Masakazu Aoki

Morphological characteristics of young, maternal care behaviour and microhabitat use by caprellid amphipods

Microhabitat use by nine species of caprellid amphipods in the genus Caprella inhabiting a Japanese Sargassum bed could be divided into three major categories. Differences in the patterns of use were closely related to the nature of the mother–young association and morphological characteristics of young. Pattern A was shown by three species Caprella danilevskii , C. okadai and C. subinermis that produced large first instar young (>1.6 mm in body length, BL) with no maternal care, and directly use Sargassum thalli as habitat substrata throughout their lives. Two different patterns of microhabitat use occur in the six other species, which produce small first instar young ( Sargassum thalli as their primary habitat. Pattern B comprised of C. decipiens , C. monoceros and C. scaura in which females care for their young until they become large enough to disperse onto the seaweed thalli. The first instar young of these three maternal care species have specialized pereopods. In Caprella monoceros and C. scaura , which show a ‘cling-to-mother’ type of maternal care, each pereopod has a concaved margin on the propodus and a long dactylus, well adapted for clinging to the mother, and C. decipiens , which show a ‘stay-around-mother’ type of maternal care, have comparatively slender and longer pereopods to hold onto seaweed branches near their mother. Pattern C included C. arimotoi , C. glabra and C. penantis which showed no maternal care and use thin branches of epizoites such as epiphytic hydroids as their primary substrata. It is concluded that caprellids that live on seaweed with thick thalli and branches such as Sargassum patens have different solutions to the problem of where the small first instar young can live: species with larger young emerge directly onto seaweed thalli, but species with smaller young use an epiphytic secondary habitat or depend on maternal care.

Comparative Study of Mother-Young Association in Caprellid Amphipods: Is Maternal Care Effective?

ABSTRACT Mother-young association was examined in 9 species of caprellid amphipods in the genus Caprella. Five species: Caprella arimotoi, C. danilevskii, C. glabra, C. okadai, and C. penantis showed no interaction between mother and young after emergence from her brood pouch, but various degrees of prolonged cohabitation of young with their mother were observed in the other four species. In Caprella subinermis, though no maternal care was observed, the first instars remained on the body of the mother for up to 4 days, and then dispersed before the first molt. Aggregations of young protected by the mother were observed in the other 3 species. Aggregations of a mother with multiple cohorts of young were found in Caprella decipiens, while C. monoceros and C. scaura had single cohorts of young on or around the body of the mother. Based on field data and results of field-rearing experiments, the cohabiting period is up to 30 days in C. decipiens and up to 20 days in C. monoceros. For 5 species having different degrees of mother-young association, one-month field-rearing experiments under predator-free conditions were conducted. The results showed that the presence of the mother increased both the survival and growth rate of the young in C. monoceros,but the presence of the mother increased only the growth rate in C. scaura. In the other 3 species, C. danilevskii, C. decipiens, and C. subinermis, the presence of the mother did not positively affect either survival or growth of the young.

Ecological studies on the community of drifting seaweeds in the south‐eastern coastal waters of Izu Peninsula, central Japan. I: Seasonal changes of plants in species composition, appearance, number of species and size

Seasonal patterns of drifting seaweeds in the southeastern coastal waters of Izu Peninsula of central Japan were examined by sampling 966 patches from spring to autumn 1991–1993. In total, 57 plant species appeared, including 10 epiphytic algal species. Monthly totals of the number of species, excluding epiphytic aigae, were highest in May (33) and August (27), though 19–21 species of sargassaceous algae were found from May to August, The number of species, excluding epiphytic algae, in one patch of drifting seaweeds was 1 to 11 (x̄= 2.93 ± 2.06) with high richness in May a result of almost entirely sargassaceous species. The wet weight of each patch and maximum stipe length of plants varied from 5 to 6970 g and from 20 to 840 cm (x̄= 536.1 ± 782,3 g and 110.6 ± 76.8 cm), respectively, with highs in April and May. Out of 18 species common to all years, 10 species dominated the top or second rank in monthly pooled frequency of appearance. Seasonal changes of these 10 major species were examined, Sargassum horneri (Turner) C. Agardh and Hizikia fusiformis (Harvey) Okamura were abundant in April, but were replaced partly by Sargassum muticum (Yendo) Fensholt in May and largely by Sargassum yamamotoi Yoshida in June. In July, Sargassum nipponicum Yendo and Sargassum piluliferum (Turner) C. Agardh dominated. Subsequently, the major species shifted to Sargassum ringgoldianum Harvey and S. yamamotoi in August, Sargassum micracanthum (Kützing) Endlicher, Sargassum macrocarpum C. Agardh and Zostera marina Linnaeus in September, and S. ringgoldianum and S. micracanthum in October. However, the occurrence of S. yamamotoi, S. nipponicum and S. piluliferum in June or July were particularly heterogeneous compared with other areas of Japan. Dendrogram analysis was done based on frequency of appearance. Pooled monthly samples were divided into three groups characterized from the dominant species, degree of domination, weight, length and number of species of drifting seaweeds as well as the degree of diversity or evenness in appearance. This characterization indicated that the diversity and abundance of drifting seaweeds were higher from April to June than in later months.

