Morphogenesis of the digestive system during metamorphosis of the nudibranch Doridella steinbergae (Gastropoda): Conversion from phytoplanktivore to carnivore

The structural changes undergone by the digestive system of a phytoplanktotrophic nudibranch larva during metamorphosis into a benthic carnivore are described using histological and electron microscopic techniques. The relative positions of the stomach, digestive gland, and distal end of the intestine are rearranged at metamorphosis by the actions of the larval retractor muscle and the accessory pedal retractor muscle. Although the anus and distal end of the intestine are secondarily displaced to the posterior end of the gastropod, the stomach undergoes further torsional displacement at metamorphosis. The tissues of the larval stomach and distal end of the larval esophagus undergo drastic alteration at metamorphosis. The larval stomach consists of a ciliated vestibule, which receives the openings of the esophagus and left digestive gland, a gastric shield, a style sac, and an intestinal groove. All of these areas, except the vestibule, are destroyed by cell dissociation at metamorphosis. The vestibule becomes the ventral stomach of the benthic stage and the proximal end of the intestine becomes enlarged and muscularized to form the dorsal stomach of the benthic stage. The metamorphic changes involving the distal end of the esophagus include the continued development of the radula and oral lip glands, which both appear in rudimentary form during the larval stage, and differentiation of the buccal pump, salivary glands, and oral lips.     

Temperature and development in larvae of the turbot Scophthalmus maximus

Turbot (Scophthalmus maximus L.) were reared at 12 and 16°C until 26 d after hatching. At both temperatures, starting at the neural plate stage, somites were initially formed every 75 min. Expressed as a percentage of development time (DT, fertilisation to 90% larvae hatching) somite formation occurred relatively earlier during embryogenesis at 12°C (45% DT) than at 16°C (55% DT). At 12°C, after the 32-somite stage the rate of somite formation decreased to one every 300 min. The larvae hatched after 6 d at 12°C and 3 d at 16°C at a relatively primitive stage of development, prior to the opening of the mouth and anus, with unpigmented eyes, and a straight gut. Temperature altered the relative timing of organogenesis in the larval stages. At 12°C, the following characters appeared (in this order): swimbladder>loop in the gut (at the time of yolk exhaustion)>caudal fin. In contrast, at 16°C, the caudal fin appeared at the same time as the loop in the gut. At 16°C, spines formed on the head in the region of the otic capsule at the time the swimbladder formed and the yolk was exhausted, but were absent in 12°C larvae. At both temperatures, in 1 d-old larvae the myotomes just behind the yolk-sac contained 200 inner muscle fibres (presumptive white muscle). The initial growth of inner muscle was largely due to hypertrophy, but by 26 d at 12°C and 11 d at 16°C hyperplastic growth became important, as evidenced by a significant increase in the number of small fibres (<10 m²). By 26 d the average number of inner muscle fibres had increased to 341 at 12°C and 988 at 16°C. New muscle fibres were added in distinct germinal zones at the dorsal and ventral apices of the myotomes. Metamorphosis was associated with a thickening of the superficial (presumptive red) muscle layer and the appearance of tonic muscle fibres.     

Gut pigment accumulation and destruction by arctic copepods in vitro and in situ

The results presented here were obtained at six locations during three cruises in 1985 (off the coast of Labrador), 1986 (at the eastern end of Viscount Melbourne Sound) and 1988 (off the coast of Labrador). In situ chlorophyll maximum concentrations were >7 gl^-1 at depths of between 0 and 30 m in all sampling areas. In feeding experiments copepods attained higher gut pigment concentrations the longer they had been previously starved and higher concentrations when fed in the dark than when fed in the light. Community ingestion rates calculated from changes in particulate chlorophyll were higher than estimates derived from gut pigment data except when copepods had been starved for 24 h. Differences between estimates by the two methods suggested pigment destruction. In feeding experiments pigment: biogenic silica ratios in food and faecal pellets suggested that the length of starvation period affected the degree of pigment destruction differently at different stations and that feeding in the light greatly increased pigment destruction. A comparison of pigment: silica ratios in the water column, and in faecal pellets collected from copepods which had fed there, suggested that pigment destruction may occur in situ sometimes and that the degree to which it occurs may be affected by feeding history, light, diel feeding behaviour and species composition.     

