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. 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 . They are part of the class and located in the order Ophistobranchia. The pteropods are divided into the orders Thecosomata and Gymnosomata. They are small to medium sized , 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 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, , 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 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 , consisting of three families: the , the Carinariidae and the Pterotracheidae. They range in size from a few mm in the genus 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).

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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 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. Near Brava Island (chart 3) again f. lanceolata is by far the most abundant pteropod, although not as dominant at Senghor Seamount (45,78%). Other common members of the waters near Brava Island are Diacria trispinosa (10,84%), another tropical and subtropically globally distributed species and Diacavolinia deshayesi (8,43%). Diacria atlantica `dark form´ (7,23%), a species typically found in the upwelling areas off Northwestern Africa (van der Spoel, 1998) , was also found. As at this location beside Diacavolinia deshayesi also Diacavolinia limbata, a species distributed in the Atlantic Ocean especially in upwelling areas (van der Spoel & Dadon, 1999), was found, the identity of the unidentified specimens of Diacavolinia (6,02%) remains uncertain.

Clio cuspidata and f. atlantica. were only found near Brava Island and not at Senghor Seamount, but this might be caused by their rarity. While Clio cuspidata is circumglobally distributed in tropical and subtropical seas (van der Spoel & Dadon, 1999), Cuvierina columnella f. atlantica is only found in the tropical to subtropical Atlantic Ocean (van der Spoel & Dadon, 1999). Only identified off the coast of the Island of Brava are Diacavolinia limbata and Diacria atlantica which indicates upwelling events at this location.

7 Within the IKMT-material the species Diacria major was only found at Senghor Seamount, indicating the influences of the SACW transported by the NEC to the Cape Verde Islands.

Many of the pteropods collected by the Multinet were lacking the shell which caused some problems with their identification. A total of 9655 specimens of 19 species of thecosomate pteropods and 3 unidentified specimens of gymosomate pteropods were found in the Multinet catches (chart 4). The by far most common species is the minute species Limacina inflata (63,76%), followed by Limacina lesueurii (15,70%) and Clio pyramidata f. lanceolata (14,14%). Limacina inflata and Limacina lesueuri are both globally distributed in tropical to warm temperate seas. Limacina lesueuri shows a bisubtropical distribution pattern (van der Spoel, 1967).

The species f. imitans, Diacria danae, Hylocylis striata, Styliola subula, Creseis acicula f. acicula, Creseis virgula, Limacina bulimoides, Limacina inflata, Limacina lesueurii, Limacina trochiformis, Peraclis apicifulva, Peraclis reticulata, Desmopterus papilio and fragments of unidentified specimens of the family Cymbuliidae were only cought by the Multinet.

Tesch (1946) identified 5 groups of Pteropod communities, two of them, the arctic and the subarctic group, are not of interest for this study. Clio recurva, Diacria danae and uncinata are within a group of species preferably living in equatorial waters, while a second group including Cavolinia gibbosa and Styliola subula preferably inhabit subtropical waters, although they can also be found in tropical waters. A third group of species detected by Tesch is found both in subtropical as well as in tropical waters, compromising of Creseis acicula, Cavolinia inflexa and Cuvierina columnella.

Similar observation were carried out by Chen & Bé (1964). They proposed four categories of pteropods communities, two of which again is an arctic and a subarctic one. Beside those they found a subtropical, warm tolerant category and a subtropical, cold tolerant category.

Members of both categories were collected near in the present study. Members of the first category collected near the Cape Verde Islands are Limacina trochiformis, Creseis acicula, C. virgula conica, Hyalocylis striata and Cavolinia inflexa. Species of the second category are Limacina bulimoides, L. inflata, L. lesueuri, Clio pyramidata, Creseis virgula constricta, Styliola subula, and Diacria trispinosa. Which subspecies of Creseis virgula was collected near the Cape Verde Islands could not be identified.

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Figure 4. Some of the pteropods collected near the Cape Verde Islands:

1. Limacina lesueuri, 2. Limacina inflata, 3. Cavolinia uncinata uncinata, 4. Diacavolinia deshayesi, 5. Clio pyramidata f. lanceolata, 6. Creseis acicula f. acicula, 7. Creseis virgula f. virgula, 8. Cuvierina columnella f. atlantica, 9. Diacria atlantica `dark form´, 10. Diacria trispinosa trispinosa, 11. Diacria major, 12. Peraclis reticulate

Both Tesch´s as well as Chen & Bé´s species groupings show that both tropical as well as subtropical influences cause the species composition of the pteropods community around the Cape Verde Islands, which fits to the oceanic current patterns around those islands, being influenced both by the tropical NEC and the subtropical CC.

