The Year in Metabolism 1975–1976

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Spawning occurs in the autumn, triggered by a decrease in water temperature, leaving scallops virtually devoid of energy reserves and possibly contributing to the increased mortality seen at this time. Although seasonal trends in the oxygen consumption rates Bayne, ; De Vooys, ; Widdows, and ammonia excretion rates Bayne, a, b; Widdows, ; Mann, a, b of marine bivalves have been correlated to changes in annual reproductive cycles, the fact that these physiological processes do not always respond similarly to seasonal environmental changes precludes their general use as indicators of the balance in catabolism between the different nutrient reserves in the tissues Bayne, ; Bayne and Scullard, ; Mann, When carbohydrate is oxidized, all of the oxygen utilized forms carbon dioxide, resulting in a respiratiory quotient RQ of 1.

When protein and lipid are catabolized, some of the oxygen forms water, resulting in respective RQ values of 0. A n RQ greater than 1.

Reproductive energy metabolism in the bay scallop, Argopecten irradians concentricus (Say)

This provided a means of identifying periods of catabolism of specific substrates, reinforcing the information obtained in the previous two chapters. Since seasonal cycles of growth and reproduction in A. Bottom water temperature was obtained at the time of collection with a hand-held thermometer. Scallops were returned to the laboratory along with extra seawater for maintenance, physiological rate measurements, and salinity and total alkalinity determinations.

Fouling organisms were removed, and animals were placed in aquaria containing water obtained at the collection site maintained at environmental temperature. Salinity was determined conductimetrically with an Autosal and total alkalinity was measured by titration with HCl, standardized with 1.

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Determination of physiological rates began the morning after collection and was completed within 48 h Six scallops were placed in separate pyrex closed respiration chambers of about 1. Environmental temperature was maintained by a recirculating water bath Forma Scientific connected to water jackets surrounding the chambers.

Water within each chamber was magnetically stirred throughout the PAGE 71 59 Bacterial respiration was negligible over this length of time. At the end of the experimental period, the exact chamber water volume was measured and scallop tissue dry weight DW was obtained by drying to a constant weight at 60 C Oxygen uptake was measured by a polarographic electrode al.

The rate of oxygen consumption was calculated in terms of ml oxygen consumed gDw-1 h Initial and final total carbon dioxide concentrations were calculated based on the change in pH, knowing total alkalinity and the apparent dissociation constants of carbonic and boric acids Riley and Chester, Carbon dioxide production rate was calculated as ml carbon dioxide produced gDw-1 h The amount of ammonia excreted was determined by PAGE 72 subtracting final from initial chamber water ammonia concentrations, as determined with an Orion ammonia electrode calibrated with NH4Cl in conjunction with an Orion Ionalyzer.

Ammonia excretion rate was calculated in terms of pg ammania-N excreted gDw-1 h Relationships between physiological rates and environmental temperature and salinity as both separate and combined independent variables were investigated using regression analysis. Seasonal trends in the indexes were fitted to polynomial regression equations. Results Mean rates of scallop oxygen consumption, carbon dioxide production, and ammonia excretion, along with the environmental temperatures and salinities occurring on the various sampling dates are given in Table 4 Mean oxygen consumption rate showed no obvious seasonal trend, with a minimum of 0.

For most of the study, mean oxygen consumption was between 0 9 and 1 0 ml gnw-1 h PAGE 74 The mean rate of carbon dioxide production did vary seasonally increasing steadily from 0 51 ml gDw-1 h-1 on 19 May to a maximum of 1 61 ml gDw1 h 1 62 on 30 July and then consistently decreasing to a mi n imum of 0.

Mean ammonia N excretion rate generally increased over the study period, from between 75 3 June and 87 19 May pg gDW1 h-1 to 1 36 11 October and 3 September pg gDW1 h 1 then falling to 92 p g gDw1 h-1 on 9 November. All three scallop physiological rates were significantly dependent up o n the combined effects of environmental temperature and salinity P PAGE 75 , Mean carbon dioxide production rate of A.

