Electron microscopy of the skin of the teleost,Hippoglossoides elassodon

Summary The normal skin of the pleuronectid fish, Hippoglossoides elassodon, is described by light and electron microscopy. The epidermis consists of 5 to 9 layers of cells, the majority of which are squamous cells and the minority mucous cells. The squamous cells are characterized by numerous desmosomes and associated cytoplasmic filaments. The mucous cells accumulate mucous droplets in vacuoles of Golgi origin and are observed apparently in the process of releasing their content at the free surface. The dermis consists of alternating lamellae composed of typical collagen fibers. Pigment cells are of three types: melanophores, iridophores (guanophores), and lipophores.This work was supported by Public Health Service Research Grant CA-08158 from the National Cancer Institute.     

American plaice, Hippoglossoides platessoides (Fabricius), predation on green sea urchins, Strongylocentrotus droebachiensis (O. F. Muller), in eastern Newfoundland

Green sea urchins were the most abundant food species (62% by weight) in American plaice (all female) collected from an area in eastern Newfoundland where sandy bottom abuts urchin dominated bedrock. Of secondary importance were capelin (12.9% by weight), which were spawning in the area when the samples were obtained. The results suggest that American plaice may be important predators of urchins in such habitats.     

Diets of sympatric flatfishes, Hippoglossoides platessoides, Pleuronectes ferrugineus, Pleuronectes americanus, from Sable Island Bank, Canada

Prey exploitation was documented for Hippoglossoides platessoides, Pleuronectes ferrugineus , and P. americanus collected from southeast Sable Island Bank in February and June 1989. Diets varied significantly by sample and with fish species and length. Of 239 species consumed by flatfishes, 66 were determined to be principal prey. Crustaceans, particularly amphipods, were the most frequently exploited prey of all three flatfish species. Small H. platessoides fed on suprabenthic fauna, while larger fish exploited epifauna and hyperiid suprafauna. P. ferrugineus exploited epibenthic fauna, consuming crustaceans and tunicates when small and polychaetes when larger. P. americanus preyed on epifauna; smaller fish exploited polychaetes while larger fish usually consumed crustaceans associated with ectoproct colonies. Feeding intensity was dramatically lower in February, food being found in stomachs of only 36% of H. platessoides and 49% of P. ferrugineus , and lacking in the stomachs of P. americanus . In June, stomachs of 86% of H. platessoides , 94% of P. ferrugineus and 96% of P. americanus contained food. Results indicate that prey resources for the three flatfish species are partitioned by specific habitat and prey type preferences.     

A simulation of food-web interactions leading to rainbow smelt Osmerus mordax dominance in Sparkling Lake, Wisconsin

A process-based simulation model was used to examine the nature and intensity of food-web interactions that allow Osmerus mordax to dominate invaded lakes. The model simulates food-web interactions among linked populations of O. mordax, Coregonus artedi and Sander vitreus. Simulations indicated that O. mordax dominate where: (1) adult O. mordax prey on young-of-the-year (YOY) C. artedi, (2) YOY O. mordax negatively affect YOY S. vitreus through competition and (3) adult S. vitreus experience moderate fishing mortality. Osmerus mordax dominated simulations across a broad range of variable values that regulated competition and predation, and displayed threshold responses to increasing angler harvest. Consequently, angler harvest should be carefully managed in lakes susceptible to O. mordax invasions because the alternative could lead to fishery collapse.     

Effects of age, sex and abundance on the bathymetric pattern of American plaice in the southern Gulf of St Lawrence

American plaice Hippoglossoides platessoides density in September in the southern Gulf of St Lawrence was strongly related to depth, with peak densities at intermediate depths (70–90 m). Bathymetric pattern was similar for all ages of plaice (3–12 + year) and varied little among years. Densities were highest at similar depths for males and females, but depth distribution tended to be more spread out for females. Depth distribution was not significantly density dependent except for 10-and 11-year-old males and for females aged 12 years and older. In the three significant cases, depth distribution tended to spread out as abundance increased. Water temperature and depth were closely linked in the southern Gulf, but plaice depth preferences could not be attributed solely to temperature selection.     

