Biodiversity and taxonomy of harpacticoid copepods associated with coral substrates of tropics and deep sea = Biodiversiteit en taxonomie van harpacticoide copepoden geassocieerd met koraalsubstraten van tropen en diepzee
Gheerardyn, H. (2007). Biodiversity and taxonomy of harpacticoid copepods associated with coral substrates of tropics and deep sea = Biodiversiteit en taxonomie van harpacticoide copepoden geassocieerd met koraalsubstraten van tropen en diepzee. PhD Thesis. Universiteit Gent, Vakgroep Biologie: Gent. 251 pp. |  |
| Available in | Author | | Document type: Dissertation
|
| Abstract | Tropical coral reefs are one of the most diverse habitats in the world’s oceans. By providing substrata for sedentary organisms, and food and shelter for mobile organisms, corals create a rich series of habitats for great numbers of species (Paulay, 1996). However, the true extent of the diversity of coral reef associated organisms is still poorly understood. Best studied groups are macro- and megafauna, especially fishes and macrocrustaceans (e.g., Findley & Findley, 2001; Hughes et al., 2002; Roberts et al., 2002). Knowledge of the associated meiofauna (organisms between 32 µm and 1 mm) is at present limited. Nevertheless, a wide range of potential microhabitats is available for benthic fauna in the large variety of dead coral substrates, which originate from physical and biological breakdown of coral skeletons.Although the existence of cold-water corals has been known to science since the 18th century (Pontopiddan, 1755), the occurrence, biology, and diversity of the associated communities has by no means been studied as intensively as for tropical coral reefs. Cold-water coral reefs occur in the upper bathyal zone throughout the world. Like tropical coral reefs, they are characterised by high habitat diversity (Rogers, 1999). It has even been stated that the biodiversity of Lophelia pertusa reefs is of a similar order of magnitude as that of some shallow-water tropical coral reefs (Rogers, 1999). Studies dealing with associated fauna on either living or dead L. pertusa have mainly focused on the macro- and megafauna (Jensen & Frederiksen, 1992; Mortensen et al., 1995; Fosså & Mortensen, 1998; Rogers, 1999). More recently, the meiofauna (at higher taxon level) and nematofauna associated with dead coral substrates have been investigated by Raes & Vanreusel (2005, 2006).The present study focuses on the associated harpacticoid copepod fauna of dead coral substrates. The order of Harpacticoida is one of the ten orders of the subclass Copepoda. Harpacticoid copepods are ubiquitous in marine soft-sediments and generally the second most abundant meiobenthic taxon after the numerically dominant Nematoda (Coull & Bell, 1979; Hicks & Coull, 1983; Higgins & Thiel, 1988; Giere, 1993). The associated harpacticoid assemblages of dead coral substrates are investigated in a shallow, tropical lagoon along the eastern coast of Zanzibar (Tanzania). The associated assemblages of cold-water coral substrates are investigated from Lophelia pertusa reefs in the Belgica Mound Province (Porcupine Seabight, NE-Atlantic), at a depth of about a 1000 m.In chapter 2, the harpacticoid fauna associated with tropical coral substrates is investigated. The main aim of the study was to assess the influence of microhabitat type on assemblage structure and diversity. Three different microhabitat types were distinguished, namely dead coral fragments, coral gravel and coral sand. The coral assemblage was significantly different from that in gravel and every sediment layer, and consisted of typical ‘phytal’ taxa with an addition of eurytopic and sediment-dwelling forms. We assume that the sediment trapped by the coral provides a habitat for sediment-dwellers, while the complex microtopography of the coral branches provides a suitable substratum for epibenthic or even ‘phytal’ taxa. The assemblages of coral gravel and upper sediment layer did not differ significantly from each other and contained mostly the same dominant genera. Differences in sediment granulometry were important in structuring the sediment assemblages. We assume that the primary factors affecting composition of the associated fauna are most likely the nature and structure of the primary substrate. Furthermore, assemblages might experience differences in environmental conditions. Especially at Makunduchi, the coral assemblage was significantly more diverse than gravel and sediment. Coral form and complexity, with implications for habitable space, nutritional resources and level of predation might be important in structuring diversity of the associated assemblage. Chapter 3 presents the first characterisation of the harpacticoid