Lampetra Richardsoni Research Articles

Differential effects of bicarbonate on severe hypoxia- and hypercapnia-induced cardiac malfunctions in diverse fish species.

We tested in six fish species [Pacific lamprey (Lampetra richardsoni), Pacific spiny dogfish (Squalus suckleyi), Asian swamp eel (Monopterus albus), white sturgeon (Acipenser transmontanus), zebrafish (Danio rerio), and starry flounder (Platichthys stellatus)] the hypothesis that elevated extracellular [HCO3-] protects spontaneous heart rate and cardiac force development from the known impairments that severe hypoxia and hypercapnic acidosis can induce. Hearts were exposed in vitro to either severe hypoxia (~ 3% of air saturation), or severe hypercapnic acidosis (either 7.5% CO2 or 15% CO2), which reduced heart rate (in six test species) and net force development (in three test species). During hypoxia, heart rate was restored by [HCO3-] in a dose-dependent fashion in lamprey, dogfish and eel (EC50 = 5, 25 and 30mM, respectively), but not in sturgeon, zebrafish or flounder. During hypercapnia, elevated [HCO3-] completely restored heart rate in dogfish, eel and sturgeon (EC50 = 5, 25 and 30mM, respectively), had a partial effect in lamprey and zebrafish, and had no effect in flounder. Elevated [HCO3-], however, had no significant effect on net force of electrically paced ventricular strips from dogfish, eel and flounder during hypoxia and hypercapnia. Only in lamprey hearts did a specific soluble adenylyl cyclase (sAC) inhibitor, KH7, block the HCO3--mediated rescue of heart rate during both hypoxia and hypercapnia, and wasthe only species where we conclusively demonstrated sAC activity was involved in the protective effects of HCO3- on cardiac function. Our results suggest a common HCO3--dependent, sAC-dependent transduction pathway for heart rate recovery exists in cyclostomes and a HCO3--dependent, sAC-independent pathway exists in other fish species.

River Network and Reach‐Scale Controls on Habitat for Lamprey Larvae in the Umpqua River Basin, Oregon

AbstractThis study developed a spatially explicit framework to support the conservation of Western Brook Lamprey Lampetra richardsoni and Pacific Lamprey Entosphenus tridentatus in the Umpqua River basin, Oregon. This framework identified locations within the river network likely to support “potential burrowing habitat” for lamprey larvae based on geomorphic conditions and evaluated the overlap of potential burrowing habitat with water temperatures suitable for the nonnative, piscivorous Smallmouth Bass Micropterus dolomieu. The study also documented reach‐scale factors that create heterogeneity in potential burrowing habitat to guide on‐the‐ground habitat restoration. Based on criteria for mean annual suspended sediment loads and channel slope, 18% of the Umpqua River network was classified as potential burrowing habitat. Existing mean August water temperatures of ≥20°C were suitable for Smallmouth Bass for 32% of the potential burrowing habitat. This percentage increased to 41% of the potential burrowing habitat using projected mean August water temperatures for year 2040, suggesting that water temperatures in the future will facilitate upstream expansion of Smallmouth Bass into the potential burrowing habitat. At finer spatial scales, potential burrowing habitat was influenced by channel features, such as large wood, pools, and local channel slope and width. These results provide an initial template for identifying locations in river networks likely to have potential burrowing habitat, considering the overlap between threats and lamprey habitats, and planning conservation actions to support native lampreys.

