Distribution patterns of pelagic gastropods at the Cape Verde Islands Holger Ossenbrügger* Semester thesis 2010 *GEOMAR | Helmholtz Centre for Ocean Research Kiel Marine Ecology | Evolutionary Ecology of Marine Fishes Düsternbrooker Weg 20 | 24105 Kiel | Germany Contact: [email protected] Contents 1. Introduction . .2 1.1. Pteropods . 2 1.2. Heteropods . 3 1.3. Hydrography . 4 2. Material and Methods . 5 3. Results and Discussion . 7 3.1. Pteropods . 7 3.1.1. Species Composition . 7 3.1.2. Spatial Density Distribution near Senghor Seamount . .. 9 3.1.3. Diel Vertical Migration . 11 3.2. Heteropods . 17 3.2.1. Species Composition . .17 3.2.2. Spatial Density Distribution near Senghor Seamount . .17 3.2.3. Diel Vertical Migration . 18 4. Summary and directions for future research . 19 References . 20 Acknowledgements . 21 Attachment . .22 1. Introduction 1.1. Pteropods Pteropods belong to the phylum of the Mollusca. They are part of the class Gastropoda and located in the order Ophistobranchia. The pteropods are divided into the orders Thecosomata and Gymnosomata. They are small to medium sized animals, ranging from little more than 1mm for example in many members of the Genus Limacina to larger species such as Cymbulia peroni, which reaches a pseudoconch length of 65mm. The mostly shell bearing Thecosomata are known from about 74 recent species worldwide and are divided into five families. The Limacinidae are small gastropods with a sinistrally coiled shell; they can completely retract their body into the shell. Seven recent species of the genus Limacina are known. The Cavoliniidae is the largest of the thecosomate families with about 47 species with quite unusually formed shells. The family Peraclidae compromises 8 species with a sinistrally coiled shell. Species of the genus Peraclis are only found in meso- and bathypelagic waters of the ocean. The fourth thecosomate family, the Cymbuliidae, consists of medium to quite large animals with the 3 genera, Cymbulia, Corolla and Gleba and 9 species that typically bear a gelatinous, so called pseudoconch. The last family, the Desmopteridae, is a very small family with 3 species that have no shell and are usually very small. The order Gymnosomata is a group of about 45 to 50 species which have no shells. They consist of 7 families, the Clionidae, Cliopsidae, Hydromylidae, Laginiopsidae, Notobranchaeidae, Pneumodermatidae and Thliptodontidae. Because only few unidentifiable specimenss of the Gymnosomata were found during this study this group will not be discussed in detail. The pteropods belong to those gastropods that have a holoplanctonic lifecycle, together with a few other groups, the Heteropods (see 1.2.), the raft drifting Janthinidae and a few species of the nudibranch families Glaucidae and Phylliroidae. Pteropods are mostly oceanic epi- to mesopelagic organisms and can reach quite high densities, which makes them an important part of the planktonic foodweb. As in many other oceanic groups species richness is highest in the lower latitudes, while relative abundances increases with higher latitudes. In the Arctic Ocean as well as in the 2 Southern Ocean Limacina helicina and Clione limacina are the dominant members of the Thecomomata and the Gymnosomata, respectively. Pteropods are efficient grazers, using a mucus net to gather food particles which is the case in most thecosomate pteropods or predators, like the gymnosomate pteropods, that prey on their thecosomate relatives. Pteropods are prey of a wide range of animals, ranging from chaetognaths, medusae, ctenophores, heteropods, siphonophores and the cephalopod Argonauta boettgeri to fishes of various kind, sea birds and baleen whales. 1.2. Heteropods Heteropods are holoplanktonic animals. The gastropods formerly united in the order Heteropoda are members of the prosobranchian superfamily Pterotracheoidea, consisting of three families: the Atlantidae, the Carinariidae and the Pterotracheidae. They range in size from a few mm in the genus Atlanta to about 50 cm in the Indopacific species Carinaria cristata. Of the Atlantidae, a group of small gastropods that can completely retract their entire body into their small dextrally coiled shell, about 20 recent species are known. While two genera, Oxygyrus and Protatlanta are monospecific, the identification of the animals of the genus Atlanta in quite complicated due to great similarity in body and shell forms and small size. The family Carinariidae has 7 species in 3 genera: Carinaria, Pterosoma and Cardiopoda. They are usually quite large and have no shell at all or only a small one that can only hide their visceral mass. In the third family, the Pterotracheidae, a shell is only present in the larval phase, while the adult has no shell at all. The family consists of two genera, Pterotrachea and Firoloida and 5 species. As all heteropods are opical predators, they live mostly in the euphotic zone. In contrast to the pteropods the heteropods are only warm water animals, living in the tropical to temperate oceans of the world. They mostly prey on thaliaceans, chaetognaths and copepods. 