S. Kasahara

Calanoid copepod eggs in sea-bottom muds

We have found numerous eggs of neritic copepods in sea-bottom sediments. Eggs of 6 species of calanoid copepods: Tortanus forcipatus Giesbrecht, Calanopia thompsoni A. Scott, Acartia erythraea Giesbrecht, A. clausi Giesbrecht, Centropages abdominalis Sato and C. yamadai Mori are described and illustrated. The possibility that these eggs “rest” in the sediments, and their biological significance, are briefly discussed.

Calanoid copepod eggs in sea-bottom muds. II. Seasonal cycles of abundance in the populations of several species of copepods and their eggs in the Inland Sea of Japan

The seasonal cycles of abundance of populations of dominant calanoid copepods in the water column and of their eggs recovered from the bottom sediment in the central part of the Inland Sea of Japan are described. The numbers of both copepods and eggs fluctuated markedly with season in an essentially similar pattern among the 6 species studied (Tortanus forcipatus Giesbrecht, Calanopia thompsoni A. Scott, Acartia erythraea Giesbrecht, A. clausi Giesbrecht, Centropages abdominalis Sato, C. yamadai Mori). The density of eggs in the sea bottom was highest shortly before the population of adults and late copepodids disappeared from the plankton; the numbers of eggs then gradually decreased until the appearance of the next planktonic population.

Calanoid Copepod Eggs in Sea-Bottom Muds. IV. Effects of Some Environmental Factors on the Hatching of Resting Eggs

The hatching of resting eggs of 6 species of marine calanoid copepods,Calanopia thompsoni, Labidocera bipinnata, Acartia erythraea, A. clausi, Centropages abdominalis andC. gamadai, recovered from neritic sea-bottom muds in the central part of the Inland Sea of Japan was examined under various environmental conditions (temperature, salinity, oxygen concentration, illumination and presence of bottom mud). Temperature and oxygen concentration were found to be important factors affecting hatching. The respective ranges of temperature in which the eggs of each species hatched correspond closely to the range of temperature at which the planktonic population of that species was observed in the natural environment. Extremely low oxygen concentrations in the water completely inhibited hatching in all species. A wide range or, salinity and the presence or absence of illumination did not prevent hatching.

Growth and Production of the Inshore Marine Copepod Pseudodiaptomus marinus in the Central Part of the Inland Sea of Japan

The growth and production of the inshore marine copepod Pseudodiaptomus marinus was studied in the central part of the Inland Sea of Japan. The stage-specific growth rate was determined under controlled laboratory conditions by examining the length-weight relationship and development rates at various temperatures. The stage duration was short and constant from NII to CII, beyond which development was retarded. Males developed faster than females in CIV and CV. The specific growth rate was highest in copepodite stages followed by the nauplii and adult females (=egg production rate). The daily production of P. marinus was estimated from the stage-specific growth rate and stage-specific abundance in nature as the sum of the individual stages. The production changed seasonally with water temperature and population biomass. Daily production and biomass (P/B) ratios increased linearly with temperature. Total annual production was 20.7 mg C m-3 yr-1.

Calanoid copepod eggs in sea-bottom muds. III. Effects of temperature, salinity and other factors on the hatching of resting eggs of Tortanus forcipatus

The “resting” eggs of a marine neritic copepod, Tortanus forcipatus Giesbrecht, recovered from sea-bottom sediment were hatched in the laboratory. Hatching occurred at temperatures of 13° to 30°C, no eggs hatched at 10°C. Temperatures around 25°C were found to be optimal for hatching, although the range of optimal temperatures for hatching was approximately 5°C lower in eggs stored for 14 to 15 months than in those stored for 1 to 2 months. A wide range of salinity, from 18 to 54%S, was favourable for hachting. Eggs failed to hatch within the sediment mud, which suggests that they are in a state of dormancy in the mud. Hatching was successful under both light and dark conditions.

Calanoid copepod eggs in sea-bottom muds. V. Seasonal changes in hatching of subitaneous and diapause eggs of Tortanus forcipatus

A warm-water neritic calanoid copepod, Tortanus forcipatus, has been found to lay two physiologically different types of eggs (subitaneous and diapause eggs), which are separately spawned in response to different seasonal environmental conditions. Subitaneous eggs are produced when the planktonic populations are at their numerical maxima, and are only shed until late summer. After peak spawning, true resting eggs are spawned which undergo diapause for 1 to 3 months. The eggs recover from their diapause around mid-winter, but hatching does not take place until the water temperature at the sea bottom exceeds 15°C in early summer.