. However, hagfish present a challenge to fishries management agencies. Nardi (1993) proposed that to maintain the hagfish fishery as a sustainable fishery would require specific biological and resource information that is still lacking a decade later. Population dynamics, relationships of length, age and maturity, age of recruitment, extent of resource and reproductive potential are only a few of the areas that must be addressed before an effective management strategy can be formulated for this fishery. Our research has focused on one aspect of hagfish life history, reproduction, in particular the reproductive steroids and GnRH associated with reproductive cycles in other animals. It is not known what cues are responsible for this reproductive cycle. The cycle may be influenced by internal signals or external signals from the environment or a combination of the two. Changes in water temperature, food availability or water currents could all influence the reproductive cycle of hagfish in the Gulf of Maine. Identification of the factors influencing reproduction in hagfish will be made more difficult due to their unique life history. Traditional methods of population assessment used by the fisheries industry cannot be applied to the hagfish. If the population is to be assessed and managed for a sustainable fishery, nontraditional methods of stock assessment will have to be employed. Quantification of reproductive steroid concentrations throughout the year can be used to identify periods when there is a greater potential for reproductive activities. Protecting hagfish populations during potential reproductive periods may allow them to recover from the pressures of a commercial fishery. It may also help to target collection times to increase the probability of obtaining reproductively mature hagfish that may help unravel the mystery of reproduction in this ancient vertebrate ancestor.
We would like to dedicate this paper to the late Dr. Aubrey Gorbman for his invaluable assistance during this project. We would also like to thank Cari Bourn, Mihael Freamat, Nathaniel Nucci, Adam Root, Matt Silver, JoAnne Davis, Jen Glieco, Emily Violette, Jocelyn Sanford, Greg Blaisdell, Kara Lee and Byron Peddler for assistance in collection of animals and processing of samples and the crew of the RV Gulf Challenger, Captain-Paul Pelletier, Bryan Scares, Debbie Brewitt for the use of the boat and other assistance. This research was supported by UNH/UME NOAA Sea Grant R/FMS-168 and NSF 0090852 to SAS. This was Scientific Contribution Number 2226 from the New Hampshire Agricultural Experiment Station.
1 From the Symposium EcoPhysiology and Conservation: The Contribution of Endocrinology and Immunology presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 5-9 January 2004, at New Orleans, Louisiana.
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MICKIE L. POWELL,* SCOTT I. KAVANAUGH,* AND STACIA A. SOWER2,*
* Department of Biochemistry and Molecular Biology, University of New Hampshire, College of Life Sciences and Agriculture, Rudman Hall, 46 College Road, Durham, New Hampshire 03824-2617
2 Correspondence; e-mail: firstname.lastname@example.org
Copyright Society for Integrative and Comparative Biology Feb 2005
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