Labile protein reserve

Since the physiological rates were determined at environmental temperatures and salinities within 48 h of collection, the metabolic rates measured were probably closest to "routine" rates rather than "standard" starved or "active" swimming or feeding rates Thompson and Bayne, Bivalve physiological rates such as those investigated in this study are dependent upon a number of factors including body size, salinity, temperature, and reproductive state, as well as activity level and acclimation history Bayne and Scullard, ; Shumway, In this study, bay scallop oxygen consumption, carbon dioxide production, and ammonia excretion rates are dependent upon seasonal variations in temperature and salinity.

However, to attribute changes in these physiological rates only to environmental factors would be an over-simplification of a complex relationship. Bay scallop reproductive development as well as oxygen consumption rate and carbon dioxide production rate parallels average water temperature at Anclote Chapter 3. Reproductive development also covaries with environmental salinity as well as ammonia excretion in that the lower salinities occur later in the year when gametes are maturing and the adductor muscle is undergoing a large PAGE 81 decrease in weight and protein content Chapter 4.

Both of these factors in themselves could contribute to the observed variations in physiological rates. On the basis of this study it is impossible to separate environmental factors from reproductive events as determinants of bay scallop physiological rates. Oxygen consumption rates V02 have now been measured for three widely separated bay scallop populations.

At 20 C, V02 in a Massachusetts population is 0. According to the regression equation found in this study relating oxygen consumption to temperature, Florida bay scallops have a V02 of 0.

However, no information as to the acclimation history and reproductive state of animals in the Van Dam study is given. Also, a water temperature of 20 C occurs in Florida in March and November when only small, resting stage and large, post-spawn scallops are found Chapter 3. In Massachusetts, 20 C is close to the yearly maximum and occurs when gametes are fully developed. Thus, because of differences in acclimation and reproductive states in study animals, there is little that can be concluded from these studies regarding the ability of A.

Resting stage animals exhibit increasing indexes from May to June, indicative of a shift from lipid to carbohydrate as the primary substrate supporting metabolism. Although the two physiological indexes indicate the same general trends i.

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During gametogenesis from July-September the RQ exceeds 1. After spawning, however, both indexes indicate that protein is the primary catabolic substrate. This could be the result of a greater anaerobic capacity and need for glycolytic energy reserves in the intertidal M. The seasonal cycle of mean RQ values exhibited by A irradians also reflects changes in reproductive energy metabolism similar to those found for other species. Monthly RQ values for M edulis increase from a low of 0.

The metabolism of resting stage animals is lipid based as indicated by the indexes. Maximum digestive gland lipid level also occurs at this time, and its utilization is associated with the initiation of gametogenesis. Early in the gametogenic cycle, maximum physiological indexes are obtained, signifying that metabolism is carbohydrate based. This corresponds with maximum adductor muscle glycogen content which is utilized as gametogenesis proceeds, perhaps being incorporated directly into the lipid of developing oocytes.

As gametogenesis is completed and the gonad attains maximum weight and lipid and protein contents, physiological indexes decrease. A decrease in adductor muscle weight and protein content over this period coincides with this shift to a protein based metabolism.

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Minimum indexes occur as spawning commences. Nucleic acids are required in the male for sperm production, and lipid and protein are mobilized in the female for the synthesis of eggs Giese, The utilization of energy reserves often accompanies the gametogenic process Bayne, ; Sastry, Seasonally monitoring the distribution of a radiotracer within the body of an animal, however, can provide direct information regarding the storage and utilization of specific nutrient pools in response to gamete development. The use of radiotracers to investigate bivalve reproductive energy metabolism has been limited.

The most PAGE 87 75 comprehensive study involved the seasonal distribution of 14c and 32p in acid soluble, lipid, and protein fractions of several body components of Mytilus edulis Thompson, The translocation of radiolabel from digestive gland to gonad components in conjunction with reproductive development was demonstrated for the scallops Argopecten irradians Sastry and Blake, and Chlamys hericia Vassallo, Allen , studied the incorporation and release of 32p orthophosphate into tissues of several bivalve species.

