Ontogeny of feeding in two native and one alien fish species from the Murray-Darling Basin, Australia

Investigations into the feeding of the early stages of fishes can provide insights into processes influencing recruitment. In this study, we examined ontogenetic changes in morphology and feeding behaviour of two native Australian freshwater species, Murray cod, Maccullochella peelii peelii, and golden perch, Macquaria ambigua, and the alien species, common carp, Cyprinus carpio. Murray cod free embryos are large and well developed at the onset of feeding, whereas the other two species begin exogenous feeding much younger and are smaller and less-developed. Carp commence exogenous feeding 3 days earlier than golden perch, and show more advanced development of the eyes and ingestive apparatus. We conducted feeding experiments, presenting larvae of the three species with a standardised prey mix (comprising equal numbers of small calanoid copepods, large calanoid copepods, small Daphnia, and large Daphnia). Larvae of most tested ages and species showed a preference for mid-sized prey (300–500 μm wide). This was true even when their gapes substantially exceeded the largest prey offered. Daphnia were consumed more than similar-sized copepods. The results of this study suggest that survival through their larval period will be threatened in all three species if catchable prey <500 μm in width are not available throughout such time. They also suggest that interspecific competition for prey may occur, especially when larvae are very young. The precocious development of structures involved in feeding and the extended transition from endogenous to exogenous feeding of early carp larvae are likely to have contributed to the success of this species since its introduction to Australia.     

Relating the ontogeny of functional morphology and prey selection with larval mortality in Amphiprion frenatus

Survival during the pelagic larval phase of marine fish is highly variable and is subject to numerous factors. A sharp decline in the number of surviving larvae usually occurs during the transition from endogenous to exogenous feeding known as the first feeding stage in fish larvae. The present study was designed to evaluate the link between functional morphology and prey selection in an attempt to understand how the relationship influences mortality of a marine fish larva, Amphiprion frenatus, through ontogeny. Larvae were reared from hatch to 14 days post hatch (DPH) with one of four diets [rotifers and newly hatched Artemia sp. nauplii (RA); rotifers and wild plankton (RP); rotifers, wild plankton, and newly hatched Artemia nauplii (RPA); wild plankton and newly hatched Artemia nauplii (PA)]. Survival did not differ among diets. Larvae from all diets experienced mass mortality from 1 to 5 DPH followed by decreased mortality from 6 to 14 DPH; individuals fed RA were the exception, exhibiting continuous mortality from 6 to 14 DPH. Larvae consumed progressively larger prey with growth and age, likely due to age related increase in gape. During the mass mortality event, larvae selected small prey items and exhibited few ossified elements. Cessation of mass mortality coincided with consumption of large prey and ossification of key elements of the feeding apparatus. Mass mortality did not appear to be solely influenced by inability to establish first feeding. We hypothesize the interaction of reduced feeding capacities (i.e., complexity of the feeding apparatus) and larval physiology such as digestion or absorption efficiency contributed to the mortality event during the first feeding period. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Muscle development in the Japanese flounder,Paralichthys olivaceus,with special reference to some of the larval‐specific muscles

We investigated muscle development in the Japanese flounder Paralichthys olivaceus, focusing primarily on the cranial muscles, using a whole mount immunohistochemical staining method. It is well established that during the very early stages of morphogenesis, until 4 days post hatching (dph), muscles required for feeding develop. Later, between 8 and 16 dph, the muscle composition in the dorsal branchial arches changes to the adult form. We discovered the presence of larval‐specific muscles in this ontogenetic period, termed the larval branchial levators 2 and 3, located in the dorsal branchial arches. The larval branchial levators 2 and 3 disappear during the course of development, whereas the others remain as levator internus 1 and levator posterior, which have also been described in adult fish. In place of these regressed muscles, the levatores externi and levator internus 2 develop and regulate the branchial arches. In addition, we found that the levator posterior, which is thought to represent the fifth levator externus, and the levatores externi exhibit different origins. We also found that at least a part of the caudal fin musculature develops from the trunk myotome. J. Morphol. 2010. © 2010 Wiley‐Liss, Inc.     

