Length Of Larval Life Research Articles

Biological Bulletin Virtual Symposium: Biology of Marine Invertebrate Larvae

Larval forms have been central to the study of evolution-ary and developmental biology since the inception of thesefields in the mid-19th century, yet they still offer contem-porary biologists tremendous opportunities for decipheringfundamental biological principles. Larvae are found be-tween the embryonic and juvenile stages of those metazoanswith indirect development, which includes the vast majorityof marine invertebrates and many marine fishes. As devel-opmental stages that interact behaviorally with their envi-ronments, they play important roles in development, ecol-ogy, and evolution and are therefore of interest to a widecommunity of biologists. The recent rise of “Evo Devo”(Evolutionary Developmental Biology) has propelled thestudy of larval forms to center stage in organismal biology.However, many marine invertebrate larvae are still unde-scribed and the variety of developmental types is greatlyunderestimated, especially in phyla that historically re-ceived little attention. Increased application of confocalmicroscopy in combination with fluorescent dyes is nowrevealing a previously unsuspected diversity of develop-ment and facilitating comparative studies of larval neuro-genesis, myogenesis, and anatomy.Most larvae are small, on the order of 0.5 to a fewmillimeters in size, and they generally occupy differenthabitats from those of the juveniles into which they meta-morphose. For planktonic larvae, small size and slow ratesof movement with cilia or muscle-driven appendages re-quire them to have shapes and functions different fromthose of juveniles and also leave them more or less at themercy of the water currents that disperse and mix them.Because many marine invertebrates have planktonic larvalstages and benthic adults, larvae link adult populationsgenetically and contribute to local population sizes anddynamics. A recent renewed interest in marine conservationfocuses on larvae as the dispersal stage that provides theall-important connectivity among metapopulations, includ-ing those in marine reserves. Larval stages are also key tothe understanding of recruitment, a critical issue in this ageof declining fisheries resources.As transitional stages between embryos and juveniles,larvae have structures, physiology, and behaviors that allowthem to operate in their own environments while developinganatomical and physiological attributes that will be used bythe juvenile stages in a new environment. In addition tosimilarities and differences of invertebrate larval formsamong major clades such as phyla and classes, many lowerlevel clades also show large variations in larval form, lengthof larval life, and timing of expression of traits, all of whichprovide opportunities to study how the differences arose.In the context outlined above, larval biology is a verybroad, very active, and very rich area for exploration ofbasic scientific questions, so it is fitting that the fourthvirtual symposium in

Temporal and spatial variation in length of larval life and size at settlement of the Hawaiian amphidromous goby Lentipes concolor

Larvae of the Hawaiian amphidromous goby Lentipes concolor settled after a mean length of larval life (LLL) of 86·2±8·5 days (n=236, range=63–106 days) at a mean size of 16·0±0·7 mm LT (n=154, range=14·1‐17·9 mm). Mean LLL for L. concolor was about twice that typically reported for tropical marine gobiids. Variation in LLL (CV=10%) and size at settlement (CV=4%) were low, and comparable to that for marine gobiids. LLL and LT were weakly positively correlated (Pearson's correlation coefficient r=0·50, P<0·0001). Larvae settled after shorter planktonic lives and at smaller sizes during months with warmer ocean temperatures. Inter‐island variation in LLL did not support a dominant south‐east to north‐west larval drift, following the dominant south‐east to north‐west flow of prevailing currents in the Archipelago. Instead, recruits on Maui Island, centrally located in the archipelago, had shorter LLL than recruits to upstream Hawai'i and downstream Kaua'i islands. These findings have important implications for understanding the complex life history dynamics of amphidromous fishes as well as their management.