Changes in the digestive gland of Euphausia superba during short-term starvation: lipid class,fatty acid and sterol content and composition

During a period of short-term (19 d) starvation, total lipid in the digestive gland of Euphausia superba Dana decreased from 21 to 9% dry weight. Total lipid per digestive gland decreased significantly during starvation compared to Day 0 individuals, falling from 1960 (±172) to 385 (±81) g. Polar lipid was the major lipid class utilised during starvation, falling from 1510 (±225) to 177 (±46) g per digestive gland (76 to 45%). Absolute levels of triacylglycerol fell from 300 (±41) to 76 (±5) g; however, relative levels remained unchanged. The relative level of free fatty acid increased significantly with starvation (4 to 39%) with absolute levels ranging from 79 (±1) to 156 (±20) g per digestive gland. Absolute levels of all fatty acids per digestive gland declined continually until the end of the starvation period. The long-chain polyunsaturated acids eicosapentaenoic (20:53) and docosahexaenoic (22:63), decreased with starvation from 37 to 26% and 15 to 10%, respectively whereas the saturated fatty acid, palmitic acid (16:0), increased from 15 to 20%. Cholesterol, the major sterol in this organ, increased from 17 (±20) to 44 (±13) g per digestive gland by Day 3, and by Day 19 had returned to levels found in the digestive gland of Day 0 individuals. Desmosterol followed a similar pattern to cholesterol, increasing from 3 (±1) g per digestive gland on Day 0 to 11 (±4) g on Day 3, and falling to 2 (±1) g on Day 19. Other sterols in the digestive gland, predominantly of algal origin, fell from the levels found in Day 0 individuals to near zero amounts by Day 6. The digestive gland of E. superba plays a dynamic role during shortterm starvation in terms of lipid content and composition. The relative levels of polar lipids, free fatty acids and cholesterol in the digestive gland may provide reliable indices of the nutritional condition of E. superba in the field. Sterols in the digestive gland are indicative of recent dietary composition of krill, and may also be used to quantify dietary input from individual phytoplanktonic species.     

RNA:DNA ratio during the critical period and early larval growth of the red drum Sciaenops ocellatus

Eggs from two separate spawning stocks of the red drum Sciaenops ocellatus (Linnaeus) were hatched, and the larvae were reared in the laboratory for 2 wk under closely controlled conditions. Total RNA, DNA, and soluble protein were measured in each population daily in triplicate pooled samples of larvae from each of three tanks. Growth rate in mm d^-1 was determined for each population at 2 d intervals. Growth rate explained 72 and 95% of the variation in the RNA:DNA ratios of the two populations individually, and 86% of the variation in the RNA:DNA ratio when data from the two populations were combined. The RNA:DNA ratio appeared to be most effective as an indicator of growth in rapidly growing larvae, and to lose some resolution when growth was intermittent. The rates of deposition of RNA, DNA, and protein into tissue were all highly correlated with growth rate and with each other. Mean population RNA:DNA ratios of red drum yolk-sac larvae decreased from Day 1 post-hatch until larvae initiated successful feeding behavior, and then increased steadily throghout the remainder of the experimental period. This pattern of change in the RNA:DNA ratios during the yolk-sac stage appears to be an intrinsic developmental pattern of red drum ontogeny. The lowest values for the RNA:DNA ratio were observed just prior to the initiation of feeding or during the critical period, indicating that red drum larvae experience a decrease in capacity for protein synthesis as they initiate feeding. Intrinsic variation in the RNA:DNA ratio during development suggests that caution be used when comparing the RNA:DNA ratios of yolk-sac larvae to a critical ratio calculated from Buckley's general model.     