An experimental study to evaluate predation threats on two native larval lampreys in the Columbia River Basin, USA

AbstractExperimental predation studies were conducted to evaluate and compare the predation threats of 10 species of native and non‐native fishes on larvae of Pacific Lamprey, Entosphenus tridentatus, and Western Brook Lamprey, Lampetra richardsoni. The relative predatory threats were examined over four sets of binary factors, including the following: (a) short (2‐day) or long (7‐day) duration, (b) presence/absence of fine sediment, (c) live or dead larvae and (d) species of lampreys. Our short‐term results showed a positive correlation with the sizes of predator fishes and consumed lamprey larvae. Also, most predator fishes had a significantly higher propensity to prey on lamprey larvae when sediment was absent. Conversely, this demonstrated the importance of sediment in protecting lamprey larvae from predation. Based on the predatory behaviour for live and dead larvae, predator fishes were classified into four groups using principal component analysis. Predation rates of larvae in sediment by piscivorous predators including Northern Pikeminnow Ptychocheilus oregonensis and Smallmouth Bass Micropterus dolomieu were lower and showed no differences even when the study duration was extended. In contrast, predation rates by benthic predators including White Sturgeon Acipenser transmontanus and Common Carp Cyprinus carpio, increased when we extended the study duration. This suggests that when given longer duration, benthic predators are more adept at consuming larvae within the sediment. These results provide important context for assessing the potential threat of predation on lampreys in streams, which is a key knowledge gap for lampreys.

Contemporary and historical river connectivity influence population structure in western brook lamprey in the Columbia River Basin

The western brook lamprey Lampetra richardsoni (WBL) is a small non-parasitic lamprey that inhabits rivers and streams from southern Alaska to northern California. WBL remain in fresh water throughout their entire life and show limited dispersal. Although adults may migrate short distances upstream to spawn, most movement likely occurs through passive drifting of larvae downstream. Genetic differentiation among populations is thus expected to be high, even within a single basin, but WBL population structure has received little attention. The present study examined population connectivity of WBL from 23 sites throughout the Columbia River Basin and coastal Washington, using eight microsatellite loci and cytochrome b sequence data. Although population structure generally corresponded to contemporary river connectivity, there were some cases where genetic patterns were better explained by historical connections. Microsatellite genetic differentiation among populations separated by < 570 km was moderate to high; FST values ranged from − 0.0026 to 0.7117 and averaged 0.2929. Tributary distance was the best predictor of FST, suggesting that most gene flow takes place in tributaries rather than through the mainstem of the Columbia River. As predicted, gene flow occurred primarily in a downstream direction, resulting in lower genetic diversity in upstream sites. WBL populations in these areas may be particularly vulnerable to local extinction. Therefore, whereas anadromous lamprey management efforts are focused on improving passage at mainstem dams, conservation of WBL will require protection of individual watersheds with particular emphasis on headwater areas.

Bioenergetics Models to Estimate Numbers of Larval Lampreys Consumed by Smallmouth Bass in Elk Creek, Oregon

AbstractNonnative fishes have been increasingly implicated in the decline of native fishes in the Pacific Northwest. Smallmouth Bass Micropterus dolomieu were introduced into the Umpqua River in southwest Oregon in the early 1960s. The spread of Smallmouth Bass throughout the basin coincided with a decline in counts of upstream‐migrating Pacific Lampreys Entosphenus tridentatus. This suggested the potential for ecological interactions between Smallmouth Bass and Pacific Lampreys, as well as freshwater‐resident Western Brook Lampreys Lampetra richardsoni. To evaluate the potential effects of Smallmouth Bass on lampreys, we sampled diets of Smallmouth Bass and used bioenergetics models to estimate consumption of larval lampreys in a segment of Elk Creek, a tributary to the lower Umpqua River. We captured 303 unique Smallmouth Bass (mean: 197 mm and 136 g) via angling in July and September. We combined information on Smallmouth Bass diet and energy density with other variables (temperature, body size, growth, prey energy density) in a bioenergetics model to estimate consumption of larval lampreys. Larval lampreys were found in 6.2% of diet samples, and model estimates indicated that the Smallmouth Bass we captured consumed 925 larval lampreys in this 2‐month study period. When extrapolated to a population estimate of Smallmouth Bass in this segment, we estimated 1,911 larval lampreys were consumed between July and September. Although the precision of these estimates was low, this magnitude of consumption suggests that Smallmouth Bass may negatively affect larval lamprey populations.Received September 20, 2016; accepted March 31, 2017Published online June 6, 2017