copepod fauna associated with cold-water coral substrates. As in the previous chapter, the main aim was to assess the influence of microhabitat type on copepod assemblage structure and diversity. Three different microhabitat types were distinguished, namely dead coral fragments, glass sponge skeletons and underlying sediment. Apart from some subtle differences, it appears that coral fragments and underlying sediment do not harbour distinctly different assemblages. Only two sponge skeletons were collected and conclusions about its assemblage were considered as provisional. Several factors might be important in explaining the apparent lack of difference in assemblage structure. Sediment, retained between the coral branches, might attract sediment-dwellers, which obscure the presence of true epibenthic taxa. Active migration by swimming and the close contact between the substrates may facilitate considerable exchange. Also, the high evenness, typical of the deep sea fauna, in combination with limited sample sizes, undoubtedly influences the pattern observed. At least at the family level, the copepod fauna of the Porcupine Seabight does not seem to differ markedly from other deep-sea studies, in which essentially the same families are dominant. At the genus and species level, it is however clear that coral fragments provide a specific habitat suitable for typically ‘phytal’ taxa, with prehensile first legs and modified body shapes. Coral fragments and sediment were both characterised by high species diversity and low species dominance, and did not differ markedly in this. This might indicate that copepod diversity is not substantially influenced by hydrodynamical stress, which, however, is the main structuring factor of the associated nematode assemblages. In the second part of this study, we investigate taxonomy and morphological adaptations to coral substrates within the harpacticoid family Laophontidae. This family is considered highly successful in terms of species richness and number of habitats explored. Laophontidae show a high degree of morphological plasticity and therefore are model organisms to study the relation between habitat and morphology. In chapter 4, eight new species of the harpacticoid family Laophontidae, from different locations in the Indo-West Pacific Ocean, are described and placed in a new genus, Peltidiphonte gen. n. The new genus is clearly characterised by the extremely depressed body shape, the presence of distinct processes on the proximal antennular segments and the absence of sexual dimorphism in the endopods of the swimming legs. Most of the specimens were collected from dead coral substrates, suggesting a close affinity between the members of the new genus and this substrate. The dorsoventral flattening of Peltidiphonte represents an adaptation to an epifaunal life style on the surface of dead coral fragments, and should decrease the risk of being swept away by strong currents. The new genus has a distribution covering the Indo-West Pacific Ocean. Furthermore, a key to the eight species of the genus is provided. In chapter 5, two new monospecific genera of Laophontidae are established. They differ from most other laophontid genera in the absence of sexual dimorphism in the endopods of the swimming legs. Both new species resemble each other closely in habitus, integumental ornamentation, chaetotaxy of the swimming legs and absence of sexual dimorphism in the endopods. However, the detailed characteristics of A1, maxilla and male P5 show that the species are not congeneric. The new genus Propephonte, described from the northern coast of Papua New Guinea, is closely related to Peltidiphonte, based on shape and setation of the fifth pereiopods and the detailed structure of the first antennular segment. Furthermore, we assume that Indolaophonte and Langia are more derived genera within this lineage, wherein setation and segmentation of the swimming legs became more reduced as an adaptation to an interstitial life style. The new genus Apistophonte is described from the Kenyan coast. Based on the detailed characteristics of maxilla, fifth pereiopods and first antennular segment, we conclude that Apistophonte branched off from a different stock than the lineage grouping Propephonte and Peltidiphonte. The exact affinities of the genus, however, remain difficult to assess. A new species of Paralaophonte is described in chapter 6. Its most distinguishing feature is the robust, enlarged and specialised maxilliped, formerly unseen in Paralaophonte. Paralaophonte harpagone sp. n. does not show any sexual dimorphism in the endopodite of P3 nor in the exopodites of P2 to P4. However, it is a true representative of the genus