Bertil Åkesson

Morphology and life cycle of Ophryotrocha diadema, a new polychaete species from California

Abstract A new Ophryotrocha species is described from the harbour of Los Angeles. It has a maximum size of 4.6 mmand a maximum of 25 segments. The maxillary plates are of the Pvtype. The new species is a simultaneous hermaphrodite with segments 1 and 2 sterile, 3 and 4 male, and the remaining ones female. The chromosome number is 2n = 8. The larvae develop inside the transparent membrane of a fusiform egg case until the 4-setiger stage. They have no pygidial stylus. The reproductive rate is age dependent reaching a maximum during the fourth week of reproduction. There are two homozygotes and one heterozygote for a colour gene, all with different reproductive patterns.

Reproduction and Larval Morphology of Five Ophryotrocha Species (Poly cha eta, Dorvilleidae)

Reproduction and larval morphology of Ophryotrocha puerilis, O. labronica, O. nologlandulata, O. hartmanni, and O. bacci are described. All species have been cultivated in the laboratory. The morphology of adults and larvae is uniform, but there is a considerable variation in the shape of the egg masses. The interrelations between members of the genus Ophryotrocha are discussed: Gonochoric species have the same type of egg mass and the same chromosome number. O. puerilis is unique as a protandrous hermaphrodite. Among contemporary hermaphrodites with male and female segments, fusiform egg masses with a rigid surface are correlated with one larval type, gelatinous egg masses with another larval type.

Morphology and Life History of Ophryotrocha maculata sp. n. (Polychaeta, Dorvilleidae)

The type locality of the new species is a muddy bottom at a depth of 25 meters in the Gullmarsfjord on the Swedish west coast. On that locality the fauna is recovering from previous sulphite pulp mill pollution. The shape and morphology of O. maculata conform with most other members of the species. It is distinguished by segmental transverse rows of dark spots. It is a contemporary hermaphrodite with anterior male and posterior female segments. Different from other known species the germ cells are released directly into the water. As a consequence the brood is not protected by the parents.

Two new DorvNea species (Polychaeta, Dorvilleidae) with obligate asexuaI reproduction

Two new species, Dorvillea bermudensis sp.n. and D. albomaculata sp.n., with obligate asexual reproduction are described. Both are closely related to sexually reproducing members of the same genus. The new species undergo repeated fission of the architomy type, with the rate of regeneration and subsequent production of segments being positively correlated with the energy content of the fragments. Regeneration and reproductive potentials vary between head and tail fragments with tail lineages producing higher rates than head lineages. Both species are very tolerant of starvation which may cause considerable degrowth and initiation of dispersal behavior.

Temperature and life cycle in Ophryotrocha labronica (Polychaeta, Dorvilleidae)

Abstract Under experimental conditions the life cycle of Ophryotrocha labronica La Greca & Bacci can be completed at temperatures from 12to 28°C. The cycle is 83 days at 14.5 °C and 16.5 days at 28°C. Maximum growth rate occurs at 28 °C. Maximum fecundity and reproductive rate are obtained at 20°C and 23 DC, respectively. The temperature preference for reproduction is 21°C. At all temperatures the mortality rate of males is higher than that of females. In adults the best survival occurs in the lower part of the temperature range. For larvae and juveniles the best survival is obtained at 25°C. In spite of the high mortality and a low reproductive rate, 28°C appears to be close to the optimum temperature for a population of O. labronica. The negative factors are more than compensated for by the advantages of a short generation time.

Ophryotrocha socialis n. sp., a link between two groups of simultaneous hermaphrodites within the genus (Polychaeta, dorvilleidae)

Abstract Ophryotrocha socialis n. sp. is described from laboratory cultures. Its requirements are those of a boreal and opportunistic species which does not survive at salinities below 20-25‰. It is a simultaneous hermaphrodite combining traits which otherwise are restricted to two different species groups of Ophryotrocha with the same type of sexuality. The new species lives gregariously in tube systems and is capable of homing. Its eggs are not shed in a matrix. Connected with this are specialized brood care, facultative external self-fertilization and a high level of inbreeding. Feeding and spawning in the species are inhibited by light.