Expression of the oligopeptide transporter, PepT1, in larval Atlantic cod (Gadus morhua)

The intestinal absorption of di- and tri-peptides generally occurs via the oligopeptide transporter, PepT1. This study evaluates the expression of PepT1 in larval Atlantic cod (Gadus morhua) during the three weeks following the onset of exogenous feeding. Larval Atlantic cod were fed either wild captured zooplankton or enriched rotifers. cDNA was prepared from whole cod larvae preceding first feeding and at 1000 each Tuesday and Thursday for the following three weeks. Spatial and temporal expression patterns of PepT1 mRNA were compared between fish consuming the two prey types using in situ hybridization and quantitative real-time PCR. Results indicated that PepT1 mRNA was expressed prior to the onset of exogenous feeding. In addition, PepT1 was expressed throughout the digestive system except the esophagus and sphincter regions. Expression slightly increased following first-feeding and continued to increase throughout the study for larvae feeding on both prey types. When comparing PepT1 expression in larvae larger than 0.15-mg dry mass with expression levels in larvae prior to feeding, no differences were detected for larvae fed rotifers, but the larvae fed zooplankton had significantly greater PepT1 expression at the larger size. In addition, PepT1 expression in the zooplankton fed larvae larger than 0.15-mg dry mass had significantly greater expression than rotifer fed larvae of a similar weight. Switching prey types did not affect PepT1 expression. These results indicate that Atlantic cod PepT1 expression was slightly different relative to dietary treatment during the three weeks following first-feeding. In addition, PepT1 may play an important role in the larval nutrition since it is widely expressed in the digestive tract.     

Oxygen uptake in developing eggs and larvae of the cod, Gadus morhua L.

Unfertilised cod eggs showed a mean oxygen uptake rate at 5°C of 0.089 μl O₂, dry wt.⁻¹ h⁻¹; this gradually rose to 0.768 μl O₂ mg dry wt.⁻¹ h⁻¹ in eggs about to hatch. From hatching to complete yolk absorption larvae respired at 1.6 μl O₂, mg dry wt.⁻¹ h⁻¹. During starvation following yolk absorption, uptake fell significantly to 1.1 μl O₂, mg dry ⁻¹ h⁻¹. Much of this decrease in oxygen consumption was shown to be caused by reduction in activity. Loss of weight during the embryo and larval phases could not easily be reconciled with total oxygen consumption; it is suggested that cod embryos and larvae may not rely solely upon endogenous energy reserves during development.     

Feeding habits and diet overlap of marine fish larvae from the peri-Antarctic Magellan region

The Magellan region is a unique peri-Antarctic ecosystem due to its geographical position. However, the knowledge about the distribution and feeding ecology of fish larvae is scarce. Since this area is characterized by low phytoplankton biomass, we hypothesize that marine fish larvae display different foraging tactics in order to reduce diet overlap. During austral spring 2009–2010, two oceanographic cruises were carried out along southern Patagonia (50–56°S). Larval fish distribution and feeding of the two most widely distributed species were studied, the smelt Bathylagichthys parini (Bathylagidae) and black southern cod Patagonotothen tessellata (Nototheniidae). Larvae of B. parini showed a lower increase in the mouth gape at size, primarily feeding during daytime (higher feeding incidence during the day) mostly on nonmotile prey (invertebrate and copepod eggs, appendicularian fecal pellets, diatoms). They showed no increase in feeding success (number, total volume of prey per gut and prey width) with increasing larval size, and the niche breadth was independent of larval size. Larvae of P. tessellata showed a large mouth gape at size, which may partially explain the predation on motile prey like large calanoid copepods (C. simillimus) and copepodites. They are nocturnal feeders (higher feeding incidence during night) and are exclusively carnivorous, feeding on larger prey as the larvae grow. Nonetheless, niche breadth was independent of larval size. Diet overlap was important only in individuals with smaller mouth gape (<890 μm) and diminished as larvae (and correspondingly their jaw) grow. In conclusion, in the peri-Antarctic Magellan region, fish larvae of two species display different foraging tactics, reducing their trophic overlap throughout their development.     

