BackgroundArthropod diversity is unparalleled in the animal kingdom. The study of ontogeny is pivotal to understand which developmental processes underlie the incredible morphological disparity of arthropods and thus to eventually unravel evolutionary transformations leading to their success. Work on laboratory model organisms has yielded in-depth data on numerous developmental mechanisms in arthropods. Yet, although the range of studied taxa has increased noticeably since the advent of comparative evolutionary developmental biology (evo-devo), several smaller groups remain understudied. This includes the bizarre Pycnogonida (sea spiders) or “no-bodies”, a taxon occupying a crucial phylogenetic position for the interpretation of arthropod development and evolution.ResultsPycnogonid development is variable at familial and generic levels and sometimes even congeneric species exhibit different developmental modes. Here, we summarize the available data since the late 19th century. We clarify and resolve terminological issues persisting in the pycnogonid literature and distinguish five developmental pathways, based on (1) type of the hatching stage, (2) developmental-morphological features during postembryonic development and (3) selected life history characteristics. Based on phylogenetic analyses and the fossil record, we discuss plausible plesiomorphic features of pycnogonid development that allow comparison to other arthropods. These features include (1) a holoblastic, irregular cleavage with equal-sized blastomeres, (2) initiation of gastrulation by a single bottle-shaped cell, (3) the lack of a morphologically distinct germ band during embryogenesis, (4) a parasitic free-living protonymphon larva as hatching stage and (5) a hemianamorphic development during the postlarval and juvenile phases. Further, we propose evolutionary developmental trajectories within crown-group Pycnogonida.ConclusionsA resurgence of studies on pycnogonid postembryonic development has provided various new insights in the last decades. However, the scarcity of modern-day embryonic data – including the virtual lack of gene expression and functional studies – needs to be addressed in future investigations to strengthen comparisons to other arthropods and arthropod outgroups in the framework of evo-devo. Our review may serve as a basis for an informed choice of target species for such studies, which will not only shed light on chelicerate development and evolution but furthermore hold the potential to contribute important insights into the anamorphic development of the arthropod ancestor.
A complete account on the shallow-water Pycnogonida known up to now from depths between 0-30 m in the Caribbean Sea of Colombia is presented. Almost all the information included herein is based upon the data obtained by the first author during a 14-month fieldwork programme carried out at the Instituto de Investigaciones Marinas de Punta de Betin (INVEMAR) in Santa Marta, Magdalena department. 5312 specimens of 50 species, including 7 species new to science in the genera Ammothella (2), Tanystylum (1), Callipallene (2), Anoplodactylus (1) and Endeis (1), were collected from 179 samples at 45 stations. The area covered by this research ranges in the west from Punta Brava, just near the airport of Santa Marta, eastward to Punta el Diamante at the eastern border of the Tayrona National Park, extending over about 70 km of coastline. Additionally, 6 other pycnogonid species reported previously from outside this area, which are also known from the Caribbean Sea of Colombia are briefly reviewed. The species from the Santa Marta area are described in detail, with comments on their intraspecific affinities, habitat preference, phenology, vertical distribution and geographic distribution. Quantitative samples were taken approximately monthly at three stations at Bahía Concha over one year to compare the species composition from substrata of different structure, namely stands of the brown algae Digenia simplex and Sargassum cymosum on a dead Porites reef, and a stand of Thalassia testudinum, which was interspersed with dead coral substratum and coralline algae. Number of species at all of these stations was found to be similar and species composition of the two species of brown algae almost identical. Achelia sawayai was by far the most numerous species in samples of Digenia simplex, whereas Tanystylum acuminatum and T. isabellae were most numerous in Sargassum cymosum. Species composition in Thalassia was rather different from that of the algal vegetation. In Thalassia, Ammothella appendiculata and A. exornata were the commonest species which were not found in Digenia and Sargassum. No evidence was found that reproduction of pycnogonids in this tropical area is limited to certain periods of the year. One might presume a shorter reproductive cycle only for Tanystylum acuminatum, because ovigerous males appeared at the end of the rainy season in December and remained until May, therefore for most of the duration of the dry season. Generally, number of species and specimens collected at all three quantitative sampling stations was rather variable from month to month, implying a heterogenous distribution within the substratum, which surely depend on the distribution of their food. Zoogeographic patterns are at present almost impossible to interpret, owing to the limited information available on the distribution of most species. However, it can be seen that the Santa Marta region has a very high number of shallow-water pycnogonid species, compared with the fauna of Panamá and Belize, where only 34 and 33 species have been recorded, respectively. However, the high number of species found in the Santa Marta area may be a result of the extensive collecting efforts made over a period of more than one year.
D N A L i b r a r y o f L i f e , r e s e a r c h a r t i c l eurn:lsid:zoobank
Material of Pycnogonida was collected by scuba diving and snorkelling in the eastern Mediterranean Sea from the Aegean Sea to northern Cyprus in 1995–2002; some additional samples were taken in 2006 (mainly in Gencelli Cove, Turkey). This yielded the first record of Ammothella appendiculata for the region. A. longioculata and Callipallene spectrum are newly recorded for Turkey. A hitherto undescribed and tiny species of Anoplodactylus was found. The insufficiently known adult stage of Ammothella uniunguiculata is figured by SEM photograph.
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