In the present study, incorporation of 14c into lipid, carbohydrate, and protein fractions of gonad, digestive gland, and adductor muscle body components of the bay scallop, Argopecten irradians concentricus, was monitored seasonally to characterize the intra-organ biochemical transformations associated with reproduction.

Upon return to the laboratory, fouling organisms were removed and scallops were placed in an aquarium containing water obtained at the time of collection, adjusted to environmental temperature C , which ranged from Six scallops were dissected for determination of mean body component wet weight WW and dry weight DW.

PAGE 88 76 The next day 24 scallops were placed into a separate feeding tank containing 20 1 water. After feeding, the scallops were returned to the holding tank where they were maintained with non-radioactive Tetraselmis at the rate of ml animal-1 day-1 for the duration of the experiment. Six scallops were sacrificed on each of 1, 4, 7, and 10 days after ingestion of the radiolabel, and 0.

Labile protein reserve

Each tissue p iece was homogenized with a tissue grinder i n 5 ml chloroform:methanol in a test tube. After separating overnight in a refrigerator, the lower phase lipid fraction was removed with a Pasteur pipette and transferred to a tared scintillation vial. After evaporating to dryness, vials were reweighed to obtain m g DW lipid. The tubes were then heated in a PAGE 89 77 boiling water bath for 30 min, cooled, and centrifuged. The precipitate protein fraction was rinsed with distilled water into a tared scintillation vial, evaporated to dryness, and reweighed to obtain mg DW protein.

Mg DW carbohydrate was derived from the difference between calculated total dry tissue weight and the sum of lipid and protein weights. Each of the biochemical fractions was solubilized in the scintillation vial with 1 ml NCS tissue solubilizer Amersham Corp. The radioactivity in each vial was counted in an Isocap liquid scintillation counter Nuclear-Chicago Corp.

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Using appropriate corrections for quenching, results were expressed as counts per minute CPM mgDW-1 for each of the body components fractions. Total CPM for a particular body component was obtained by adding its respective lipid, carbohydrate, and protein fraction counts. Results Bay scallop carbon turnover was rapid enough that biochemical transformations conversion, transfer, utilization, etc. After Day 4 i. Therefore, results were expressed as the rate of change slope of CPM mgDw-1 between Day 1 and Day 4 on the various sampling dates.

A positive slope indicated a net gain in radiolabel relatively slow carbon turnover , and a negative slope indicated a net loss o f radiolabel relatively fast carbon turnover. For plotting purposes a log transformation was performed on the slopes. The digestive gland and gonad had considerably higher Day 1 counts than the adductor muscle in all months except November when adductor muscle CPM were almost double those of the digestive gland. In all months except September and October, gonad counts were higher than digestive gland counts on Day 1, probably accountable for by the fact that scallop intestine is intertwined throughout the female go nad and was impossible to exclude completely.

Figure 16 illustrates the dynamics of 14c incorporation by the three body components over the reproductive cycle. In June and July, l4c losses between Day l and Day 4 in the gonad and digestive gland were offset by gain s in the adductor muscle. Turnover started to increase in August as oogenesi s commenced. By November, after spawning had occurred, 14c was being lost from all components very rapidly. Cl -, This loss was relatively minor in June and July, but increased steadily from July through November.

Protein Lipid Carbo.

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Protein June DG AM 33 15 96 39 82 FG July DG 80 54 83 AM 96 12 7 63 59 32 FG Aug. DG 34 73 41 AM 44 26 11 51 16 3 35 14 FG Sept. The digestive gland protein fraction always exhibited the least 14c incorporation. The lipid fraction had the second highest CPM mgDW-1 at all times except for the Day 1 June sample and the November Day 4 sample in which protein was higher than lipid.