Temporal and spatial variation in length of larval life and size at settlement of the Hawaiian amphidromous gobyLentipes concolor

Temporal and spatial variation in length of larval life and size at settlement of the Hawaiian amphidromous gobyLentipes concolor

Reproduction and dispersal at vents and cold seeps

Reproductive cycles are determined from samples taken at regular intervals over a period of time related to the assumed periodicity of the breeding cycle. Fiscal, ship time and sampling constraints have made this almost impossible at deep-sea vents and seeps, but there is an accumulating mass of data that cast light on these processes. It is becoming apparent that most reproductive processes are phylogenetically conservative, even in extreme vent and seep habitats. Reproductive patterns of species occurring at vents and seeps are not dissimilar to those of species from the same phyla found in non-chemosynthetic environments. The demographic structure of most vent and seep animals is undescribed and the maximum ages and growth rates are not known. We know little about how the gametogenic cycle is initiated, though there is a growing body of data on the size at first reproduction. Gametogenic biology has been described from seasonal samples for only one organism from vent/seep environments. For other species, the pattern of gametogenesis has been described from serendipitous samples that allow determination of reproductive effort, but such samples reveal little about energy partitioning during the gametogenic process. Some notable adaptations have been described in mature gametes, including modified sperm. Spawning has been observed for a number of species both in situ and in vitro. Knowledge of the larvae of vent/seep organisms has been derived from laboratory fertilizations, from field collections over vent and seep areas and, for molluscs, from protoconch or prodissoconch size and shape. Larval dispersal has been perhaps the most intractable aspect of reproduction. Because the length of larval life is known for only a single seep organism and no vent organism, we cannot infer dispersal distance from a knowledge of current velocities. Modelling has been used to assess the maximum larval distance that allows effective migration between vent sectors. An indirect approach has been to estimate gene flow within, and between, vent sites using DNA sequencing and electrophoretic techniques. Although data are still equivocal, there are indications of considerable mixing among populations within and between vent sectors of the same ridge. Our knowledge of reproductive biology in vent and seep organisms remains fragmentary, but with molecular and biochemical techniques, emerging larval culture techniques, and increased sampling effort, the pieces of the jigsaw will eventually form an overall picture.

Ultraviolet Radiation and Distribution of the Solitary Ascidian Corella inflata (Huntsman).

The solitary ascidian Corella inflata is a common fouling organism in many areas of Puget Sound and the San Juan Archipelago, Washington, USA. Despite its abundance, it is conspicuously absent from areas that receive direct sunlight. Previous work suggests that ascidians in unshaded habitats can be overgrown and killed by algal overgrowth. In this study, we tested the hypothesis that UV irradiation contributes to C. inflata distribution by killing individuals exposed to direct sunlight. To test this, we exposed C. inflata embryos, larvae, juveniles, and adults to UV irradiation and measured the responses. We also tested for UV-absorbing compounds in larvae, juveniles, and adults. In the laboratory, UV significantly damaged all life stages; the earliest stages were most vulnerable. A 3-week UV exposure significantly shortened adult life span. Juveniles suffered 100% mortality after only 3 days. Tadpole larvae decreased settlement and metamorphosis after 1 day of UV exposure, and embryos exhibited developmental abnormalities after only 30 minutes of exposure. None of the life-history stages had apparent UV-absorbing compounds. Given the vulnerability of this species to UV, we suggest that its unique life-history traits (i.e., time of spawning, brooding behavior, length of larval life) help it persist in its preferred habitat and avoid dispersal into inappropriate, UV-exposed areas.

Effects of food diet on the survival, development and growth rates of two cultured echinoplutei (Paracentrotus lividusandArbacia lixula)

Summary We examined the effects of phytoplanktonic food on the survival rate, growth, development, length of larval life and competence for metamorphosis for two species of echinoids from the northeastern part of the western Mediterranean Sea. For larvae of Arbacia lixula seven diets were tested and for those of Paracentrotus lividus two. A diet of Cricosphaera elongata yielded a high survival rate (80%) and rapid larval growth; development was shorter, rudiment diameter was larger and metamorphosis rate was higher. There was a maximum feeding level beyond which no change in growth and development rate was observed. In response to a poor diet, larvae increased the larval period and the length of the larval arms; the survival rate decreased and development was slow and more variable. However, there was no change in the post-larval diameter regardless of diet history.