Paralaophonte by the typical sexually dimorph P2 endopodite with its modified distal inner seta on the second endopodal segment. The maxilliped is similar in robustness and position to the highly specialised maxilliped of the genus Namakosiramia Ho & Perkins, 1977, of which the two members exist as ectoparasites on holothurians. This similarity is attributed to convergence and we can only speculate whether the specialised maxilliped of Paralaophonte harpagone sp. n. is an adaptation to live as an associate of another invertebrate. In chapter 7, a new monospecific genus, Spiniferaphonte, is described from coral gravel along the Kenyan coast and is particularly characterised by strong hook-like processes on the caudal rami. The new genus is closely related to Laophontina and Wellsiphontina, as shown by following synapomorphies: a denticulate operculum, a sexually dimorphic P4 exopod (reduced chaetotaxy of the ultimate segment in the male), and the absence of sexual dimorphism in the P2 and P3 endopods. The two-segmented exopod of P1 and the presence of a seta on the endopodal part of the male P5 are less derived, indicating that the new genus represents a separate lineage within this group. Interstitial genera in the Laophontidae show similar adaptations to an interstitial life style, namely a cylindrical body shape and reduced setation and segmentation of the swimming legs. Furthermore, it is striking that the presence of distinct, thorn-like processes on caudal rami is limited to interstitial genera. Distinct processes on the proximal antennular segments and a proximally thickened caudal seta V also appear to be associated with this interstitiality. We assume that these structures may play a role in the movement and anchoring of the animals in their interstitial habitat. Chapter 8 presents a revision of the laophontid genus Tapholeon Wells, 1967, which until now consisted of two species. In the present contribution, two new species are described from the coast of Kenya, T. inconspicuus sp. nv. and T. tenuis sp. n. Furthermore, a redescription of the type species T. ornatus Wells, 1967, based on the type material, is provided. Two species, formerly attributed to Asellopsis Brady & Robertson, 1873 (viz. A. arenicola Chappuis, 1954 and A. chappuisius Krishnaswamy, 1957), are allocated to Tapholeon based on the absence of sexual dimorphism in the swimming legs P2 to P4. The former of the two species is redescribed based on additional material from the Comoros. As a consequence of the transfer of these two species, the genera Asellopsis and Tapholeon have distinct distributions, with Asellopsis frequently reported from the Mediterranean Sea (including the Black Sea) and the eastern shores of the North Atlantic Ocean. The genus Tapholeon (now containing six species) shows a limited distribution confined to the southwestern part of the Indian Ocean and the Bay of Bengal. The similarities of both genera in body shape and caudal rami are attributed to convergence. An updated generic diagnosis and a key to the six species of Tapholeon are included. The results of this PhD thesis demonstrate that hard coral substrates provide a specific epifaunal habitat for benthic fauna. Especially in the tropics, it has been shown that harpacticoid community structure and diversity is influenced by the presence of these degradation products. Tropical coral fragments support a specific assemblage composed of epibenthic or phytal taxa with an addition of sediment-dwelling species. The addition of microhabitats contributes significantly to total species richness. In the cold-water coral degradation zone, only small differences could be detected in harpacticoid community structure of coral fragments and underlying sediment. This may partly be due to high evenness in combination with small sample sizes, but the presence of typical ‘phytal’ taxa nevertheless demonstrates the importance and specific nature of the habitat provided by hard biogenic substrates in the deep sea. Harpacticoid studies of neighbouring Atlantic regions are necessary to assess the impact of cold-water coral degradation zones on regional harpacticoid diversity. Especially in the tropical lagoon, it was demonstrated that coral substrates provide a variety of habitats exploited by different Laophontidae with specialised morphologies. This family shows a high degree of morphological plasticity and certain adaptations to the habitat clearly have evolved several times independently. The number of new taxa found, in the lagoon on Zanzibar and the cold-water coral degradation zone in the Porcupine Seabight, is a clear indication of our insufficient knowledge of copepod diversity in the tropics and the deep sea. |
|
