W. Huntting Howell

Structure and evolution of teleost mitochondrial control regions

We amplified and sequenced the mitochondrial control region from 23 species representing six families of teleost fish. The length of this segment is highly variable among even closely related species due to the presence of tandemly repeated sequences and large insertions. The position of the repetitive sequences suggests that they arise during replication both near the origin of replication and at the site of termination of the D-loop strand. Many of the conserved sequence blocks (CSBs) observed in mammals are also found among fish. In particular, the mammalian CSB-D is present in all of the fish species studied. Study of potential secondary structures of RNAs from the conserved regions provides little insight into the functional constraints on these regions. The variable structure of these control regions suggests that particular care should be taken to identify the most appropriate segment for studies of intraspecific variation.

Growth responses of suspension-feeding bivalve molluscs to changes in water flow: differences between siphonate and nonsiphonate taxa

Abstract Experiments were conducted in a multiple-flume apparatus to determine the effects of water flow (current speed) on individual growth of the infaunal siphonate Mercenaria mercenaria (hard clam) and the epibenthic nonsiphonate Crassostrea virginica (eastern oyster). During summer 1990 there were no significant differences in shell growth rate or final soft tissue weights of either species over a 24-day period for mean flow treatment levels of ≈2, 4 and 8 cm s −1 mid-depth (≈ffree-stream) speeds. The overall trends were a positive relation between growth and flow speed for clams, and a negative relation for oysters. A 28-day experiment in summer 1991 tested for the effects of four flow speed levels: 0, 1, 2 and 4 cm s −1 . There were substantial and marginally significant differences in shell growth rates for oysters (ANOVA, P = 0.042) and clams (ANOVA, P = 0.063). Growth response patterns were different for the two taxa, with clams showing a consistent positive relation between flow speed and growth (maximal growth at 2 to 4 cm s −1 ), and oysters showing maximal growth (average three-fold increase over other flow speeds) at 1 cm s −1 with decreased growth at 0 cm s −1 and > 1 cm s −1 . We hypothesize that the differences in growth response patterns may be explained by differences in inhalant pumping speeds, with maximal individual growth expected for suspension-feeding bivalves at ambient flow speeds that approximate the inhalant pumping speed. Because siphonate taxa typically have greater inhalant pumping speeds, we predict that ambient flow speeds optimal for individual growth will generally be several cm s −1 greater than those for nonsiphonate taxa.

Age and size at sexual maturity for the winter skate, Leucoraja ocellata, in the western Gulf of Maine based on morphological, histological and steroid hormone analyses

SynopsisWe determined age and size at sexual maturity in male and female winter skates, Leucoraja ocellata, from the western Gulf of Maine. Age estimated from vertebral band counts resulted in an Index of Average Percent Error (IAPE) of 5.6%, suggesting that this method represents an accurate approach to the age assessment of L. ocellata. Size at sexual maturity was assessed by evaluating three endpoints: steroid hormone concentrations, and morphological and histological criteria. Our results suggest that 50% maturity in males occurs at a total length of 730 mm and at 11 years of age. For females, our results suggest that 50% maturity occurs at a total length of 760 mm and between 11 and 12 years of age. Collectively, our study suggests that analyzing a combination of reproductive parameters offers an accurate estimation of sexual maturity in the winter skate. Moreover, our results indicate that L. ocellata is a late-maturing and long-lived species, characteristics which make it highly susceptible to over-exploitation by commercial fisheries.