Large Wood and Instream Habitat for Juvenile Coho Salmon and Larval Lampreys in a Pacific Northwest Stream

AbstractThe influences of large wood on Pacific salmon are well‐studied, but such studies on nonsalmonid species such as lampreys are uncommon. To address this need, we evaluated the potential effects of large wood on larval lampreys (Pacific Lamprey Entosphenus tridentatus and potentially Western Brook Lamprey Lampetra richardsoni) as well as on juvenile Coho Salmon Oncorhynchus kisutch in a small, coastal, Oregon stream. Our objectives were to (1) identify instream habitat characteristics associated with the presence of larval lampreys and abundance of juvenile Coho Salmon and (2) evaluate how these characteristics were associated with instream wood. To address habitat use, we quantified presence of larval lampreys in 92 pools and abundance of juvenile Coho Salmon in 44 pools during summer low flows. We focused on a study reach where large wood was introduced into the stream between 2008 and 2009. Results indicated that presence of larval lampreys was significantly associated with the availability of fine sediment and deeper substrate. The abundance of juvenile Coho Salmon (fish per pool) was strongly associated with pool surface area and to a weaker extent with the proportion of cobble and boulder substrates in pools. Pools with wood, regardless of whether they were formed by wood, had significantly greater coverage of fine sediment, deeper substrate, and greater pool surface area. Taken together, these results suggest that instream wood can provide habitat associated with the presence of larval lampreys and greater abundance of juvenile Coho Salmon.Received September 11, 2016; accepted March 25, 2017 Published online May 31, 2017

Growth and annual survival estimates to examine the ecology of larval lamprey and the implications of ageing error in fitting models.

This study used existing western brook lamprey Lampetra richardsoni age information to fit three different growth models (i.e. von Bertalanffy, Gompertz and logistic) with and without error in age estimates. Among these growth models, there was greater support for the logistic and Gompertz models than the von Bertalanffy model, regardless of ageing error assumptions. The von Bertalanffy model, however, appeared to fit the data well enough to permit survival estimates; using length-based estimators, annual survival varied between 0·64 (95% credibility interval: 0·44-0·79) and 0·81 (0·79-0·83) depending on ageing and growth process error structure. These estimates are applicable to conservation and management of L. richardsoni and other western lampreys (e.g. Pacific lamprey Entosphenus tridentatus) and can potentially be used in the development of life-cycle models for these species. These results also suggest that estimators derived from von Bertalanffy growth models should be interpreted with caution if there is high uncertainty in age estimates.

River Lamprey (Lampetra ayresii) Outmigrant Upstream of the John Day Dam in the Mid-Columbia River