The complete mitochondrial genome of Caprella scaura (Crustacea, Amphipoda, Caprellidea), with emphasis on the unique gene order pattern and duplicated control region

The nucleotide and amino acid sequences and the gene order of the mitochondrial genome are highly informative for studying phylogeny, population genetics, and phylogeography. This study determined the complete mitochondrial genome of the caprellid species Caprella scaura. The mitochondrial genome of C. scaura has a total length of 15,079 bp, with an AT content of 66.43%. The mitochondrial genome contains typical gene components, including 13 protein-coding genes, 2 rRNA genes, and 22 tRNA genes. In comparison with the mitochondrial genome of a gammarid, some distinct characteristics were found. For example, the order of the two conserved gene blocks is inverted between Gammaridea and C. scaura. In addition, two copies of almost identical control regions were found in the mitochondrial genome of C. scaura. These unique characteristics will be useful for determining the evolutionary history of the Caprellidea.

Phylogenetic Analysis of Caprellid and Corophioid Amphipods (Crustacea) Based on the 18S rRNA Gene, With Special Emphasis on the Phylogenetic Position of Phtisicidae

Members of the amphipod suborder Caprellidea exhibit degenerated abdomens and pereopods 3 and 4. Some genera of Podoceridae (Gammaridea, Corophioidea) such as Dulichia also show reduced abdomens and pereopods and thus are generally regarded as a sister group of the Caprellidea. In addition, one of the caprellid families, the Caprogammaridae, exhibits abdominal segments that are similar to those of the podocerids, as well as rudimentary pereopods 3 and 4, which are more consistent with those of other caprellids. Therefore, an evolutionary scheme has been suggested on the basis of the gradual degeneration of the pereopods and abdomen: [Dulichia, (caprogammarids, caprellids)]. However, the Phtisicidae (Caprellidea) contradict this hypothesis because they exhibit well-developed pereopods 3 and 4, along with degenerated abdomens. Therefore, previous studies have suggested that the Phtisicidae and other caprellids may be polyphyletic. We examined the phylogenetic position of the Phtisicidae and other caprellid amphipods, using 18S rRNA gene sequence data. The results strongly indicate that the Phtisicidae and other caprellid families form a monophyletic clade. However, a close phylogenetic relationship among Dulichia (Corophioidea) and taxa belonging to the Caprellidea was not definitively supported. This study is the first to use molecular data to investigate the phylogenetic relationships among the Caprellidea.

Seasonal changes in the epifaunal community on the shallow-water gorgonian Melithaea flabellifera

Periodic surveys of the subtidal epifaunal community on the gorgonian Melithaea flabellifera were conducted over a five year period at the Izu Peninsula, southern Japan. The occurrence patterns of epifaunal species on M. flabellifera were examined. The epifauna consisted of 16 species representing five phyla. The amphipod Incisocalliope symbioticus dominated (usually >80%) and occurred throughout the study.

Seasonal traits of reproduction in a gnathiid isopod Elaphognathia cornigera (Nunomura, 1992).

Abstract Field studies on the reproductive traits of a population of a gnathiid isopod Elaphognathia cornigera (Nunomura) were conducted at a rocky intertidal shore on the Izu Peninsula, southern Japan. Fortnightly surveys for two years showed some peculiar seasonal trends in the female reproductive characteristics. While adult males and juveniles of the species occurred throughout the year, semelparous adult females occurred only from spring to autumn. Based on the occurrence patterns of the females in six different reproductive stages, there seemed to be four generations in 1994 and three in 1995. Though the reproductive investment of a female showed no significant variation through each year, there were marked seasonal variations in the female broods. Female body size and brood size became the largest in early summer, but egg size reached its maximum in autumn. Maximum food availability (high density of gobies) in spring and the unfavorable environmental conditions in winter were thought to be the major controlling factors of the size and number of female broods.