Effects of feeding history on mobilisation and deposition of body constituents and on growth in juvenile Ambassis vachelli (Pisces: Chandidae)

Young Ambassis vachelli (Richardson) 40 to 50 d old, were used in a laboratory experiment to test the effects of starvation and subsequent re-feeding on body constituents and growth. Fish in three laboratory treatments (fed continuously; fed for 9 d and then starved for 15 d; starved for 9 d and fed for 15 d), were compared to fish from a local field population. Different body stores were mobilised at different times and rates during starvation. Carbohydrates were mobilised from the onset of starvation and were depleted after 3 d. Lipid and protein were mobilised at an increasing rate from the onset of starvation. The mortality in starved fish was relatively high (up to 70%) until re-feeding. Upon feeding, all body stores were restored rapidly in fish that were starved, with carbohydrate levels displaying an overshoot (carbohydrate level exceeding normal levels) in comparison to the levels in continuously fed fish. After 15 d of feeding, the starved fish had levels of constituents similar to those in continuously fed fish. Fish that were fed and subsequently starved were able to maintain themselves for at least 15 d prior to death, indicating a better degree of starvation resistance than fish without a history of feeding. This implies that feeding history in the early life of a fish is important in growth and survival but that young fish may have growth regimes flexible enough to survive relatively long periods of starvation.     

Feeding of laboratory-reared larvae of the sea bream Archosargus rhomboidalis (Sparidae)

Feeding by larvae of the sea bream Archosargus rhomboidalis (Linnaeus) was investigated from late September, 1972 to early May, 1973 using laboratory-reared larvae. Fertilized eggs were collected from plankton tows in Biscayne Bay, and the larvae were reared on zooplankton also collected in plankton nets. Techniques were developed to estimate feeding rate, food selection, gross growth efficiency, and daily ration. Daily estimates of these were obtained through 16 days after hatching at rearing temperatures of 23°, 26°, and 29°C. Feeding rate increased exponentially as the larvae grew, and increased as temperature was raised. At 23°C larvae began feeding on Day 3, at 26° and 29°C larvae began feeding on Day 2. Feeding rates at initiation of feeding and on Day 16 were, respectively: 23°C, 7.16 food organisms per larva per hour (flh) and 53.78 flh; 26°C, 7.90 flh and 168.80 flh; 29°C, 17.62 flh and 142.07 flh. Sea bream larvae selected food organisms by size. At initiation of feeding they selected organisms less than 100 m in width. As larvae grew they selected larger organisms and rejected smaller ones. The major food (more than85% of the organisms ingested) was copepod nauplii, copepodites, and copepod adults. Minor food items were barnacle nauplii, tintinnids, invertebrate eggs, and polychaete larvae. Mean values for gross growth efficiency of sea bream larvae ranged from 30.6% at 23°C to 23.9% at 29°C. Mean values for daily ration, expressed as a percentage of larval weight, ranged from 84% at 23°C to 151% at 29°C and tended to decline as the larvae grew.This paper is a contribution from the Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA     

Food and daily ration of the rock soleLepidopsetta bilineata (Pleuronectidae) in the eastern Bering Sea

A total of 129 specimens ofLepidopsetta bilineata (Ayres) were collected in the eastern Bering Sea (57°02–58°11N; 161°58–164°38W) between 28 August and 7 September 1984. Examination of its stomach and intestine contents revealed that polychaetes, gammarid amphipods and echiuroids constitute the bulk of its diet. Prey type is related to fish size. Juveniles consume more mobile prey, such as small crustaceans (gammarid amphipods, cumaceans, carideans), while adults, although still continuing to feed on gammarid amphipods, preferentially feed on infaunal organisms such as polychaetes, echiuroids and mollusks. Some prey, such as echinoderms, fishes and urochordates, were encountered only in stomachs, which resulted in a higher index of feeding diversity for this organ. Rock sole feeds primarily during daylight hours, peaking at dusk. Its daily ration was estimated as 0.49, 1.12, and 1.61% of fish body weight from contents of stomach, intestine, and stomach plus intestine, respectively.     