The anadromous River Lamprey (Lampetra ayresii) is known to occur in North America from Alaska to central California, generally in association with larger estuarine systems (such as the Fraser River in British Columbia, the Sacramento River in California, and the Columbia River in Washington-Oregon; Vladykov and Follett 1958; Weitkamp and others 2015). Its occurrence in the Columbia River basin, however, has not been well documented. Although small numbers of juvenile (feeding phase) River Lamprey have been captured incidentally in the Columbia River Estuary (Bond and others 1983; Weitkamp and others 2012, 2015), there are few records of this species in freshwater (Kostow 2002). The farthest upriver report in the Columbia River basin is a March 1940 record at Bonneville (presumably at Bonneville Dam), 235 river kilometers (rkm) from the Pacific Ocean (Vladykov and Follett 1958). However, adult River Lamprey (117to 324-mm total length; Vladykov and Follett 1958; Bond and others 1983; Wydoski and Whitney 2003; Weitkamp and others 2015) are less noticeable than the larger Pacific Lamprey (Entosphenus tridentatus) (700to 800-mm total length; Clemens and others 2010) and might spawn in deep water in large river systems similar to other lamprey species (Dawson and others 2015). Thus, even when abundant, they are rarely observed in freshwater (Beamish 1980; Beamish and Youson 1987). Furthermore, during its prolonged freshwater larval stage, it is difficult or impossible to distinguish River Lamprey, either morphologically or genetically, from the closely related and widely distributed Western Brook Lamprey (Lampetra richardsoni) (Docker 2009). The nonmigratory, non-trophic adult Western Brook Lamprey (90to 180-mm total length) can be distinguished from adult River Lamprey by morphological and dentition characters (Wydoski and Whitney 2003; Docker 2009). There is a growing conservation concern for lampreys worldwide. Like other species, the River Lamprey is susceptible to various freshwater threats such as dredging and barriers to migration (Maitland and others 2015). The Columbia River Basin is currently fragmented by many mainstem hydropower, navigation, and flood-control dams, as well as numerous additional dams and irrigation diversions in tributary systems. Many of these dams do not provide fish passage and those that do are likely problematic for migratory adult lampreys (Moser and others 2015). Documentation of River Lamprey in the Columbia River basin would help inform management of this highly regulated system, as well as elucidate distribution patterns to aid conservation. A current effort is underway to increase the monitoring of outmigrating juvenile lampreys through the operation of salmonid smolt monitoring systems at mainstem Columbia River dams (Fish Passage Center 2014). Subsamples of juvenile (metamorphosed to outmigrating phase) and larval (unmetamorphosed) lampreys are collected, anesthetized, identified (to species for juvenile lampreys and to genus for larval lampreys), measured, and assessed for body condition. A putative juvenile (recently metamorphosed) River Lamprey was collected at the John Day Dam smolt bypass facility on 28 May 2013 from a sample time frame of 13:00 to 07:00. The fish was 144-mm total length and weighed 3.1 g. Its dentition pattern clearly distinguished it from the relatively more common Pacific Lamprey (Fig. 1). Pacific Lamprey have 3 large cusps on the supra-oral lamina, 4 pairs of lateral teeth, and 1 row of posterior teeth; whereas the River Lamprey has 2 cusps on the supra-oral lamina, 3 pairs of lateral teeth, and no posterior teeth (Wydoski and Whitney 2003). Its caudal pigmentation was likewise consistent with Lampetra spp. (Fig. 2, Fig. 3); lampreys in the Entosphenus genus possess a lighter caudal ridge, whereas the caudal ridge of Lampetra spp. is uniformly dark (Goodman and others GENERAL NOTES

Evaluation of Methods to Measure Condition in Pacific Northwest Larval Lampreys

Pacific lamprey (Entosphenus tridentatus) populations are declining and western brook lamprey (Lampetra richardsoni) status is unknown in the Pacific Northwest. Accurate measures of fish condition are a basic requirement to monitor health and status of fish populations. Fulton's condition factor has traditionally been used to assess condition of larval lamprey but alternative field-based and laboratory-based measures have not been assessed. We compared condition factor, body density, and lipid content of sympatric larval Pacific lamprey and western brook lamprey in 2010 and 2011 from two rivers of the Pacific Northwest. Condition factor was higher for Pacific lamprey than it was for western brook lamprey. All measures were variable and correlations among condition indices were weak. Body density and lipid content were higher in western brook lamprey in one year suggesting the possibility of increased energy requirements of metamorphosis to a nontrophic adult stage. The body density technique was onerous and likely has little field-based practicality. Body density and lipid content had a negative relationship as increased lipids leads to a more buoyant and less dense fish. Condition factor did not appear to be a good predictor of lipid content in our study and a suite of complex factors, including lipid dynamics prior to metamorphosis and nutritional resources, may underlie these results. This study provides one of the few empirical datasets on sympatric lampreys with different life history strategies.