Effect of Benthic Microhabitat Cues on the Metamorphosis of Pueruli of the Spiny Lobster Panulirus Argus

To determine whether settlement microhabitat induces metamorphosis in the puerulus stage of the Caribbean spiny lobster (Panulirus argus) and to identify the specific features of the habitat that might elicit the response, we monitored the metamorphic progress of more than 200 pueruli exposed to 6 different settlement substrates: sea water alone (no substrate), red algae, sea grass, artificial algae, algae-treated sea water, and artificial algae plus algae-treated sea water. Initial pigmentation followed settlement by approximately 1 day and began 3-6 days (mean = 5 days) after the swimming pueruli were intercepted entering the Florida Bay nursery. Presence of red algae, Laurencia spp., a preferred settlement substrate, accelerated the rate of pigmentation, but by < 1 day. Metamorphosis to the first benthic juvenile stage occurred 7-9 days (mean = 8 days) after pueruli entered the bay and was unaffected by any of the substrates tested. Rates of both pigmentation and metamorphosis varied by as much as 2 days among replicate experiments conducted during the summers of 1986-1988. These results suggest that metamorphosis of pueruli of P. argus is essentially determinant; thus, physiological constraints may limit the distance that pueruli can disperse into the nursery and may force many pueruli to settle in inappropriate habitats where survival is improbable. Settlement and metamorphosis of planktonic larvae of many marine invertebrates can be induced by physical or chemical cues associated with settlement habitat (see Chia and Rice, 1978; Burke, 1983, for reviews). Specific environmental cues are thought to stimulate neural or hormonal processes that trigger larval settlement and metamorphosis in habitats that offer greater survival for subsequent life stages (Burke, 1983). Larvae that can retard development until favorable conditions are encountered are capable of increased dispersal, which is a function of both transport processes and length of larval life (Sulkin and Van Heukelem, 1986). Such physiological plasticity in the onset of metamorphosis may be a desirable trait for widely dispersing larvae or larvae with ecological requirements that differ from adults, because the probability of encountering suitable settlement habitat within a specific locality or period of time may be low. Alternatively, marine animals living in regions where currents are seasonally predictable and can reliably transport larvae to appropriate nursery grounds may evolve more canalized larval development with physiologically constrained metamorphic rates (Lipcius et al., 1990). For-sessile or sedentary invertebrates (e.g., hydroids, ascidians, echinoids, molluscs, barnacles), on which most research on this topic has focused, selection of a settlement site ultimately reflects the habitat requirements of the adult stage (Meadows and Campbell, 1972; Highsmith, 1982; Morse et al., 1988; and many ot ers). But for invertebrates such as decapod crustaceans, with more complex life cycles and mobile adult s ages, habitat choice by larvae or postlarvae should be influenced by factors that affect juvenile survival, factors that may be unrelated to adult habitat requirements. Despite their widespread abundance and distribution in the marine environment, little information is available concerning metam rphic induction in decapod crustaceans (Felder et al., 1985). Induction or inhibition of metamorphosis from the larval or puerulus stage to he first benthic stage in resp nse to the presence/absence of natural substrates has been recorded for a brachyuran crab (Castro, 1978), a homarid lobster (Botero and Atema, 1982), and a palinurid lobster (Serfling and Ford, 1975). The absence of metamorphic induction for a pagurid crab was reported by Roberts (1971). In contrast to observations made by Serfling and Ford (1975), who reported that the pueruli of the California spiny lobster Panulirus interruptus apparently required surf

Developmental Arrest During Larval Life and Life-Span Extension in a Marine Mollusc

The length of larval life in the nudibranch Phestilia sibogae is determined by a chance encounter with a specific metamorphic stimulus associated with the post-larval benthic habitat. A developmental hiatus begins at the onset of larval metamorphic competence and ends at metamorphosis; aging is suspended during this hiatus. Because the duration of post-larval life is unaffected by the duration of larval life, total life-span varies with the length of the larval period. Developmental control of the timing of expression of life-history stages is an important factor regulating aging and senescence in animals with complex life cycles.