Seasonal variation in the balance between physiological mechanisms of feeding and digestion in Mytilus edulis (Bivalvia: Mollusca)

Seasonal variations in the balance between physiological mechanisms of feeding and digestion are considered among 45 to 57 mm shell length Mytilus edulis L. from southwest England. Mussels had been acclimated to standardised conditions of food availability in March, June and October, 1981. Results indicate that despite showing a negative relationship (P<0.001) with the efficiency of ¹⁵N absorption, coincident alterations of both ingestion rate (0.17 to 0.54 mg total Phaeodactylum tricornutum h^-1) and organic gut content (1.00 to 1.80 mg) represent significant mechanisms effecting the seasonal regulation of nutrient acquisition. Concurrent changes in absorption efficiency alone clearly exerted a relatively minor influence upon seasonal rates of absorption. Associated gut residence times of ¹⁵N-labelled material varied between 4 and 10 h. Furthermore, despite the ensuing deleterious effects, together with increased gut contents, absorption efficiencies among mussels that had been starved throughout acclimation were consistently maintained equivalent to those among individuals feeding normally. Such constancy was at least partially achieved by an increased residence time of material within diverticulae, relative to the digestive system as a whole. Finally, comparison of isotopic absorption efficiencies with those recorded for total organics and organic nitrogen has demonstrated differences of as much as 30% between gross and net uptake due, we suggest, to excretion of metabolically derived material within the alimentary canal.     

Effect of feeding regime and irradiance on the photophysiology of the symbiotic sea anemone Aiptasia pulchella

To determine how the animal and algal components of the symbiotic sea anemone Aiptasia pulchella respond to changes in food availability and culture irradiance, sea anemones from a single clone were maintained at four irradiance levels (320, 185, 115, and 45 E m^-2 s^-1) and either starved or fed for 5 wk. Changes in protein biomass of sea anemones maintained under these conditions were not related to the productivity of zooxanthellae, since the protein biomass of fed A. pulchella decreased with increase in irradiance and there was no difference in protein biomass among starved sea anemones at the four irradiance levels. Except for the starved high-light sea anemones, the density of symbiotic zooxanthellae was independent of culture irradiance within both starved and fed. A. pulchella. Starved sea anemones contained over twice the density of zooxanthellae as fed sea anemones. Within both starved and fed individuals, chlorophyll per zooxanthella increased with decreasing culture irradiance while algal size remained constant (in fed sea anemones) at about 8.80 m diameter. Chlorophyll a: c ₂ ratios of zooxanthellae increased with decreasing culture irradiance in zooxanthellae from starved sea anemones but remained constant in zooxanthellae from fed sea anemones. As estimated from mitotic index data, the in situ growth rates of zooxanthellae averaged 0.007 d^-1 and did not vary with irradiance or feeding regime. Photosynthesis-irradiance (P-I) responses of fed A. pulchella indicated an increase in photosynthetic efficiency with decreasing culture irradiance. But there was no consistent pattern in photosynthetic capacity with culture irradiance. Respiration rates of fed sea anemones also did not vary in relation to culture irradiance. The parameter I _k , defined as the irradiance at which light-saturated rates of photosynthesis are first attained, was the only parameter from the P-I curves which increased linearly with increasing culture irradiance. The daily ratio of net photosynthesis to respiration for A. pulchella ranged from 1.6 to 2.8 for sea anemones maintained at the three higher irradiances, but was negative for those maintained at 45 E m^-2 s^-1. Since the final protein biomass was greatest for sea anemones maintained at the lowest irradiance, these results indicate that sea anemone growth cannot be directly related to productivity of zooxanthellae in this symbiotic association.     