Distribution of Pacific LampreyEntosphenus tridentatusin Watersheds of Puget Sound Based on Smolt Monitoring Data

Abstract Lamprey populations are in decline worldwide and the status of Pacific lamprey (Entosphenus tridentatus) is a topic of current interest. They and other lamprey species cycle nutrients and serve as prey in riverine ecosystems. To determine the current distribution of Pacific lamprey in major watersheds flowing into Puget Sound, Washington, we sampled lamprey captured during salmonid smolt monitoring that occurred from late winter to mid-summer. We found Pacific lamprey in 12 of 18 watersheds and they were most common in southern Puget Sound watersheds and in watersheds draining western Puget Sound (Hood Canal). Two additional species, western brook lamprey (Lampetra richardsoni) and river lamprey (L. ayresii) were more common in eastern Puget Sound watersheds. Few Pacific lamprey macrophthalmia were found, suggesting that the majority of juveniles migrated seaward during other time periods. In addition, “dwarf” adult Pacific lamprey (< 300 mm) were observed in several watersheds and may represent ...

Genetic diversity, endemism and phylogeny of lampreys within the genus Lampetra sensu stricto (Petromyzontiformes: Petromyzontidae) in western North America

Phylogenetic structure of four Lampetra species from the Pacific drainage of North America (western brook lamprey Lampetra richardsoni, Pacific brook lamprey Lampetra pacifica, river lamprey Lampetra ayresii and Kern brook lamprey Lampetra hubbsi) and unidentified Lampetra specimens (referred to as Lampetra sp.) from 36 locations was estimated using the mitochondrial cytochrome b gene. Maximum parsimony and Bayesian inferences did not correspond with any taxonomic scheme proposed to date. Rather, although L. richardsoni (from Alaska to California) and L. ayresii (from British Columbia to California) together constituted a well-supported clade distinct from several genetically divergent Lampetra populations in Oregon and California, these two species were not reciprocally monophyletic. The genetically divergent populations included L. pacifica (from the Columbia River basin) and L. hubbsi (from the Kern River basin) and four Lampetra sp. populations in Oregon (Siuslaw River and Fourmile Creek) and California (Kelsey and Mark West Creeks). These four Lampetra sp. populations showed genetic divergence between 2.3 and 5.7% from any known species (and up to 8.0% from each other), and may represent morphologically cryptic and thus previously undescribed species. A fifth population (from Paynes Creek, California) may represent a range extension of L. hubbsi into the Upper Sacramento River.

Short Communication Characterization of 12 microsatellite loci for the Pacific lamprey, Entosphenus tridentatus (Petromyzontidae), and cross-amplification in five other lamprey species

The Pacific lamprey (Entosphenus tridentatus) is an anadromous fish that is of conservation concern in North America and Asia. Data on Pacific lamprey population structure are scarce and conflicting, impeding conservation efforts. We optimized 12 polymorphic microsatellite loci for the Pacific lamprey. Three to 13 alleles per locus were observed in a sample of 51 fish collected from the West Fork Illinois River, Oregon. Observed heterozygosity ranged from 0.235 to 0.902 and expected heterozygosity ranged from 0.214 to 0.750. Cross-species amplification produced 8 to 12 polymorphic loci in four other Entosphenus species and in the western brook lamprey (Lampetra richardsoni). Two loci appear to be diagnostic for distinguishing Entosphenus from Lampetra. These markers will be valuable for evaluating population structure and making conservation decisions for E. tridentatus and other lamprey species.

Characterization of ten microsatellite loci for western brook lamprey Lampetra richardsoni

We describe the isolation and development of ten polymorphic microsatellite loci for western brook lamprey (Lampetra richardsoni). Two to nine alleles were observed per locus in a sample of 35 fish. Gene diversity ranged from 0.000 to 0.792 in six populations. Cross-species amplification was successful as nine, seven and eight loci were polymorphic in Lampetra pacifica, Entosphenus hubbsi, and Entosphenus tridentatus, respectively. These markers will aid in evaluating the population structure of lamprey that are native to western North America.