Behavioral responses of Atlantic cod to sea temperature changes

Understanding responses of marine species to temperature variability is essential to predict impacts of future climate change in the oceans. Most ectotherms are expected to adjust their behavior to avoid extreme temperatures and minimize acute changes in body temperature. However, measuring such behavioral plasticity in the wild is challenging. Combining 4 years of telemetry‐derived behavioral data on juvenile and adult (30–80 cm) Atlantic cod (Gadus morhua), and in situ ocean temperature measurements, we found a significant effect of sea temperature on cod depth use and activity level in coastal Skagerrak. During summer, cod were found in deeper waters when sea surface temperature increased. Further, this effect of temperature was stronger on larger cod. Diel vertical migration, which consists in a nighttime rise to shallow feeding habitats, was stronger among smaller cod. As surface temperature increased beyond ~15°C, their vertical migration was limited to deeper waters. In addition to larger diel vertical migrations, smaller cod were more active and travelled larger distances compared to larger specimens. Cold temperatures during winter tended, however, to reduce the magnitude of diel vertical migrations, as well as the activity level and distance moved by those smaller individuals. Our findings suggest that future and ongoing rises in sea surface temperature may increasingly deprive cod in this region from shallow feeding areas during summer, which may be detrimental for local populations of the species.     

Algae as first food for cod larvae,Gadus morhuaL.: filter feeding or ingestion by accident?

Cod larvae. Gadus morhua L., 24 days posthatch, were fed different species of algae in order to evaluate both rates and mechanisms of ingestion. The results were compared with cod larvae feeding on a natural assemblage of algae in a lagoon. Small algae (Nannochloris atomus Butcher, 14 pm) were found to enter the larval gut in accordance with drinking rate. In contrast, the cod larvae concentrated larger algae [Dunaliella salina (Dunal) Teodor. 6 10 pm] in the gut at rates from 492 to 7251 times the drinking rate. D. salina was captured in the slits between the visceral arches, and increased activity when the algae was added indicate that the cod larvae are active filter feeders during early larval stages. In the gut of cod larvae from the lagoon the fraction of algal material with diameter larger than 8 pm was 39.2% in 7-day-old larvae while, in 12-day-old larvae it had decreased to 12.6%. The most conspicuous characteristics of gut content in the youngest larvae were green spheres (10 μn) and a naked dinoflagellate (20 μn). Short chains of the diatom Skeletonemu cosrarum (Greville) Cleve were also frequently found.     

The Lympho-Hemopoietic Organs of the Anadromous Sea Lamprey,Petromyzon marinus. A Comparative Study throughout its Life Span

We have analyzed morphological changes affecting the lympho-hemopoietic organs of the anadromous sea lamprey, Petromyzon marinus throughout its life span. For this analysis, ammocoetes (2–4 years), premetamorphosing lampreys (nearly 5 years), metamorphosing lampreys, macrophtalmia stages (young adults) and parasitic adults (nearly 7 years) were used. The principal lympho-hemopoietic organs in the ammocoete are typhlosole, larval opisthonephros and nephros-associated adipose tissue. After metamorphosis, these organs degenerate, and their lympho-hemopoietic tissue is replaced by dense connective tissue. The supraneural body and to a lesser degree, the definitive opisthonephros, are the main blood-forming organs in adult lampreys. During larval life, lympho-hemopoietic cells appear in the branchial area, associated with pharyngeal epithelium. These loci are not morphologically homologous to the thymus gland of jawed vertebrates. These results are discussed, with special emphasis on the importance of cell microenvironments in eluciding changes in different blood-forming loci throughout the life cycle and their significance for the lamprey's immune capacity.     

Embryogenesis and early larval development in wild-caught Levantine scraper,Capoeta damascina (Valenciennes, 1842)

Levantine scraper, Capoeta damascina is a candidate species for future stock assessments, conservation studies, and hatchery efforts. Herein, we documented embryonic and early larval development, from egg activation to the exogenous feeding period, using morphological and histological landmarks. Embryos were obtained by in vitro fertilization from hormonally induced wild-caught broodstock, and subsequent development was monitored at temperatures coinciding with native conditions. Embryonic development from fertilization to hatch lasted ~105–110 hr. Larvae emerged with unpigmented eyes and body morphology, as well as an undifferentiated digestive tract. The mouth was closed at hatch by the oropharyngeal membrane and opened by the early endogenous feeding period. Trabeculae cartilage, quadrate bone, and Meckel's cartilage of the endoskeleton were present during the endogenous feeding period. During this period, the larvae underwent considerable changes in craniofacial morphology, locomotion, and organogenesis of the digestive tract. The cartilaginous floor of the neurocranium developed and the first four ceratobranchials appeared simultaneously at the end of endogenous feeding period. The digestive tract was differentiated into buccopharynx, esophagus, and small intestine during the endogenous feeding period. The intestinal valve and numerous longitudinal folds at the posterior region of the intestine formed together by the endo–exogenous feeding period. Major developmental events in retinogenesis occurred during the endogenous feeding period. When larvae entered exogenous feeding the mouth was fully-functional. Additionally, liver size and eye diameter increased. Our analysis of embryonic and early larval development in Levantine scraper aligned with other freshwater fishes.     