Development of the pH in the intestinal tract of larval turbot

The pH in the gut of turbot larvae and juveniles of turbot was studied from day 11 until the completion of metamorphosis. Dietary effects on the gut pH were estimated when larvae were offered live feed, a microdiet, only microalgae or no feed. The pH in the gut was weakly alkaline until day 24 after hatching with no differences between the foregut, midgut and hindgut. The foregut contents started to turn acidic from day 28 after hatching when the larvae were already weaned successfully, which indicates that an acidic pH is not necessary for the digestion and utilisation of formulated feed. During the following 20 days the pH in the foregut/stomach decreased further to a minimum of pH 3.5, while the pH in the midgut and the hindgut increased slightly to a maximum of pH 9.0. Larvae receiving live feed, microdiet or microalgae had a similar pH in the midgut on day 11, while starved larvae exhibited a lower gut pH. This suggests bicarbonate secretion from the larval pancreas stimulated by ingested microalgae or feed particles.     

A microinjection technique using a pH-sensitive dye to determine the gut pH of Calanus helgolandicus

By applying a microinjection technique previously used in cell biology, a pH-sensitive dye, 2,7-bis(2-carboxyethyl)-5,6-carboxyfluorescein) (BCECF), was injected into the gut of the copepod Calanus helgolandicus to determine pH under a range of feeding conditions. The median gut pH of the fore- and hindguts of starved individuals was 6.86 and 7.19, respectively. This was a consistently lower pH than that observed in copepods feeding on either the diatom Thalassiosira weissflogii, the dinoflagellate Prorocentrum micans, or the coccolithophorid Emiliania huxleyi (Strain 92D), all of which had a median gut pH>7.0. In all treatments, the median pH of the foregut was more acidic than the median of the hindgut, implying that the foregut is the site of acid secretion. The results, which demonstrate pH values down to 6.11, are discussed in terms of coccolith dissolution, pigment degradation and digestive enzyme activity.     

Physiology of the digestive tract of the sedentary polychaete Terebellides stroemi

The digestive tract of the sedentary polychaete Terebellides stroemi, collected from 88 m depth on the French Catalan Coast, was studied using histological, biochemical, cytological and spectrographic techniques. The ciliated oesophagus possesses a ventral pharyngeal pouch with a muscle bulb. The stomach is divided into two parts, the fore stomach is ciliated, but the hind stomach takes the place of a gizzard with a thick peritrophic membrane and a thick muscle mantle. The digestive gland enters the digestive tract at the boundary of the two stomachal areas; it is comprised of epithelial lamellae with a glandular epithelium. The gland cells contain a spherocrystal secretion with a high calcium and phosphorus content. The cells are also absorbant due to their microvillar brush border and to the storage of glycogen and lipids in the basal epithelium. A biochemical study using the Apizym System confirmed the secretion of enzymes such as phosphatases, esterases and various carbohydrases. Tryptic and chymotryptic enzymes were visualized by biochemical hydrolysis of synthetic substrates. The epithelium of the fore intestine is similar in appearance to the epithelium of the digestive gland, but consists of a single epithelial layer. Hind intestine and rectum are only excretory systems. The presence of this digestive gland does not seem to have been described elsewhere in polychaetes but is similar to the Morren's gland of some oligochaetes. The study was carried out during 1982 and 1983.     

Effects of feeding frequency and symbiosis with zooxanthellae on nitrogen metabolism and respiration of the coral Astrangia danae

Colonies of the temperate coral Astrangia danae occur naturally with and without zooxanthellae. Basal nitrogen excretion rates of nonsymbiotic colonies increased with increasing feeding frequency [average excretion rate was 635 ng-at N (mg-at tissue-N)^-1 h^-1]. Reduced excretion rates of symbiotic colonies were attributed to N uptake by the zooxanthellae. Nitrogen uptake rates of the zooxanthellae averaged 8 ng-at N (10⁶ cells)^-1 h^-1 in the dark and 21 ng-at N (10⁶ cells)^-1 h^-1 at 200 Ein m^-2 s^-1. At these rates the zooxanthellae could provide 54% of the daily basal N requirement of the coral if all of the recycled N was translocated. Basal respiration rates were 172 nmol O₂ cm^-2 h^-1 for starved colonies and 447 nmol O₂ cm^-2 h^-1 for colonies fed three times per week. There were no significant differences between respiration rates of symbiotic and nonsymbiotic colonies. N excretion and respiration rates of fed (symbiotic and nonsymbiotic) colonies increased greatly soon after feeding. N absorption efficiencies decreased with increasing feeding frequency. A N mass balance, constructed for hypothetical situations of nonsymbiotic and symbiotic (3×10⁶ zooxanthellae cm^-2) colonies, starved and fed 15 g-at N cm^-2wk^-1, showed that the presence of symbionts could double the N growth rate of feeding colonies, and reduce the turnover-time of starved ones, but could not provide all of the N requirements of starved colonies. Rates of secondary production, estimated from rates of photosynthesis and respiration were similar to those estimated for reef corals.     