3.1.2. Spatial Density Distribution near Senghor Seamount

When looking at the distribution of pteropods at Senghor Seamount itself (figures 5 to 10) it can be seen that most pteropods have their highest densities over the peak of the seamount, while they are most scarce on its flanks. Getting into the open ocean in most species the abundance increases again. This pattern was found in the genera Cavolinia (figure 5) and

9 Diacria (figure 6) and in the species Clio pyramidata f. lanceolata (figure 7) and Limacina inflata (figure 8). Exceptions from this pattern can be found in Limacina lesueuri (figure 9) and specimen of the genus Peraclis (figure 10). Limacina lesueuri was found in similar abundances on the seamount peak, the open ocean and the flanks of the seamount. The same is true for the genus Peraclis, but in this taxon the abundances are generally higher west of the seamount than east of it. The high abundance of pteropods over the top of the seamount is caused by the pumping effect of the taylor column. Many of the pteropods drifting from the open ocean near to the flank of the seamount they probably get into the ring current of the taylor column and are transported to the center of the ring current over the top of the seamount.

Figure 5. Cavolinia at Senghor Seamount Figure 6. Diacria at Senghor Seamount

Figure 7. Clio pyramidata at Senghor Seamount Figure 8. Limacina inflata at Senghor Seamount

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Figure 9. Limacina lesueuri at Senghor Seamount Figure 10. Peraclis at Senghor Seamount

3.1.3. Diel Vertical Migration

For the rarer taxa a comparison between night and day hauls was carried out only on genus level, while the more common species were compared on their own. In the figures the dots represent the abundances of the taxons at the single stations, while the bars are the average for all stations. The white line indicates the position of the pycnocline that was measured to be in around 50 to 60m depth.

The average abundance of the Cavolinia species (fig. 11) in the upper water layers is much higher at night than during the day. During the night highest average densities of 146 per 1000m³ are reached in surface waters between the surface and 25m depth. The highest locally observed density was 224 per 1000m³ at this depth. Below 50m depth only small numbers of 0 to 12 per 1000m³ were found. During the day the average density of Cavolinia in the surface waters is much less. The highest average densities of 36 per 1000m³ are found in 25 to 50m depth. Nevertheless the animals can be found in all sampled depth and even have the highest observed single during the day density of 142 per 1000m³ in 75 to 100m depth. Although a diel vertical migration was observed it becomes clear that Cavolinia is a mainly epipelagic genus. Of the three Cavolinia species collected in this study both Cavolinia uncinata uncinata and Cavolinia gibbosa f. flava are mainly found in epipelagic waters (van der Spoel & Dadon, 1999). For Cavolinia uncinata uncinata diel vertical migrations were observed in the Florida Current (Wormelle, 1962) At Sengor Seamount this species was collected mainly near the

11 pycnocline, also individual specimen were found in all sampled depth. Cavolinia gibbosa f. flava was only collected in the surface waters down to 25m. Cavolinia inflexa f. imitans has been observed to migrate into deeper waters in the Florida Current by Wormelle (1962) with a mean day level of 88m and a mean night level of 98m. This species has been observed even deeper by Menzier (1958) in the Mediterranean Sea in 200 to 400m depth. Near Senghor Seamount it was found in small numbers in all sampled depth with no special preference for any depth strata.

The average abundance of the Diacavolinia species (fig. 12Fehler! Verweisquelle konnte nicht gefunden werden.) in the upper water layers is much higher at night than during the day, although they were generally quite rare in the samples. During the night highest average densities of 7 per 1000m³ are reached in surface waters between the surface and 25m depth. The highest locally observed density was 17 per 1000m³ at this depth. In no other depth strata Diacavolinia were observed during the nighttime sampling. During the day Diacavolinia was found between the surface and 100 to 150m depth in quite low numbers. The highest average during the day density of less than 2 per 1000m³ was reached in the surface waters. The highest observed single during the day density of 7 per 1000m³ has been recorded in the 0 to 25m depth strata. So a diel vertical migration has been observed for Diacavolinia migrating during the night to the surface waters probably in search for food. Studies carried out by other workers show Diavavolinia being a genus typical for epipelagic waters which matches the observed distribution pattern at Senghor Seamount (van der Spoel & Dadon, 1999).