Diet composition of larval and young-of-year shovelnose sturgeon in the Upper Missouri River

Obtaining food following the transition from endogenous to exogenous feeding and during the first year of life is a critical event that strongly influences growth and survival of young‐of‐year fishes. For shovelnose sturgeon Scaphirhynchus platorynchus, limited information is available on food habits during the first year of life. The objective of this study was to quantify diet components of shovelnose sturgeon during the transition from endogenous to exogenous feeding and during the young‐of‐year life stage in the North Dakota and Montana portions of the Missouri River. Young‐of‐year shovelnose sturgeon were sampled between early August and early September 2003. Shovelnose sturgeon initiated exogenous feeding by 16 mm, and individuals 16–140 mm fed exclusively on two macroinvertebrate orders (Diptera and Ephemeroptera). Young‐of‐year shovelnose sturgeon exhibited an apparently high feeding success as 99 of 100 individuals contained food in the gut. The number of organisms in the gut increased exponentially with fish length for larval Diptera (r² = 0.73, P < 0.0001) and linearly (r² = 0.12, P = 0.0006) for larval Ephemeroptera, but the number of Diptera pupae in the gut was not significantly related (P = 0.55) to length of young‐of‐year shovelnose sturgeon. The length of ingested prey was linearly related to fish length for Diptera larvae (r² = 0.20, P = 0.002), whereas the relationship between lengths of ingested Ephemeroptera larvae and lengths of young‐of‐year shovelnose sturgeon was best described by a power function (r² = 0.50, P < 0.0001). These results provide the first quantification of feeding dynamics for young‐of‐year shovelnose sturgeon in a natural river environment.     

The effects of starvation and 20-hydroxy-ecdysone on feeding and pupariation of early 3rd-instar Calliphora vicina larvae

At a larval body weight of 15–16 mg, young 3rd-instar Calliphora vicina larvae pass from the obligatory feeding phase, when starvation results in cessation of growth or development, to the facultative feeding phase when starvation leads to subsequent pupariation. During normal larval development the transition from obligatory to facultative feeding phase appears to be independent of 20-hydroxyecdysone but absence (or insufficiency) of the hormone is the likely reason why starved larvae in the obligatory feeding phase fail to pupariate. 20-Hydroxy-ecdysone induces termination of feeding by normal, facultative phase larvae and is the probable signal for larval commitment to pupariation.     

Development of feeding structures in larval fish with different life histories: winter flounder and Atlantic cod

The size at which feeding structures developed and shifts in head proportions occurred, differed between Atlantic cod Gadus morhua and winter flounder Pseudopleuronectes americanus. The sequence and timing of the development of feeding structures may not be dependent on size, but may occur because they are necessary to meet specific requirements offish larvae feeding in the plankton. In early larval stages development of feeding structures was similar in number and type and was necessary for first-feeding in both species. In later stages, significant differences between species occurred in the timing of the development of feeding structures. In cod differentiation of new structures and changes in head proportions occurred at about two-thirds of the way through larval life, which coincided with an increase in growth. In flounder changes in feeding morphology did not occur during the symmetrical larval stage, but occurred only after metamorphosis to the asymmetrical demersal juvenile stage. Differences between cod and flounder in the size at which feeding structures develop may reflect life history adaptations expressed in the duration of the pelagic larval stage, as well as differences in juvenile habitat and feeding ecology.     

Predation by a carnivorous marine copepod,Euchaeta norvegica Boeck,on eggs and larvae of the North Atlantic cod Gadus morhua L.