Feeding,starvation and metabolic rate in the shore crab Carcinus maenas

The relationship between food intake and metabolic rate (as measured by oxygen consumption) of Carcinus maenas (L.) was studied. The metabolic rate of feeding crabs which had been starved for a short period increased, and several days were required for it to return to its original level. During prolonged starvation, the metabolic rate of C. maenas fell in two stages. The first reduction was to about 60% of the feeding level; this occurred during the first week of starvation. There was then a further reduction to about 40% of the feeding level and, at this level of metabolism, the crabs were able to survive for 3 months, with only 50% mortality; most of this occurred in the last 2 weeks. Metabolic rate was found to affect food intake; crabs acclimated to 24°C took 2.4 times as much food as crabs acclimated to 10°C, although the metabolic levels of the two groups differed by a factor of only 1.4. From the results obtained when the crabs were starved and when starved crabs were fed, it is suggested that, during starvation, the metabolic rate of C. maenas first drops from the elevated feeding level to a level at which carbohydrate reserves are utilised, and subsequently to a minimum level at which lipid reserves and proteins are used.     

Digestive mechanics and gluttonous feeding in the feather starOligometra serripinna (Echinodermata: Crinoidea)

The movement and digestion of food in the gut ofOligometra serripinna (Carpenter) were studied at Lizard Island (14°3842S; 145°2710E) in the austral winter of 1986. Feather stars in the laboratory were fed a brief, small meal of brine shrimp nauplii and killed at increasing time intervals thereafter. Histological reconstructions showed that the ingested nauplii progressed along the digestive tract surprisingly quickly. Some nauplii were found in the mid and hind intestine in only 30 min, and all of the nauplii had reached the hind intestine and rectum in 1 h. Digestion of the nauplii had started at 1 h, and only a few fragments of naupliar exoskeleton remained in the hind intestine and rectum 5 h after the start of feeding. Videotape analysis showed that no fecal pellets were released during this experiment. In the natural environment ofO. serripinna, ingested particles may similarly be transported quickly to the hind part of the gut and digested there — when feather stars were fixed in the field, most of the gut contents were found in the hind intestine and rectum.O. serripinna, which efficiently rejects inert particles before they are ingested, usually defecates infrequently (probably not more than once over a span of many hours) and differs from some other feather stars that ingest numerous inert particles and defecate much more frequently. When specimens ofO. serripinna were fed continuously on brine shrimp nauplii,Artemia sp. (San Francisco strain), in the laboratory, the feather stars fed gluttonously, packing their guts with several hundred nauplii in 1 to 2 h. Thereafter, superfluous feeding began (i.e., further ingestions appeared to force undigested nauplii, some of them still living, out of the anus). These observations suggest thatO. serripinna usually feeds at relatively modest rates in its natural habitat, but can feed gluttonously to take advantage of infrequent patches of highly concentrated, nutritious particles (e.g. copepod swarms, migrating demersal zooplankton, and invertebrate gametes from mass spawnings). It is likely that such patches of nutritious particles are usually small enough to drift out of reach of the feather stars before gluttonous feeding proceeds to superfluous feeding. Opportunities for superfluous feeding in nature are probably very infrequent (e.g. ingestion of coral gametes and embryos after a mass spawning), and the feather stars evidently have no behavior that stops further ingestions after the gut becomes filled to capacity.     