The influence of food concentration and temperature on growth and morphological differentiation of blue mussel Mytilus edulis L. larvae

The relationship between growth rate, rate of morphological development, and length of larval life, was examined for larvae of the blue mussel Mytilus edulis L. The larvae were reared at phytoplankton ( Isochrysis galbana, clone T-ISO) concentrations between 0.5 × 10 4 and 30 × 10 4 cells · ml −1 at either 12 or 16°C. Growth rates generally increased with increasing food concentration and were highest at the higher temperature; maximum shell growth rate was ≈ 8 μm · day −1. The number of days required for larvae to develop recognizable eyespots sometimes varied proportionately with changes in growth rate, but often did not; temperature was especially effective at uncoupling rate of growth from rate of morphological development. Maintained in clean glass dishes at 16°C, many eyespotted larvae survived until the end of the study, as long as 8 wk after they first developed eyespots (≈80 days after fertilization). Nearly 30% of those larvae that survived under these conditions eventually metamorphosed in the absence of filamentous substratum, suggesting that metamorphosis can be postponed at least 45 days at 16°C. Food concentration had no effect on 1. (1)the time elapsed between eyespot development and noninduced attachment; 2. (2) the average shell length of individuals that did attach; or 3. (3) the mean size of larvae remaining at the end of the study.

Post-metamorphic change in activity metabolism of anurans in relation to life history.

Newly-metamorphosed individuals of some species of frogs and toads differ from adults in behavior, ecology, and physiology. These differences may be related to broader patterns of the life histories of different species of frogs. In particular, the length of larval life and the size of a frog at metamorphosis appear to be significant factors in post-metamorphic ontogenetic change. These changes in performance are associated with rapid post-metamorphic increases in oxygen transport capacity. Bufo americanus (American toads) and Rana sylvatica (wood frogs) spend only 2-3 months as tadpoles and metamorphose at body masses of 0.25 g or less. Individuals of these species improve endurance and aerobic capacity rapidly during the predispersal period immediately following metamorphosis. Increases in hematocrit, hemoglobin concentration, and heart mass relative to body mass are associated with this improvement in organismal performance. Rana clamitans (green frogs) spend from 3 to 10 months as larvae and weigh 3 g at metamorphosis. Green frogs did not show immediate post-metamorphic increases in performance. Rana palustris (pickerel frogs) are intermediate to wood frogs and green frogs in length of larval life and in size at metamorphosis, and they are intermediate also in their post-metamorphic physiological changes.American toads and wood frogs appear to delay dispersal from their natal ponds while they undergo rapid post-metamorphic growth and development, whereas green frogs disperse as soon as they leave the water, even before they have fully absorbed their tails. The very small body sizes of newly metamorphosed toads and wood frogs appear to limit the scope of their behaviors. The brief larval periods of these species permit them to exploit transient aquatic habitats, but impose costs in the form of a period of post-metamorphic life in which their activities are restricted in time and space compared to those of adults.

RELATIONSHIP BETWEEN GROWTH, DIFFERENTIATION, AND LENGTH OF LARVAL LIFE FOR INDIVIDUALLY REARED LARVAE OF THE MARINE GASTROPOD,CREPIDULA FORNICATA