Major changes in fecundity and the effect on population egg production for three species of north-west Atlantic flatfishes

Temporal and spatial variability in fecundity was investigated for three species of north-west Atlantic flatfishes: American plaice Hippoglossoides platessoides , yellowtail flounder Limanda ferruginea and witch flounder Glyptocephalus cynoglossus . Significant log₁₀ linear relationships existed between fecundity and total length ( L _T) for all three species and for all combinations of area (NAFO regulatory areas) and year. Fecundity differed between areas for yellowtail flounder and witch flounder and changed significantly for all three species since the last reported fecundity studies in the region 30–40 years ago. Three populations experienced decreases in fecundity (3LNO American plaice and yellowtail flounder, 3Ps witch flounder), one experienced an increase (3NO witch flounder) and one did not change (3L witch flounder). Significant yearly variability in fecundity at length and relative fecundity existed for all three species within the 6 year sampling period (1993–1998). Estimates of population egg production based on new and historic fecundity data revealed that using the old, invariant fecundity and L _T relationship resulted in overestimation of total annual egg production by up to 24% for 3LNO American plaice and as high as 41% for 3LNO yellowtail flounder. These results clearly demonstrate the variable nature of fecundity for flatfishes in Newfoundland waters and suggest that the use of an invariant fecundity and L _T relationship may result in inaccurate estimates of stock reproductive potential.     

Variation with age in the timing and duration of spawning in American plaice

Spawning time of female American plaice Hippoglossoides platessoides varied significantly with age. This effect, however, was not consistent across years. Generally, younger fish spawned earlier than older fish, but in the 1990s they spawned later than older fish. Spawning duration also varied with age, with younger fish generally having a longer spawning period than older fish. Changes in depth distribution were related to changes in both spawning time and duration. As depth increased, spawning time became later and duration became shorter. Fish <11 years old showed the largest change in both spawning time and depth distribution.     

Parasites of flatfish in relation to sewage sludge dumping

Variations in the occurrence of gastrointestinal metazoan parasites in flatfish were examined in relation to sewage sludge dumping at an 'accumulating' and two 'dispersing' dump sites in Scottish coastal waters. The gross disturbance of the benthic communities at the Garroch Head disposal site in the Firth of Clyde and absence of the intermediate hosts of particular parasites was not reflected in the parasite fauna of the long rough dab, Hippoglossoides platessoides . Results indicate that this flatfish host is highly mobile and intermixing within the population masked any localized effects of sludge dumping on parasite transmission at the dump site. The common dab, Limanda limanda , is considered to be relatively static at the St Abb's Head and Bell Rock disposal sites off the Firth of Forth where no evidence of effects of sewage sludge dumping on its metazoan parasite fauna was detected. This is consistent with the results of chemical and biological monitoring of the sites. The value of a parasite-based index of the biological effects of sewage sludge dumping is discussed.     

Interactions between substrate and temperature preference in adult American plaice (Hippoglossoides platessoides)

Laboratory experiments were conducted to determine the substrate preference of American plaice and the interaction between substrate and temperature preference. When given a choice of several substrate types, American plaice clearly preferred substrates that had a finer particle size than gravel, but did not totally avoid any substrate type. When given a choice between sand and gravel, they clearly preferred sand. Preference for substrate took precedence over temperature preference down to a temperature of‐1.0°C. An examination of field data showed that American plaice were widely distributed across both gravel and sand substrates at temperatures above and below‐1.0°C. Broad tolerances of substrate and temperature are probably a factor in this species’ wide distribution throughout the northwest Atlantic.     

Immunocytochemical localization of hypocalcin in the endocrine cells of the corpuscles of Stannius in three teleost species (trout,flounder and goldfish)

Summary In order to identify the cell-type responsible for the production of hypocalcin (the recently isolated hypocalcemic hormone of teleost fish), the corpuscles of Stannius (CS) of trout, flounder and goldfish, were immunocytochemically stained with antisera raised against trout hypocalcin. The secretory granules of the type-1 cells of the CS, considered to be the hypocalcin-producing cells, showed intense immunoreactivity in all species examined. However, in trout and flounder, the secretory granules produced by the type-2 cells, which have been suggested to represent a functionally different cell-type, also showed an intense immunoreactivity. In goldfish, no type-2 cells were observed. We tentatively conclude that type-1 and type-2 cells represent structurally different forms of the same functional cell-type.