The predatory behavior of a carnivorous marine copepod, Euchaeta norvegica Boeck, feeding on eggs and larvae of the North Atlantic cod Gadus morhua L. was examined. In the laboratory, adult females of Euchaeta norvegica did not feed on eggs. Predation rates on yolk-sac larvae and starved post-yolk-sac larvae did not vary significantly with age up to 14 days old because of little change in size or activity of the larvae. This differs from E. elongata Esterly, a temperate congener, which selectively feeds on middle yolk-sac-stage larvae of the Pacific hake Merluccius productus Ayres. The subarctic congener Euchaeta norvegica appeared to detect tailbeats of the cod larvae. The functional response was measured for E. norvegica feeding on 2–4-day-old yolk-sac larvae. Maximum ingestion was achieved at 5 larvae · 1⁻¹ with a rate of 6.3 ± 1.2 larvae·copepod⁻¹·day⁻¹ or 10.5% of its body weight. Estimates of short-term feeding rates, determined from gut-evacuation curves, indicate that E. norvegica, when preying on cod larvae only, must feed for at least 4 h to achieve this maximum ingestion rate. Presence of copepods as alternative prey for E. norvegica depresses its predation rate on cod, although the ingestion of cod greatly supplements the ration consumed. Copepods fed cod larvae form black melanin-pigmented fecal pellets in which larval cod otoliths have been found. Approximately 0.5 larva was required to form one fecal pellet. The last three developmental stages of the predatory copepod were able to ingest larvae and form dark-pigmented fecal pellets. The feeding of this carnivorous marine copepod may contribute to the mortality noted in the larval stages of cod because E. norvegica is numerous in the center of the cod-spawning area of Skrova in the Lofoten Islands, northern Norway.     

Effects of bacterial treatment at early stages of Atlantic cod (Gadus morhua L.) on larval survival and development

Aims: To assess the effects of bacterial treatment at the earliest stages of cod rearing on the microbial load, larval development and performance, testing three bacterial strains (Carnobacterium divergens V41, Arthrobacter sp. and Enterococcus sp.) in vivo that were previously shown to have inhibitory potential towards fish pathogens in vitro. Methods and Results: A bacterial mixture was added eight times to the rearing water from the prehatch to the mid‐larval stage (a 38‐day period). Microbiological analysis of ova, larvae and rearing water was performed regularly. Larval performance and development were evaluated by survival rate, hypersalinity tolerance and physiological measurements. Different larval survival rates were observed within and between treatments, and possibly explained by variations in larval microflora and established probionts. Larvae from one silo, which had been bathed in the bacterial suspension, showed the highest survival rate (42·1%), lowest Vibrio levels, and were significantly heavier (19·3%) and more stress tolerant than control larvae (P < 0·01). This coincided with the intestinal establishment of two of the tested bacteria. Conclusions: Arthrobacter and Enterococcus strains added regularly to the rearing water from the postfertilized egg stage can become established in larval gastrointestinal tract. The Enterococcus strain was associated with increased larval growth, performance and microflora control, indicating its probiotic nature. Significance and Impact of the Study: Regular application of autochthonous probionts may promote larval welfare, development and stress tolerance at early stages, hence increasing production yield in intensive cod larviculture.     

Growth and fatty acid composition of freshwater prawn, Macrobrachium rosenbergii, larvae fed diets containing various ratios of cod liver oil–corn oil mixture

A feeding trial was conducted to determine the effect of replacing costly cod liver oil with corn oil as a source of dietary lipid on the growth and fatty acid composition of the larval freshwater prawn, Macrobrachium rosenbergii de Man. Prawn larvae were weaned to artificial diets containing cod liver oil and corn oil either singly or in various combinations (2 : 1, 1 : 1, 1 : 2, w/w). Weaning to artificial diets from Artemia nauplii commenced at larval stage III with complete substitution by stage X. The reference group was reared solely on Artemia nauplii during the entire experiment. Incorporation of corn oil at 33–67% of dietary supplemental oil did not have significant effects on the post‐larval production. However, larvae fed with corn oil alone revealed a significantly lower post‐larval production compared to other experimental diets as well as to the reference group. No significant differences (P > 0.05) were observed in dry weight, protein and lipid concentration among larvae fed on various dietary treatments. Palmitic (16 : 0) and oleic/vaccenic (18 : 1) acids were the dominant saturated and monounsaturated fatty acids in larval tissues, respectively, whereas the polyunsaturated fraction was dominated by eicosapentaenoic (20 : 5n‐3) acid. The polyunsaturated fatty acid composition was dominated by n‐3 acids rather than n‐6 fatty acids. The fatty acid composition of the prawn in general reflected that of the diet. Larvae on diets containing higher concentrations of corn oil rich in linoleic (18 : 2n‐6) acid showed a higher concentration of this acid in their tissues. No evidence of de novo synthesis of linoleic (18 : 2n‐6) acid was found. Higher levels of stearic (18 : 0), arachidonic (20 : 4n‐6) and eicosapentaenoic (20 : 5n‐3) acids found in larvae as compared with those fed Artemia and artificial diets strongly indicated the larval ability in chain elongation and desaturation of palmitic (16 : 0), linoleic (18 : 2n‐6) or linolenic (18 : 3n‐3) acids, respectively. Despite a large variation of n‐3 to n‐6 ratios of the live and artificial diets, larval n‐3 to n‐6 ratios were relatively stable among different dietary treatments, possibly indicative of the importance of such a ratio in the larval fatty acid metabolism.     