Larvae of the gastropod Crepidula fornicata were reared individually through spontaneous metamorphosis in clean glass containers at constant temperatures ranging from 15°C to 29°C; each larva was examined daily. Growth rates were determined from periodic measurements of individual shell length. Differentiation rates were estimated as (days to development of gill rudiments)-1 and as (days until shift from larval to adult shell geometry)-1. Growth ceased abruptly in a majority of the larvae in each treatment, over the size range 900-1100 µm shell length. Larvae continued to ingest phytoplankton during this period, and growth resumed at a normal rate following spontaneous metamorphosis. An inverse correlation was observed between rates of larval growth and length of larval life through spontaneous metamorphosis; faster-growing larvae generally had shorter larval lives than did slower growing larvae. Individual growth rate (µm/day) prior to competence was significantly correlated with rate of individual differentiation. However, rates of differentiation and growth as measured in this study were comparable predictors of when spontaneous metamorphosis would occur. The results are consistent with the hypothesis of a pre-programmed end to larval life in the planktotrophic larvae of C. fornicata, although the factors responsible for initiation of gill development and the shift in shell morphology are apparently not directly related to progress towards the point at which the larva spontaneously metamorphoses to the benthos.

Influence of temperature and temperature shifts on the development of chiton larvae,Mopalia muscosa

Summary Larvae of the mossy chiton, Mopalia muscosa, were reared either at 10–12°C or 16°C throughout larval development, or were reared at one of the two temperatures for about 400 h (17 days) and then transferred to the other temperature for the remainder of larval development. Temperature regime was found to influence developmental rate, maximum duration of planktonic existence and incidence of spontaneous metamorphosis. The effect of temperature on length of larval life may reflect exploitation of a finite nutritional source by these lecithotrophic larvae. More rapid development was correlated with shorter larval life. The effect of temperature on incidence of spontaneous metamorphosis probably does not have a nutritional basis. In 3 of the 4 treatments, at least 80% of the larvae eventually died, although some larvae metamorphosed spontaneously in all of the treatments. Approximately 50% spontaneous metamorphosis was attained in the fourth treatment, in which larvae initially were reared at the coole...

Invasion and upstream migration by glass-eels of Anguilla australis australis Richardson and A. reinhardtii Steindachner in Tasmanian Freshwater Streams

The recruitment of glass-eels into fresh water was investigated by hand-netting and electrofishing at the lowest permanent freshwater riffle on several streams in eastern Tasmania. Measurements of the forward extent of the dorsal fin distinguished the short-finned eel, A. a. australis, from the long-finned eel, A. reinhardtii; this separation was verified by vertebral counts and A. a, australis glass-eels were found to be larger than A. reinhardtii. A. a. australis glass-eels were collected at the first riffle during all seasons of the year except mid-summer. Numbers in the catch declined during mid-winter, probably as a result of an effective seaward movement of the freshwater-estuarine interface during periods of high river flow; A. a. australis glass-eels were still found to be abundant near estuary mouths at such times. A. reinhardtii glass-eels exhibited a more restricted movement into fresh water during late summer and autumn with no collections recorded after mid-winter. For both species, the stage of pigmentation was found to advance as the season progressed, and length, weight and condition factor declined with advancing pigmentation. The otoliths of invading glass-eels of both species appeared similar with a single summer ring, suggesting a larval life of 1-1½ years. The restricted invasion period of A. reinhardtii and the similar size throughout the species range suggests a short and precise larval life. The length of larval life of A. a. australis is probably quite variable, resulting in a more substantial and prolonged influx of glass-eels into Tasmanian waters.

Length of larval life in twelve families of fishes at ?One Tree Lagoon?, Great Barrier Reef, Australia

The length of larval life in some coral reef fishes was estimated from the number of growth increments in the otoliths of newly settled fishes. We examined 210 individuals comprising 38 species and 5 unidentified taxa, and belonging to 12 families. During 2 successive austral summers (1976–1977 and 1978–1979), specimens were collected from the lagoon at One Tree Reef, Great Barrier Reef, Australia. By assuming that growth increments in otoliths are laid down on a regular daily cycle commencing near the time of hatching, we calculated typical ages ranging from 3 to 6 wk with a minimum of just over 2 wk and a maximum of 12 wk. The otoliths also contain distinctive microstructural features which can serve as approximate temporal markers for the change from the postlarval to juvenile life stage.