3 1.3. The Hydrography near the Cape Verde Islands The Cape Verde Islands are an archipelago of oceanic islands located in the Eastern Atlantic Ocean about 600km off Cape Verde, Senegal from 14°47’ to 17°13’N and 22°52’ to 25°22’W. According to Fiekas et al. (1992) in the upper water layers between 20 and 200 m depth near the Cape Verde Islands the North Equatorial Current (NEC) develops. It flows in southwesterly direction and is build by confluence of the cold Canary Current (CC) from the North and a warm current from the South, which transports South Atlantic Central Water (SACW) in westerly direction around the Cape Verde Islands (Fiekas et al., 1992). The South Atlantic Central Water, that is divided by the Cape Verde Frontal Zone from the North Atlantic Central Water, is transported by an undercurrent northward over the Equator and is found during the whole year off the coast of Dakar, Senegal (Fiekas et al., 1992). In intermediate water depth of 200 to 700 m the region around the Cape Verde Islands is dominated by eddy fields (fig. 2). In depth of 700 to 1200 m an eastward flowing current transports Antarctic Intermediate Water (AAIW) to the region west of the Cape Verde Islands (Fiekas et al., 1992). John & Zelck (1997) made a model of the currents in the Mauretanian Province in the depth layer of 30-90 m Tiefe based on hydrographic data and the distribution of fish larvae (fig. 1). 4 Figure 2. Diagram of the thermohaline Circulation in the Figure 1. Model of the Currents in the Mauretanian Canary Basin with the main currents (waved) and the province watermasses (underlined), that are found at the in the 30 to 90m depth layer, based on the distribution of southeastern flank of the subtropical gyre for three defined fish larvae anf hydrographic data. (John & Zelck, 1997) water layers: 20-200m, 200-700m and 700-1200m. (Fiekas et Al., 1992) 2. Material and Methods The zooplankton samples used in this study were taken by the RV Poseidon during cruise P 320-2 in April 2005. The cruise was a pilot study titled “Biosphere –Hydrosphere – Geosphere Interactions at Seamounts”. They were taken at the stations listed in chart 1. Two methods were used for collection, a multi opening and closing net (Hydrobios Multinet) and and Isaac Kid Midwater Trawl (IKMT). The stations sampled with the multinet are marked in figure 3 by red circles, while the IKMT stations are marked by green circles. Near the Senghor Seamount, located northeast of Sal Island, the multi opening closing net (Hydrobios multinet) was used to collect zooplankton samples. It was equipped with 8 nets 5 with an opening width of 0.5 m² and a mesh size of 35 µm. Within the gear a flowmeter was installed to determine the filtered amount of water. The material was taken in defined depth strata. The first net was not used for the current study. The remaining 7 nets sampled the following depth strata: 250 m to 200 m, 200 m to 150 m, 150 m to 100 m, 100 m to 75 m, 75 m to 50 m, 50 m to 25 m and 25 m to 0 m. At Senghor Seamount and near Brava Island also micronekton samples were taken using the Isaac Kid Midwater Trawl (IKMT) with an opening width of 6 m² and a mesh size of 4mm. The samples were presorted on board to extract key species and were afterwards stored in buffered formaldehyde. The quantitative analysis was done on land under the stereomicroscope, where the pteropods were picked out, sorted by species and separately stored in buffered formaldehyde. Afterwards the pteropods were identified to the lowest possible taxonomic level. Figure 3. Cape Verde map with stations (red circles: Multinet stations, green circles IKMT stations) 6 3. Results and Discussion 3.1. Pteropods 3.1.1. Species Composition The use of the IKMT obtained due to the mesh size only the larger species (Fehler! Verweisquelle konnte nicht gefunden werden. and 3). A total of 277 specimens of 14 thecosomate pteropod species and one specimen of an unidentified gymnosomate pteropod species were cought. At Senghor Seamount (chart 2) by far the most common species is the cavoliniid Clio pyramidata f. lanceolata (76,80%), which can be found in most of the tropical and subtropical oceans of the world, but is especially common in the Atlantic Ocean (Rampal, 2002) . It is followed in abundance by Diacavolinia deshayesi (8,25%), a species distributed in the tropical to subtropical Atlantic Ocean (van der Spoel, 1993). and Diacria major (3,61%), a globally distributed Cavoliniid that prefers Central Water Masses (van der Spoel & Dadon, 1999). The unidentified Diacavolinia (2,58%) is probably also Diacavolinia deshayesi, but due to severe damages of the shell this could not be said for sure.