The behavior of lobsters in response to reduced salinity

Abstract Two experiments were conducted to measure the behavioral responses of lobsters, Homarus americanus (Milne-Edwards), to reductions in salinity. In the first experiment animals were placed in a 3 ft diameter tank that was divided in half by plastic mesh. Spontaneously active lobsters were able to move between the two halves of the tank by passing through either of two conduits. The conduits were equipped with optical sensors to monitor the passage of animals, and a perfusion system to control the salinity of the area in, and around, the conduit. When the salinity in the vicinity of both conduits was the same (28–32 ppt), lobsters exhibited no preference for either conduit. However, when the salinity in one of the conduits was lowered, lobsters preferred to pass through the high salinity (20–25 ppt) conduit rather than the one with low salinity (10–15 ppt). In addition, females appeared to be more selective in their preference and exhibited higher overall activity than males when exposed to reduced salinity. In the second experiment, individual lobsters were placed in a shelter at one end of a long seawater table and exposed to seawater of gradually decreasing salinity. The salinity required to cause a movement out of a shelter, i.e. an avoidance response, was recorded. On average, lobsters first ventured small distances ( one body length) when levels approached 12.62 ppt ± 1.59. Although it was not statistically significant, females again seemed to be either more sensitive to salinity or found it more aversive, because they tended to initiate movements at salinities greater than those required to influence males. These behavioral data indicate that: 1. (1) adult lobsters are capable of detecting changes in salinity which are comparable to the levels found during natural fluctuations in coastal bays and estuaries; 2. (2) when exposed to low salinity of sufficient magnitude, they attempt to avoid it, and; 3. (3) females appear to be more sensitive to drops in salinity and/or they find it more aversive. Previous studies have demonstrated that estuarine lobster populations are dominated by males and that there are seasonal migrations of lobsters into, and out of, estuaries. We conclude that the behavioral responses of male and female lobsters to low salinity may determine, in part, the distribution and movements of lobsters in estuarine habitats.

The effects of reduced salinity on lobster (Homarus americanus Milne-Edwards) metabolism: implications for estuarine populations

Abstract During periods of substantial freshwater runoff, lobsters that inhabit estuaries, such as the Great Bay Estuary in NH, are exposed for several days to weeks to seawater that is diluted as low as 10 ppt. To assess the physiological stress imposed by these conditions, we measured the oxygen consumption, heart rate, ventilation rate and hemolymph osmolarity of lobsters while sequentially exposing them, for 24-h periods, to seawater of 20, 15, and 10 ppt. Measurements of hemolymph osmolarity confirmed previous results which demonstrated that at salinties below 20 ppt lobsters are limited osmoregulators; allowing their hemolymph osmolarity to drop as the environmental salinity is reduced, but always maintaining it higher than the ambient osmolarity. All animals exposed to 10 ppt, at 15 °C, were capable of surviving for at least 72 h. There was a nearly linear increase in oxygen consumption, heart and scaphognathite rates in animals exposed to dilute seawater, with almost a twofold increase in metabolic rate when animals were moved from 20 to 15 to 10 ppt. At the lowest salinity tested (10 ppt) the average oxygen consumption was higher for females than for males. We conclude that at low salinities the energetic demands of osmoregulation are greater for females than males, and for both sexes the physiological stress imposed may determine, in part, their distribution and/or movements in estuarine habitats.

The reproductive cycle of the smooth skate, Malacoraja senta, in the Gulf of Maine

The smooth skate (Malacoraja senta) is a small species of skate that is native to the waters of the north-western Atlantic. Recent assessments in the Gulf of Maine indicate that the biomass of smooth skates has declined below threshold levels mandated by the Sustainable Fisheries Act. This decline, coupled with the paucity of biological data, has prompted the National Marine Fisheries Service to prohibit the possession of smooth skates in this region. Consequently, crucial life history information is now being collected, which could be used in the formulation of a management plan. The present study describes and characterises the reproductive cycle of female and male smooth skates, based on monthly samples taken off the coast of New Hampshire, USA, from May 2001 to April 2002. Gonadosomatic index (GSI), hepatosomatic index (HSI), shell gland weight, follicle size and egg case formation were assessed for 79 female skates. In general, these reproductive parameters remained relatively constant throughout most of the year. Additionally, the size distribution of ovarian follicles in females captured each month did not vary significantly. For males (n = 81), histological stages of spermatogenesis III to VI (SIII-SVI), GSI and HSI were examined. No significant differences were detected in male reproductive parameters, and production and maintenance of mature spermatocysts within the testes were observed throughout the year. Collectively, these findings indicate that, like other north Atlantic skate species, the smooth skate is reproductively active year-round.