HPLC and ELISA analyses of larval bile acids from Pacific and western brook lampreys

Comparative studies were performed on two native lamprey species, Pacific lamprey ( Lampetra tridentata) and western brook lamprey ( Lampetra richardsoni) from the Pacific coast along with sea lamprey ( Petromyzon marinus) from the Great Lakes, to investigate their bile acid production and release. HPLC and ELISA analyses of the gall bladders and liver extract revealed that the major bile acid compound from Pacific and western brook larval lampreys was petromyzonol sulfate (PZS), previously identified as a migratory pheromone in larval sea lamprey. An ELISA for PZS has been developed in a working range of 20 pg–10 ng per well. The tissue concentrations of PZS in gall bladder were 127.40, 145.86, and 276.96 μg/g body mass in sea lamprey, Pacific lamprey, and western brook lamprey, respectively. Releasing rates for PZS in the three species were measured using ELISA to find that western brook and sea lamprey released PZS 20 times higher than Pacific lamprey did. Further studies are required to determine whether PZS is a chemical cue in Pacific and western brook lampreys.

Status of the Morrison Creek Western Brook Lamprey, Lampetra richardsoni, in Canada

Status of the Morrison Creek Western Brook Lamprey, Lampetra richardsoni, in Canada

The structure and organization of lamprin genes: multiple-copy genes with alternative splicing and convergent evolution with insect structural proteins.

Lamprin is a unique structural protein which forms the extracellular matrix of several cartilaginous structures found in the lamprey. Lamprin is noncollagenous in nature but shows sequence similarities to elastins and to insect structural proteins. Here, we characterize the structure and organization of lamprin genes, demonstrating the presence of multiple similar but not identical copies of the lamprin gene in the genome of the lamprey. In at least one species of lamprey, Lampetra richardsoni, the multiple gene copies are arranged in tandem in the genome in a head-to-tail orientation. Lamprin genes from Petromyzon marinus contain either seven or eight exons, with exon 4 being alternatively spliced in all genes, resulting in a total of six different lamprin transcripts. All exon junctions are of class 1,1. An unusual feature of the lamprin gene structure is the distribution of the 3' untranslated region sequence among multiple exons. A TATA box and cap sequence have been identified in upstream sequences in close proximity to the transcription start site, but no CAAT box could be identified. Sequence and gene structure comparisons between lamprins, elastins, and insect structural proteins suggest that the regions of sequence similarity are the result of a process of convergent evolution.

Comparison of the internal morphology of adults of a population of lampreys that contains a nonparasitic life-history type, Lampetra richardsoni, and a potentially parasitic form, L. richardsoni var. marifuga

Histological techniques were used to study adults of a single population of the brook lamprey, Lampetra richardsoni, in Morrison Creek, Vancouver Island, British Columbia, to establish whether there are any differences between the internal morphology of a "silver" variety (L. r. var. marifuga) and that of the nonparasitic variety (L. richardsoni). The results of the study support the view of two life-history types in this creek. When L. richardsoni is sexually mature and has an atrophied intestine and sparse haemopoietic tissue, most L. r. var. marifuga possess a maturing (not totally mature) gonad, a highly developed alimentary canal containing ingested food, and extensive haemopoietic tissue. Although the gills, thyroid gland, and the definitive kidneys are similar in both forms, incomplete regression of the larval kidney and the absence of a cranial pancreas in all L. r. var. marifuga imply that a slow sexual maturation is accompanied by a slow, and perhaps incomplete, metamorphosis. The incomplete metamorphosis is particularly exemplified in female L. r. var. marifuga and may account for the abnormal sex ratio of this variety. The retardation of sexual maturation in male L. r. var. marifuga permits the retention of a functional digestive system and, hence, provides an opportunity for feeding. The significance of these observations to present views on the evolution of lampreys is discussed.