Ecology of the European River Lamprey (Lampetra fluviatilis) in Northern Sweden

River lamprey is widely distributed along most of the Swedish coast and is the basis for a number of fisheries. The length of larval life in the river is uncertain. Metamorphosed lampreys (mean length: 105 mm) move downstream at night, mainly during March to May. The upstream spawning run of adults from the sea starts in August and continues for many months. During this migration large numbers of lampreys are caught in traps by fishermen. Spawning takes place in late May or early June when the water temperature reaches about 10°C. Adult lampreys have several predators in the river, especially certain birds (goosander, Mergus merganser) and fish (burbot, Lota lota).Key words: lamprey, Sweden, fishery, larvae, metamorphosis, age, length, spawning, predators, migration

Laboratory observations on the bionomics of Aedes fluviatilis (Lutz) (Diptera: Culicidae)

AbstractA laboratory colony of Aedes fluviatilis (Lutz) was established in ambient conditions in Brazil in which temperature varied from 22 to 31°C and relative humidity from 61 to 73%. Females laid eggs 3–13 days (mean 5·6 days) after a blood-meal and produced, on average, 64·3 eggs per batch. Eggs were usually deposited directly on the surface of water and preferentially on water that had previously contained fourth-instar larvae. The eggs proved to have little resistance to desiccation, hatching rates being reduced when eggs were kept on dry filter paper for only 1–3 days. Hatching took place following the detachment of a cap-like portion of the anterior end or through an irregular longitudinal split along the side of the egg. In different experiments, about 10–20% of eggs failed to hatch. Eggs usually hatched 2 days after oviposition. The average length of larval life was 10·2 days, and the highest proportion of larvae pupated on day 9. The duration of the successive immature stages increased geometrically with age. Mortality was 1–2% in each of the first 3 larval instars but rose to about 10% in the fourth. In the larval stage, males developed more rapidly than females. Crowding lengthened the duration of the larval stage, reduced the numbers surviving to pupate and resulted in a disparate sex ratio with emergent males much more abundant than females. Pupation occurred throughout the day and night with a slight peak at 05.00–06.00 h. The pupal stage lasted 1–3 days, usually 2 days, and was the same in both sexes.

Biological Observations on Inopus rubriceps (Diptera: Stratiomyidae)1

Inopus rubriceps (Macquart) is a potentially serious pest which is now infesting lawns in San Francisco and San Mateo Counties in California. It was first reported in this area in 1948. The species is indigenous to eastern Australia, where it infests such crops as maize, pastures, and sugarcane. Damage to turf and other members of the grass family results from withdrawal of sap from roots of host plants by the larvae and, possibly, injection of a toxin into the plant. In California adults occur in large numbers from late September to early November each year. In some years there is a small spring flight. Eggs are deposited in crevices in the soil and hatch in 12 days at 20°C. The average number of eggs laid per female is 230 at 20°C. The length of larval life is believed to be at least 2 years. Larvae can survive extraordinarily long periods of starvation but are subject to rapid death by desiccation. Larvae can move well both horizontally and vertically through loose soil. Core samples taken over a 2-year period averaged 87 larvae per 0.093 ft of infested turf. Pupation occurs near the surface of the soil; the pupal stage lasts about 26 days at 20°C. At 20° adult female flies survived 5 days; males survived 8 days.