The average abundance of the Diacria species (fig. 13) in the upper water layers is much higher at night than during the day and show a clear diel vertical migration. During the night highest average densities of 76 per 1000m³ are reached in 25 to 50m depth. The highest locally observed density was 113 per 1000m³ at this depth. Below 50m depth only small average numbers of 0 to 3 per 1000m³ were found. During the day the average density of Diacria in the surface waters is much less. The highest average densities of 13 per 1000m³ are found in 50 to 75m depth. Nevertheless the animals can be found in all sampled depth and have the highest observed single during the day density of 36 per 1000m³ in 0 to 25m depth. As the species is known for diurnal migration between the sea surface and 500m depth it is quite possible that the animals are in waters below 250m depth and therefore were not found in larger numbers during the day.

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Figure 11. Cavolinia at Senghor Seamount Figure 12. Diacavolinia at Senghor Seamount

Figure 13. Diacria at Senghor Seamount Figure 14. Clio pyramidata at Senghor Seamount

Figure 45. Creseis virgula at Senghor Seamount Figure 16. Limacina bulimoides at Senghor Seamount

Figure 5.Limacina inflata at Senghor Seamount Figure 18. Limacina lesueuri at Senghor Seamount

Figure 6. Limacina trochiformis at Senghor Seamount Figure 20. Peraclis at Senghor Seamount 13 The average abundance of Clio pyramidata f. lanceolata (Fehler! Verweisquelle konnte nicht gefunden werden. 14) is highest during night with 1060 per 1000m³ in 75 to 100m depth. The highest locally observed density was 1939 per 1000m³ at this depth. Below this depth only low numbers were found (0 to 120 animals per 1000m³), while between 25 and 75m depth the average numbers are still high (336 to 516 per 1000m³). During the day the average density of Clio pyramidata f. lanceolata in all sampled depth are quite low (0 to 138 per 1000m³). The highest average densities are found in 150 to 200m depth with local maximum densities of 849 per 1000m³. This indicates a mean depth of maximum density below the sampling depth of 250m during the day, while the animals rise to feed during the night to the most productive waters near the pycnocline in nighttime. A strong diel vertical migration has been observed in previous studies between the ocean surface and 1000m depth (van der Spoel, 1973). Its maximum abundance was observed in 200 to 400m depth (Jung, 1973).

The average abundance of Creseis virgula (fig. 15) is highest during the day in the upper water layers. The maximum average values of 45 per 1000m³ are reached in 25 to 50m depth, with locally observed values of 235 per 1000m³. Below 50m depth only few specimens were found and no Creseis virgula were found below the 100 to 150m depth strata. During the night Creseis virgula was only found in the 25 to 50m depth strata with very low densities of 8 per 1000m³. It seems as if Creseis virgula migrates to upper water layers during the day at Senghor Seamount. All in all only a quite small number of specimen of this species were collected, so this pattern might be only an artifact by chance. The observed pattern does not correspond to patterns observed by Stubbings (1932). He found Creseis virgula living in a mean day level of 206m and a mean day level of 98m.

If the Creseis virgula at Senghor Seamount really migrate upward during the day this might be to evade predation pressure of planktonic predators migrating downward during the day.

The average abundance of Limacina bulimoides (Fehler! Verweisquelle konnte nicht gefunden werden.. 16) is highest during the day in water layers between 50 and 100m. The maximum average values of 11 per 1000m³ are reached in 50 to 75m depth, with locally observed values of 24 per 1000m³. No specimens were found above the 50 to 75m depth strata. During the night only a single specimen of Limacina bulimoides was found in the 150 to 200m depth strata, resulting in an average density of 2 per 1000m³ in this layer. This pattern is similar to that of Creseis virgula, but in Limacina bulimoides the mean day level is not near the surface but near the pycnocline whre food supply is rhichest. Again only a small 14 number of specimen was collected which makes errors by chance possible. In the Florida Current van der Spoel (1967) observed a mean day level of 90m and a mean night level at 80m. The deeper pycnocline in the Florida Current explains its deeper occurance in the Florida Current.

The average abundance of Limacina lesueurii (fig. 18) is highest during night with 1000 per 1000m³ in 75 to 100m depth. The highest locally observed density was 2152 per 1000m³ at this depth, but during the day. During the night below this depth only low numbers were found (0 to 233 animals per 1000m³). Similarly low numbers were found in the surface near water layers between 0 and 50m (0 to 302 per 1000m³). During the day the average density of Limacina lesueurii is highest in 75 to 100m depth 561 per 1000m³). Above this depth only very low average densities were observed (3 to 45 per 1000m³). Below 100m depth the locally observed density of Limacina lesueurii varied between 0 and 292 per 1000m³. This indicates a small diel vertical migration of Limacina lesueuri near Senghor Seamount. Limacina lesueuri is found in largest numbers just below the pycnocline.