Allocation of45calcium to body components of starved and fed purple sea urchins (Strongylocentrotus purpuratus)

Several lines of evidence in the literature indicate that environmental stress such as starvation may initiate reallocation of sea urchin endoskeletal tissue. For example, Aristotle's lantern enlarges under conditions of starvation, and sea urchins tagged with tetracycline and then fed develop a distinct growth line, while starved individuals develop a diffuse pattern. We designed anin vivo system to examine stress-related changes in calcification in the purple sea urchinStrongylocentrotus purpuratus. SmallS. purpuratus (ca. 2 cm test diam) were collected from the Mission Bay jetty or Imperial Beach (San Diego, California, USA) in 1987.⁴⁵Ca was incorporated from seawater into all body fractions including the organic tissue/coelomic fluid. In an initial experiment, sea urchins were fed or starved for 4 wk and then post-incubated in isotope. Overall, starved individuals deposited new calcite more slowly than did fed individuals; however, allocation was very different and calcification of teeth of starved sea urchins was nearly as great as in fed individuals. In a second experiment,S. purpuratus were first pre-labeled with isotope and then treated by feeding or starving. More of the labeled calcium was mobilized from the soft tissues and coelomic fluid into calcite in fed than in starved individuals. Growth of the teeth in starved sea urchins was significantly greater than in those fed. We conclude that starvation changes the metabolism of calcium in order to preferentially build teeth. However, we also found no evidence that calcium was resorbed from old skeletal calcite in order to build new skeleton.     

Effects of temperature and delayed feeding on growth and survival of larvae of three species of subtropical marine fishes

In larvae of the bay anchovy Anchoa mitchilli (Valenciennes), the sea bream Archosargus rhomboidalis (Linnaeus), and the lined sole Achirus lineatus (Linnaeus), growth, survival, and starvation times were investigated at temperatures of 22° to 32°C. The rate at which hours after hatching until starvation decreased in relation to temperature for unfed larvae did not differ significantly among the 3 species, ranging from-5.4 to-6.3 h per degree increase in temperature. The total number of hours until starvation did differ for all 3 species: lined soles survived longest, bay anchovies were intermediate, and sea bream survived the least time. At 28°C, unfed sea bream could survive 90.1 h, bay anchovy 102.3 h, and lined sole 119.8 h. The eyes pigmented at nearly the same time after hatching for sea bream and bay anchovy, but took about 20 h longer at all temperatures for lined sole. Quadratic equations best described the relationship between hours after hatching when the eyes became pigmented and temperature. Eye-pigmentation times became nearly constant for all 3 species at temperatures above 28°C. At 28°C, eyes pigmented about 27 h after hatching for bay anchovy and sea bream but not until 47 h for lined sole. Hours after eye pigmentation when unfed larvae starved was a measure of the effective time that larvae had to commence feeding. Bay anchovies and lined soles were nearly alike in this respect, but sea bream starved at tewer hours after eye pigmentation. Slopes of regressions representing decrease in times to staration for increasing temperatures ranged from-3.7 to-4.4 h per degree increase in temperature, and were not significantly different among the 3 species. At 28°C, unfed lined soles starved at 70 a after eye pigmentation, bay anchovies starved at 72.5 h, and sea bream at only 62 h. Yolk absorption was most rapid for all species during the first 20 h after hatching, and was faster at higher temperatures. Amounts of yolk remaining at the time eyes became pigmented were less at higher temperatures for bay anchovy and lined sole, but were greater for sea bream, suggesting that sea bream used yolk more efficiently at higher temperatures. Either no yolk or small traces (>0.20%) remained at 24 h after eye pigmentation in all 3 species. Feeding was delayed for periods of 8, 16, 24, 32, 40 and 48 h after eye pigmentation for all species at a series of experimental temperatures from 24° to 32°C. Growth and survival were affected when food was withheld for more than 24 h at 28°C, but survival did not decrease markedly until food was withheld at least 8 h longer. At lower temperatures food could be withheld longer and at higher temperatures for less time. Feeding can be initiated by most larvae for several hours after all visible yolk reserves have been exhausted. All species tested can survive for 24 to 40 h after eye pigmentation at 24° to 28°C without food and still have relatively good growth and survival when food is offered. If the “critical period” is considered relative to time of hatching, lined soles need not find food for 3 to 3.5 days after hatching, but bay anchovy and sea bream must feed within 2.5 days of hatching.     