Early development and allometric growth patterns in Siberian sturgeon and their ecological significance

Early development of Siberian sturgeon Acipenser baeri was divided into two different phases, the prelarval stage between hatching (10·4–11·1 mm L_T) and first feeding (19·6–21·0 mm L_T), and the larval stage between the initiation of external feeding and metamorphosis (28·6–32·4 mm L_T). Morphogenesis and differentiation were more intense during the prelarval than larval and early juvenile stages; the prelarval period was characterized by the replacement of embryonic adaptations and functions by definitive ones, such as branchial respiration, exogenous feeding, and active swimming. The positive allometry of the head for feeding, sensorial and respiratory functions (inflexion point at 20·0 mm L_T), and the tail for reducing costs of transport, routine swimming and escape reactions from predators (inflexion point at 20·2 mm L_T) confirmed the hypothesis that growth patterns of early life stages closely match specific needs.     

The fate of the larval epidermis in the Desor-larva of <Emphasis Type="Italic">Lineus viridis</Emphasis> (Pilidiophora,Nemertea) displays a historically constrained functional shift from planktotrophy to lecithotrophy

Pilidiophora constitutes a clade of nemerteans characterized by a peculiar larval type, the pilidium. A characteristic of this larva is the transitory epidermis in which the juvenile develops from imaginal discs. The primary function of this larval envelope is assumed to be feeding and dispersal. When juvenile development is complete, the larval epidermis is ruptured and swallowed by the juvenile. According to recent cladistic and molecular analyses of the Nemertea, the intracapsular Desor-larva of the sibling species Lineus viridis and L. ruber is thought to have evolved from a pelagic pilidium. The general course of development has been demonstrated to be similar to that of the pilidium, in which the juvenile forms from imaginal discs under the larval epidermis. The two Lineus species, however, differ in their mode of larval feeding: L. ruber being ootrophic and L. viridis being lecithotrophic. In order to elucidate the transition from the planktotrophic pilidum to lecithotrophic development, I studied the early cleavage and metamorphosis from intracapsular Desor-larva to juvenile stages in L. viridis from the island of Sylt, using light microscopical, electron microscopical, and fluorescent staining methods. Due to the specific cleavage pattern with equally sized 1st quartet animal blastomeres and vegetal blastomeres in L. viridis, the larval epidermis later contains a considerable amount of the yolk reserve. During metamorphosis, the larval epidermis is ingested by the juvenile thus displaying behavior similar to that of the pilidium larva. In contrast to the pilidium, the function of the larval epidermis of the Desor-larva has shifted from feeding and dispersal to direct food supply. Thus, the development of L. viridis is a perfect example for strong historical constraints that prevent ancestral larval structures from being lost.     

From polar night to midnight sun: photoperiod, seal predation, and the diel vertical migrations of polar cod (Boreogadus saida) under landfast ice in the Arctic Ocean

The winter/spring vertical distributions of polar cod, copepods, and ringed seal were monitored at a 230-m station in ice-covered Franklin Bay. In daytime, polar cod of all sizes (7–95 g) formed a dense aggregation in the deep inverse thermocline (160–230 m, −1.0 to 0°C). From December (polar night) to April (18-h daylight), small polar cod <25 g migrated into the isothermal cold intermediate layer (90–150 m, −1.4°C) at night to avoid visual predation by shallow-diving immature seals. By contrast, large polar cod (25–95 g), with large livers, remained below 180 m at all times, presumably to minimize predation by deep-diving mature seals. The diel vertical migration (DVM) of small polar cod was precisely synchronized with the light/dark cycle and its duration tracked the seasonal lengthening of the photoperiod. The DVM stopped in May coincident with the midnight sun and increased schooling and feeding. We propose that foraging interference and a limited prey supply in the deep aggregation drove the upward re-distribution of small polar cod at night. The bioluminescent copepod Metridia longa could have provided the light needed by polar cod to feed on copepods in the deep aphotic layers.