A comparison of endocrine pancreatic tissue in adults of four species of lampreys in British Columbia: A morphological and immunohistochemical study

Histological techniques and immunohistochemistry with antisera against insulin and somatostatin were used in a study of the identification and the distribution of endocrine pancreatic tissue in adults of four species of lampreys in British Columbia. The fine structure of the pancreatic cells of one of the species was also examined. The four species ( Lampetra ayresi, Lampetra tridentata, Lampetra macrostoma, and Lampetra richardsoni) all possess cranial and caudal pancreatic masses and an intervening intermediate cord, but there are variations in amount and deposition of the pancreatic tissue with respect to the alimentary canal and the liver which appear to be species specific. There is also some variation in the distribution of pancreatic tissue between L. richardsoni of different stream populations. A newly identified parasitic variety of L. richardsoni in Morrison Creek, called L. richardsoni var. marifuga, usually has no cranial pancreas but has an extensive intermediate cord. The distribution of pancreatic tissue in these lampreys is of relevance to taxonomy and to variations in development of the endocrine pancreas during lamprey metamorphosis. The pancreatic tissue of all species and var. marifuga is composed of cells which are immunoreactive to either insulin or somatostatin antisera. The fine structure of cells from the cranial and caudal pancreatic tissue from L. ayresi does not vary with saltwater and fresh-water acclimation. Insulin-containing B cells and somatostatin-containing D cells are present in equal numbers in both pancreatic regions and the cells each have characteristic cytoplasmic granules. A third cell type is most abundant in the cranial pancreas but it is of unknown function.

Ophioxenos lampetrae sp. nov. (Digenea: Paramphistomidae) from ammocoetes of the western brook lamprey (Lampetra richardsoni Vladykov and Follett) in British Columbia, with comments on lamprey host–parasite relationships

Ophioxenos lampetrae sp. nov. (Digenea: Paramphistomidae) from the intestine of ammocoetes of the western brook lamprey (Lampetra richardsoni) in British Columbia, Canada, is described. It is distinguished from other members of the genus, O. dienteros and O. singularis (parasites of reptiles, primarily garter snakes, and amphibians in North America), by having relatively few vitelline folicles of limited anterior extent and a voluminous uterus filling most available intercaecal space. This appears to be the first record of a digenean utilizing a petromyzontid as a definitive host. Some aspects of lamprey host–parasite relationships are discussed.

Evolution of the Brain–Pituitary Relationship: Evidence from the Agnatha

In the brain of the lamprey there are neurosecretory cells that manufacture and carry known hormonal materials to the neurohypophysis, which is that part of the diencephalic floor that is adjacent to the adenohypophysis. The adenohypophysis is differentiated into a pars distalis and a pars intermedia. However, unlike all other vertebrates, there is no vascular or nervous connection between the neurohypophysis and the pars distalis. If there is any regulation of the adenohypophysis by the hormones released from the endings of neurosecretory axones in the neurohypophysis, it would have to be by diffusion across the intervening connective tissue. Such regulation remains to be demonstrated.It is difficult to decide whether this relationship between neurohypophysis and pars distalis in the lamprey (and hagfish) is primitive or degenerate in its evolution. However, in three groups of more primitive fishes an intermediate condition is found in which a vascular connection exists between the neurohypophysis and a part of the adenohypophysis. This being so, we may speculate that the lamprey and other cyclostomes represent the simplest and most primitive anatomical relationship between brain and adenohypophysis, one in which the two organs are merely apposed, allowing neurosecretions to diffuse between the two. In the elasmobranchs, holocephalians, and the coelacanth, part of the pars distalis has been vascularized by portal vessels beginning in the neurohypophysis. In all other vertebrates the entire pars distalis is so vascularized.Key words: brain, pituitary gland, neurosecretion, hormones, evolution, Lampetra richardsoni, L. tridentata, Petromyzon marinus