The highly specialised gut of Fritillariidae (Appendicularia: Tunicata)

The morphology and some functions of the gut of Fritillaria pellucida and Fritillaria formica (Fritillaridae) were investigated by light and electron microscopy, and also by means of histochemical and immunohistochemical techniques. Fritillarids, very important for their abundance and ecological impact in marine ecosystems, have a very simplified gut: a straight oesophagus connects the pharynx to the digestive nucleus, composed of globular stomach and rectum, connected dorsally through a very short proximal intestine. The latter is characterised by a few (two to four) extremely specialised cells, completely filled with mitochondria associated tightly with membrane infoldings showing strong ATP-ase activity, and probably involved in the osmoregulation of internal body fluids. The gut is formed of an extremely low number of cells, which, although poorly diversified, are very large in the stomach and rectum. Food transfer along the gut depends on and is regulated by well-developed cardiac and pyloric valves, and signs of general digestive and absorptive activity are recognisable all along the brush border of the main tracts. The macroscopic organisation and cytological characters of the gut in fritillarids are completely different from those of the oikopleurids. In particular, fritillarids lack specialised cells for endocytosis and intracellular digestion, like those described in the genus Oikopleura. The general simplification and specialisation observed in Fritillaria gut may account for their elevated growth rate and abundant diffusion in all oceans.     

Energetics,moult cycle and ecdysteroid titers in spider crab (Hyas araneus) larvae starved after the D0 threshold

Zoea I larvae of Hyas araneus L. (Decapoda: Brachyura: Majidae) were dredged in January 1986 from the German Bight and reared in the laboratory at constant 12°C, until they reached the transition of stages C/D₀ of the moult cycle (4 d after hatching). This developmental stage had previously been found to correspond with the point of reserve saturation (PRS) which allows autonomous (food-independent) development through the rest of the moult cycle and hence, was termed the D₀ threshold. One part of the larvae was continually fed (control), another group was starved from the D₀ threshold until moulting to the zoea II instar. In these two experimental groups, as well as in the two groups of zoea II larvae obtained from the different feeding conditions, the course of the moult cycle, biomass (dry weight, W; carbon, C; nitrogen, N; hydrogen, H; energy, E; the latter estimated from C), and ecdysteroid titers (measured with a radio-immuno-assay as ecdysone equivalents) were investigated. When the larvae reached the PRS, they had gained 90% in W, 72% in C, 32% in N, 53% in H, and 65% in E, since hatching, corresponding to an accumulation of 87% of final W and 62 to 69% of C, N, and H reached later, at the end of the mould cycle in the control. The period of starvation caused a 2.5-d delay of the moult cycle, mainly in late premoult, and significant losses of biomass and energy. Starved and fed larvae secreted similar amounts of moulting hormone per individual, but with a reduced rate in the starved group, thus causing developmental delay. Zoea II larvae moulting after starvation contained less than half of the control biomass and energy, and even less than a freshly hatched zoea I. Growth rate was only slightly enhanced in these zoea II larvae as compared to the fed control, but losses of biomass, mainly of lipids, were partly compensated by a 4-d prolongation of their moult cycle, chiefly (3 d) in stage C. Biomass curves were almost parallel in the two experimental groups of zoea II larvae, with significantly higher values in the control during all stages of the moult cycle. However, similar relative proportions (74 to 89%) of late premoult biomass and energy were reached at the D₀ threshold, regardless of different feeding history and initial or final values in a given group. The ecdysteroid titer curve of the zoea II which had moulted from starved zoea I was very similar to that in control larvae, but with a 3-d delay in the occurrence of premoult peak concentration (in both groups in stage D₁). Regulation and coordination of moult cycle, ecdysteroid titers, and growth in the larval development of decapod crustaceans are discussed, with special reference to the D₀ threshold.