FINE STRUCTURES OF TUNIC CELLS AND DISTRIBUTION OF BACTERIA IN THE TUNIC OF THE LUMINESCENT ASCIDIAN CLAVELINA MINIATA (ASCIDIACEA, UROCHORDATA)

Abstract In a colonial ascidian Clavelina miniata, physical stimulations induce strong luminescence in the tunic. We described here the tunic cell morphology and bacterial distribution in the tunic that is a luminous tissue of this species. Three types of tunic cells are morphologically discriminated as morula-like tunic cells, tunic phagocytes, and tunic granulocytes, and they correspond, respectively, to the Type I, Type II, and Type III cells described by Aoki et al. (1989). Morula-like tunic cells are similar in morphology to morula cells that are hemocytes commonly found in ascidians. Tunic phagocytes contain round granules, clear vacuoles, and occasionally phagosomes. Tunic granulocytes characterized by a number of elliptical granules and they occasionally contain phagosomes and round granules that are similar in structure to tunic phagocytes. According to the description by Aoki et al. (1989), tunic phagocytes are supposed to be luminous cells. Elongated bacteria of unique forms are found in tunic phagocytes. However, these bacteria are probably not luminous ones, since they also are distributed in tunic granulocytes and outside of the tunic cells. Because other bacteria-like inclusions are not present in tunic phagocytes, we found no structural evidence to support the bacterial origin of bioluminescence in C. miniata. The clear vacuoles of tunic phagocytes may be a possible candidate for the subcellular site producing bioluminescence.

Growth and survival rates of large-type sporophytes of Ecklonia cava transplanted to a growth environment with small-type sporophytes

Stipe lengths of sporophytes of Ecklonia cava Kjellman have been reported to be longer along the southeast than southwest coast of the Izu Peninsula, central Japan. Two bays in this region that have natural populations of E. cava, but with different stipe lengths, were chosen for transplant experiments to examine if stipe length was an environmentally controlled trait. Transplant experiments were carried out in order to determine whether large-type sporophytes of E. cava with long stipes growing in Nabeta Bay (southeast Izu Peninsula, Japan) would turn into small-type sporophytes with short stipes when transplanted to Nakagi Bay (southwest Izu Peninsula). Ten juvenile sporophytes of E. cava (stipe length < 5 cm) were collected from Nabeta Bay (large-type habitat) and transplanted to Nakagi Bay (short-type habitat) in December 1995. As a transplant control, ten juvenile sporophytes of E. cava growing in Nakagi Bay were also transplanted to the same artificial reefs. Growth and survival rates of the sporophytes were monitored monthly for 3 y until December 1998. The transplanted sporophytes showed an increase in their stipe length and diameter from winter to spring, whereas almost no increase was observed from summer to autumn. However, the elongation was greater in Nabeta sporophytes than in Nakagi sporophytes. The primary blade length increased mainly from winter to early spring and decreased largely in autumn. Average primary blade lengths were similar in both Nabeta and Nakagi sporophytes from the end of the first year of transplanting. Although ca. 70% of both Nabeta and Nakagi sporophytes survived during the first 2 y after transplantation, no Nakagi sporophytes and only two Nabeta sporophytes survived to the end of the 3 y study period. Despite transplantation to Nakagi Bay, where short sitpes are naturally present, the sporophytes from Nabeta Bay persisted in having longer stipes, which suggests that stipe length is genetically, rather than environmentally, controlled.

Complicated evolution of the caprellid (Crustacea: Malacostraca: Peracarida: Amphipoda) body plan, reacquisition or multiple losses of the thoracic limbs and pleons

The Caprellidea (Crustacea) have undergone an interesting morphological evolution from their ancestral gammarid-like form. Although most caprellid families have markedly reduced third and fourth pereopods (the walking thoracic limbs) and pleons (the posterior body parts), one family, Caprogammaridae, has developed pleon with swimming appendages (pleopods), whereas another family, Phtisicidae, possesses well-developed functional third and fourth pereopods. The unique character status of these families implies that there has been reacquisition or multiple losses of both pereopods and the pleon within the Caprellidea lineages. Although the Caprellidea are fascinating animals for the study of morphological evolution, the phylogenetic relationships among the Caprellidea are poorly understood. One obstacle to studying the evolution of the Caprellidea is the difficulty of collecting samples of caprogammarid species. In this study, we obtained live samples of a Caprogammaridae species and confirmed that its pleon and pleopods could perform similar locomotive functions and swimming movements as observed in gammarids. From the phylogenetic analyses on 18S ribosomal RNA gene sequences, we identified three distinct clades of Caprellidea. The ancestral state reconstruction based on the obtained phylogeny suggested that once lost, the third and fourth pereopods were regained in the Phtisicidae, while the pleon was regained in the Caprogammaridae, while we could not exclude the possibility of independent losses. In either case, the caprellid lineage underwent a quite complicated morphological evolution, and possibly the Caprellidea may be an exception to Dollo’s law.