Little is known of the biology of the parasite Dendrogaster (Crustacea: Ascothoracida), an obligate endoparasite of sea stars. There are some 25 species of Dendrogaster reported worldwide. The highly modified females live within the coelomic cavity of the host, and heterochronous males are parasitic within the mantle cavity of the female. Two sea star species from the Otago Coast were found to be infected with two species of Dendrogaster. These parasite populations were studied from July 1993 to December 1994 to investigate specific aspects of their biology and the interactions between these parasites and their hosts.
The parasite found in Allostichaster insignis (Asteridae) was identified as Dendrogaster argentinensis Grygier and Salvat 1984, and Asterodon miliaris (Odontasteridae) was found to contain a previously unknown species, herein described and named Dendrogaster otagoensis n. sp. The structure and function of the mantle of female Dendrogaster sp were determined using light, scanning and transmission electron microscopy. The ultrastructure of the mantle is modified, sharing characteristics found in Rhizocephala (Crustacea: Cirripedia), copepods, and Ulophysema oeresundense (Ascothoracida: Dendrogasteridae), which enable these crustaceans to absorb nutrients. Such modifications include a very thin, crenulated epicuticle, a soft, unscleratinised procuticle, and infoldings of the apical plasma membrane. In Dendrogaster the gut is likely to retain the capacity to absorb nutrients.
In general, aspects of infestation of Dendrogaster argentinensis and D. otagoensis are the same. Prevalence of Dendrogaster sp. in Otago sea stars is high and parasitism occurs throughout the year. Seventy-seven point five percent of Asterodon miliaris are infected with D. otagoensis and 97.4% of Allostichaster insignis are infected with D. argentinensis. Up to 144 females are found in a single host, and as many as 34 males in a single female. Mean number of parasites per host changes little over time, but began to decrease in D. argentinensis towards the end of the sample period. The female population is dominated by small individuals, with maximum size likely to depend on space available within the host. Males achieve a large size quickly, but their growth is also restricted by the size of the female they are within. The size and number of females per host are not related to size of the host. Females acquire new males and males grow larger with an increase in female size. Males occur in females of all reproductive stages, but occur with the greatest frequency in brooding females. Reproduction is spread evenly through females in less heavily infected hosts, and there is a reduction in mean size of females as the number of females per host increases, indicative of the crowding effect. Three indices of aggregation show that both sexes of Dendrogaster are highly aggregated in their hosts, with few hosts harbouring most of the parasites.
Dendrogaster is a parasitic castrator. Infected hosts have parasite loads ranging from 0.1% - 112% total sea star wet weight. Infected sea stars have smaller pyloric caecae and gonads, however, no gonads were present in uninfected Allostichaster insignis. Effect on the gonads and pyloric caecae increases as parasite load increases. Rather than directly affecting the germinal tissue of the host, Dendrogaster castrates its host indirectly through crowding and/or competitive castration. This castration is variable among hosts in Asterodon miliaris, but is important in Allostichaster insignis. The presence of D. argentinensis in A. insignis is associated with an increase in the incidence of asexual reproduction among larger (R>25mm) individuals. Specific growth rates of intact and regenerating arms in fed and starved A. insignis are lower in infected individuals.
Larvae are brooded in the mantle cavity of the female until reaching the infective, swimming ascothoracid II stage. The larval development of Dendrogaster argentinensis and D. otagoensis is abbreviated. Larvae moult directly from the egg into the metanauplius. Metanauplii moult into ascothoracid I stage, which moult into the ascothoracid II stage immediately upon release from the female. Ultrastructure confirms the putative sensory function of the lattice organs. Other larval cuticular structures are also sensory.
Histological examination revealed that Dendrogaster has unique system of ovulation among Crustacea. The ovarian membrane contains primary oocytes, early vitellogenic and vitellogenic oocytes, but not maturing oocytes, which are found outside the ovarian membrane. The ovarian membrane forms follicles from which vitellogenic oocytes burst into the surrounding mesodermal tissue. These oocytes mature within the mantle wall tissue until entering the mantle cavity, where they are fertilised. Ovaries are absent in most brooding females. Spermatogenesis takes place within the males posterior protrusions and is continuous. Dendrogaster are likely to be labile hermaphrodites, as sperm are found in 20% of ovaries. Female Dendrogaster display asynchronous reproduction, with females of all sizes at different stages throughout the year. Females within one host are also at different stages. Spermatogenesis is present in males of all sizes. The presence of spent males in some females is indicative of a females outliving the males.
| Identifer | oai:union.ndltd.org:ADTP/243337 |
| Date | January 2009 |
| Creators | Palmer, Penny L, n/a |
| Publisher | University of Otago. Department of Marine Science |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Penny L Palmer |
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