Life‐history studies on New Zealand Brachyura

Summary Pre‐zoea and first stage zoea larvae of the New Zealand grapsid crabs Leptograpsus variegatus (Fabricius), Planes marinus Rathbun, Hemigrapsus crenulatus (H. Milne Edwards), H. edwardsi (Hilgendorf), Cyclograpsus lavauxi H. Milne Edwards, C. insularum Campbell and Griffin, Helice crassa Dana, and Plagusia chabrus (Linnaeus) are described, and a key is given for their separation. On the basis of larval characters known grapsid zoeae fall into four groups, and these support the arrangement of adult genera into subfamilies except for a division among larvae of the subfamilies Varuninae and Sesarminae. The four groups are defined in a key and this arrangement is compared with an earlier system of larval classification. Grapsid zoeae show close affinities with those of genera in the families Ocypodidae and Gecarcinidae. Except Hemigrapsus edwardsi which breeds in late autumn and winter, all New Zealand species breed during late spring or summer. Where Cyclograpsus lavauxi and the recently rediscovered C. insularum occur together, their breeding periods do not coincide. All species have five zoeal stages and a megalopa larva, and planktonic larval life may last up to seven weeks. The length of larval life is apparently unrelated to the absence of grapsid crabs (or to the presence of certain other species) in the Chatham Islands.

The effect of a eugregarine Gregarina polymorpha (Hammerschmidt) on the mealworm larva of Tenebrio molitor (L.).

SYNOPSIS. The larva of the beetle Tenebrio molitor (Coleoptera, Tenebrionidae), commonly referred to as the mealworm, harbors considerable numbers of gregarines in its midgut. These are not necessary for normal growth, nor do they prolong the life of larvae grown under optimal conditions of temperature, relative humidity and diet. When the larvae were grown under optimal conditions there was no significant difference between the length of larval life or the final pupal weight of mealworms harboring gregarines when compared with mealworms which had been reared free from gregarines. This applied both to infected and non‐infected larvae grown singly and in communal cultures.When, however, larvae were grown on a sub‐optimal diet, the gregarines had a considerable effect on the final pupal weight and the ability of the larva to complete development.

Rates of Growth in British Ammocoetes

Three species of lamprey are found in Britain: Petromyzon marinus (Linnaeus), Lampetra fluviatilis (Linnaeus), and Lampetra planeri (Bloch). Ammocoetes of all three species formed the object of these investigations. Rates of linear growth were estimated by conventional sampling procedures. In addition ammocoetes were reared under laboratory conditions in order to arrive at more accurate parameters. Rates of change in weight as the ammocoetes developed were recorded. A marked decrease in body weight and length took place during the final months of metamorphosis in ammocoetes of all three species. In the first year of larval life, body weight increased at a greater rate than at any other period in the ammocoete life. From the third year on, there occurred wide differences between the species as far as relationship between age and weight are concerned. Both L. planeri and P. marinus metamorphose after 5'/2 years of larval life, while L. fluviatilis requires 6 years. 'One of the great difficulties attendant upon studying lampreys has always been the twofold nature of their life cycle. The larval form, or ammocoete, is very much different in appearance from the adult form. Metamorphosis from ammocoete to adult is complete in a few months. The length of time any particular species of lamprey remains as an ammocoete before metamorphosing has been variously reported, but it is generally realized that the ammocoete stage lasts for several years. Of British lampreys there are three species: the Sea Lamprey (Petromyzon marinus), the River Lamprey (Lampetra fluviatilis), and the Brook Lamprey (Lampetra planeri). Several estimates have been made of the length of larval life in L. planeri, but until the present author did so in 1959, no attempts had been made to directly determine length of larval life in L. fluviatilis and P. marinus. Hardisty (1951) examined large samples of L. planeri ammocoetes in an effort to set up year classes and thereby estimate the length of larval life in the species. He came to the conclusion that larval life lasts five years. Previous to Hardisty's estimate, Ivanova-Berg (1931) estimated a larval life of five years, while Knowles (1944) reached the conclusion that, in L. planeri, the ammocoete stage lasts from two to three years. In 1959, MacDonald, using larger samples than had previous investigators, estimated that L. planeri has a larval life of nearly, but not quite, five years. The same author, again using large samples, estimated that L. fluviatilis has a larval life of nearly six years, while P. marinus has a larval life of between five and six years. Year classes for all 3 species based on total length were established (Table I). 1 From a thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Glasgow University, Scotland. 2 Aided by grants from the Government of the Province of Quebec, Canada. 3 Present address: Department of Zoology, University of Western Australia, Nedlands, Western Australia.