Fluid regulation and physiological adjustments in the winter skate, Leucoraja ocellata, following exposure to reduced environmental salinities

Winter skates, Leucoraja ocellata, exposed to 80% and 50% seawater (SW) exhibited rapid and significant weight gains followed by a slight recovery to new steady state levels within 8 days. Skates were acclimated at each salinity (100% SW [N = 16], 80% SW [N = 8], 50% SW [N = 8]), anesthetized (MS222) and bled from the caudal vein. In 100% SW, skate plasma (930 mOsm/kg) was slightly hyperosmotic to the external medium (922 mOsm/kg). Plasma osmolality decreased with seawater dilution, but became increasingly hyperosmotic to the bathing media. The environmental dilutions resulted in significant, but disproportionate changes in plasma Cl−, P−, Na+, Ca+, Mg+, trimethylamine oxide (TMAO) and urea concentrations. Mean corpuscular [Hb] and milliliter RBC water measurements suggest that skate red cells swelled less at each dilution than predicted for a passive erythrocyte osmometer. Concentrations of the major RBC solutes K+, urea, TMAO and Cl− decreased by 8, 25, 5 and 21%, respectively in 80% SW. In 50% SW, K+, urea, TMAO and Cl− concentrations decreased by 9, 47, 36 and 15%, respectively. Quantitatively, the other measured intracellular electrolytes (Mg+, Na+, P− and Ca+) also exhibited disproportionate changes in concentration. Our results indicate that L. ocellata is a euryhaline elasmobranch that can tolerate significant reduction in the external salinity through the release of both ions and urea from the extracellular compartments while retaining electrolytes at the expense of urea in the intracellular compartment.

Predators Are Attracted to Acclimation Cages Used for Winter Flounder Stock Enhancement

Acclimation cages are used for juvenile, cultured winter flounder Pseudopleuronectes americanus so that the fish can adjust to their new environment, hone their burial skills, begin pigment change, and recover from the stress of transport to the release site, all in the absence of predation. However, there have been indications that the cages attract the predatory green crab, Carcinus maenas. Studies conducted at the release site in the Hampton-Seabrook Estuary, New Hampshire, USA, determined that green crab abundance was significantly higher (ANOVA, p < 0.01) on cages containing fish than on empty cages, proving that acclimation cages containing flounder do attract green crabs. In addition, when empty acclimation cages were deployed, crab densities significantly increased (ANOVA, p < 0.01) in the vicinity of the acclimation cages and continued to increase each day, indicating that green crabs are also attracted to empty cages. Thus, although acclimation cages are a necessary tool that allows the stocked fish to adjust to their new environment, they also may be a detriment if they attract predators to the site. Using active adaptive management, release strategies must be reevaluated to reduce such predator-prey encounters, thereby improving fish survival.

Discarding of Commercial Groundfish Species in the Gulf of Maine Shrimp Fishery

Abstract The objective of the research was to quantify discards of American plaice Hippoglossoides platessoides, witch flounder Glyptocephalus cynoglossus, yellowtail flounder Pleuronectes ferrugineus, winter flounder Pleuronectes americanus, goosefish Lophius americanus, silver hake Merluccius bilinearis, and Atlantic cod Gadus morhua in the Gulf of Maine shrimp fishery. Data were obtained from 50 tows made by commercial trawlers fishing in the southwestern Gulf of Maine, Mean discard percentages (100 × fish discarded/fish caught) per tow, on a weight basis, were 93% for silver hake, 81% for American plaice, 76% for witch flounder, 56% for Atlantic cod, 41 % for goosefish, 20% for yellowtail flounder, and 11% for winter flounder. Mean discard percentages per tow on a numerical basis were slightly higher. Neither catch condition nor tow duration affected discard percentage, but discard percentage decreased with increasing depth for American plaice.

Commercial Trawler Discards of Four Flounder Species in the Gulf of Maine

Abstract The objectives of the research were to quantify commercial trawler discards of American plaice (Hippoglossoides platessoides), witch flounder (Glyptocephalus cynoglossus), yellowtail flounder (Limanda ferruginea), and winter flounder (Pseudopleuronectes americanus) and to examine variables that potentially influence discard rates. Data were obtained from 135 tows in six areas of the Gulf of Maine. Four different types of trawls were used. Mean discard percentages per tow, on a weight basis, were 25, 18, 13, and 5% for American plaice, witch flounder, yellowtail flounder, and winter flounder, respectively. Mean discard percentages were higher on a numerical basis, with 57, 27, 25, and 12% of the respective species discarded per tow. Both trawl type and area fished affected the discard percentages of American plaice and witch flounder. Stepwise multiple regression analyses indicated that six variables combined to explain 66.8% of the variation in discard percentage of American plaice, and that thre...