Stem Cells in Asexual Reproduction of Marine Invertebrates

While sexual reproduction is conserved and almost ubiquitous, asexual reproduction in forms of parthenogenesis or agametic cloning from somatic tissue is less conserved. The phylogeny shows that agametic cloning is widespread but scattered with many different modes for asexual formation of a new animal. This suggests that independent forms of cloning have evolved later from sexual ancestors between and within different phyla. Here, we present an overview of agametic cloning in the marine animal kingdom and discuss molecular and evolutionary aspects of somatic stem cell usage for asexual cloning. The molecular tissue characterizations and the relative role of different stem cells involved in agametic cloning are only at its beginning with whole phyla largely uncovered. An emerging hypothesis is that the first somatic stem cells used in cloning were also able to form a germ-line and that the more limited lineage specific stem cells are derived. We discuss advantages and problems with agametic cloning from somatic tissue and propose that the levels of stem cell potential held in the tissue can have large consequences for the reproductive life cycle strategies and long-term fitness in clonal animals and strains. We finally describe suitable molecular experimental approaches for future research on this topic.

Incipient Reproductive Isolation between Geographic Populations of Ophryotrocha labronica (Polychaeta, Dorvilleidae)

Åkesson, B. (Department of Zoology, University of Gothenburg, Göteborg, Sweden.) Incipient reproductive isolation between geographic populations of Ophryotrocha labronica (Polychaeta, Dorvilleidae). Zocl. Scripta 1 (5): 207–210, 1972.–The mechanism of sex determination and the incipient reproductive isolation between geographic populations of Ophryotrocha labronica are studied in intra‐ and interpopulation crosses. Two populations from the Naples area and one from Leghorn are employed in the crosses. The considerable genetic difference between the Leghorn population and any of the Naples populations is reflected in some crossing combinations by changes in sex ratios, by occurrence of individuals with a reduced viability in the progeny, by a decreased mating propensity, and by discrimination of mates from alien populations. It is stated that the changes in sex ratios, the decreased mating propensity, the dwarfed individuals in the progeny, and the preferential mating all reflect the incompatibility of gene complexes. The differences between reciprocal crosses are not fully explained. The occurrence of extranuclear DNA, as well as the existence of a maternal heredity in intrapopulation crosses, indicate that cytoplasmic inheritance may be one of the factors behind these reciprocal differences.

Effects of temperature and salinity on the life cycle of Ophryotrocha diadema (polychaeta, dorvilleidae)

Abstract Ophryotrocha diadema is stenohaline. The complete life cycle has only been observed at salinities of 30, 35 and 40 ‰. At an optimum salinity, 35 ‰, reproduction occurs between 11 and 27 °c. Experiments were performed in 5 ‰ steps of salinity from 15 to 45 ‰ combined with temperatures of 15, 18,21 and 25 0c. At salinities of 15 and 20 ‰ all animals, both adults and larvae, died within two days. In the remaining 20 salinity/temperature combinations adults were kept for 3 weeks and larvae for a minimum of one month or until they were sexually mature. For adults, mortality, number and size of egg masses, and reproductive success were recorded. Corresponding parameters for larvae were mortality, growth rate and time to beginning of reproduction. Optimum conditions for survival were obtained at 35 ‰ salinity and 18 ac. The optimum for egg production (reproductive rate) was 35 ‰ and 25 °c. The maximum growth rate was observed at 30 and 35 ‰ salinity and 25°C. These two combinations also represented the ...

Studies of the budding process in Convolutriloba retrogemma (Acoela, Platyhelminthes)

Convolutriloba retrogemma Hendelberg & Åkesson is an acoelous turbellarian with symbiotic algae in its parenchyma. It can reproduce asexually by budding at the posterior edge of the body. The number and position of the buds as well as their final size was found to be correlated with the size of the mother individual. Symbiotic algae aggregate rapidly near the bud as it begins to form probably as a result of a transport from other parts of the parent animal. The buds in late stages were observed to digest some of their symbiotic algae. The still-attached propagules are oriented with their anterior end facing posteriorly from the end of the mother individual. Ultrastructural studies showed that the reversed polarity of the bud is expressed in the orientation of the ciliary rootlets of the epidermal cells even before the head of the new specimen could be identified. Thus, the ciliary rootlets can be used as indicators of the polarity.