A comparable small diel vertical migration was found by Wormelle (1962) in the Florida Current. There the mean day level is 103m while the mean night level is 85m.

Limacina inflata (fig. 17) is by far the most abundant pteropod species found during this observation. Highest average densities were observed during the night in the lower water layers, being highest in 100 to 150m depth with densities of 4835 per 1000m³, locally reaching 6838 per 1000m³. While there is a minimum of density in 50 to 100m depth, in the surface near water layers (0 to 50m depth) medium high average densities are reached (797 to 1306 per 1000m³). Limacina inflata seems to avoid the presence of Limacina lesueuri, that has its maximum density in 50 to 100m depth near Senghor Seamount. This distribution pattern of Limacina inflata and Limacina lesueuri is probably to be explained with avoiding of competition for food. During the day in all sampled water depth only low average densities of Limacina inflata were observed, being only slightly higher in the 25 to 50m depth layer with 605 per 1000m³. There is quite a high fluctuation in the observed local during the day values.

Limacina inflata probably has its mean day level below 250m depth in the waters near Senghor Seamount and is therefore found only in comparatively small numbers in the material, while they rise during the night for feeding. A comparably large diel migration was observed by Moore (1949) near the Bermudas. There the mean night level is 76m while the

15 mean day level is below 300m, but with a spread of more than 175m. In the Florida Current Limacina inflata occurs in 0 to 700m depth. Despite the large spread the mean day level of 236m and the mean night level of 232m are very near to each other in the Florida Current (van der Spoel, 1967).

The average density of Limacina trochiformis (fig. 19) varies a lot between the different observed stations and depth strata (Fehler! Verweisquelle konnte nicht gefunden werden.). Highest average as well as locally observed densities were found during the night in 75 to 100m depth with 35 and 57 per 1000m³. Medium densities were found above this layer, while no animals were found below 100m depth. During the day most specimens and highest average densities were found in the upper water layers. Highest average during the day density was observed in 25 to 50m depth with 12 per 1000m³, while highest locally observed density during the day was 56 per 1000m³ in 0 to 25m depth.

As in the species mentioned before where the distribution pattern indicates a vertical diel migration up to shallower waters during the day only quite a small number of specimen of Limacina trochiformis was found near Senghor Seamount. Therefore it is not sure if the observed pattern really represents the real behaviour of the species. Limacina trochiformis has a different pattern of diel vertical migration in the Florida Current (van der Spoel, 1967). Here the mean day level is 165m and the mean night level is at 99m depth.

Peraclis species (fig. 20) were found with maximum average densities of 47 per 1000m³ in 75 to 100m depth. The highest locally observed density was 67 per 1000m³ at this depth. Below this depth low numbers were found (0 to 9 animals per 1000m³), while above 50m depth and above the pycnocline no more specimens were found. During the day still no specimens were found shallower than 50m. The locally observed values of Peraclis density in the greater depth vary quite a lot, but the average values are quite similar varying between 7 and 15 per 1000m³ with highest locally observed densities in 100 to 150m depth with 51 per 1000m³. This indicates a diel vertical migration during the nighttime up to the pycnocline that is rich in food near Senghor Seamount. Van der Spoel (1976) mentions no observations of diel vertical migration for this meso- and bathypelagically distributed genus.

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3.2. Heteropoda

3.2.1. Species composition

At least four taxa of heteropods were collected using the IKMT (34 specimens). At Senghor Seamount (chart 5) 30 specimen were found. Atlanta were most common (60,00%), followed by Carinaria lamarcki (20,00%), Oxygyrus keraudreni (13,33%) and Pterotrachea sp. (6,67%). In the Fogo Brava Channel (chart 6) only four specimen were found. No members of the Genus Atlanta collected. Most common was Oxygyrus keraudreni (50%) followed by Pterotrachea (25%) and Carinaria lamarcki (25%).

As heteropods are generally warm water species the possible presence of upwelling of cold deep waters near Fogo Island indicated by the presence of the pteropods Diacavolinia limbata and Diacria atlantica `dark form´ the scarcity of heteropods at those stations might be explained. As Carinaria lamarcki and many Pterotrachea species are mesopelagic animals (Richter & Seapy, 1999) their presence at stations with possible upwelling is not surprising.

In the Multinet catches (chart 7) 368 specimens of heteropods were found. In those Atlanta the most common within the heteropods (90,76%). Oxygyrus keraudreni (7,61%) is the second most abundant heteropod, followed by Pterotrachea (1,09%) and Carinaria lamarcki (0,54%). Nevertheless the most diverse group of heteropods, the genus Atlanta, could not be identified to species level, as the shells were always destroyed. In a small sediment sample from Senghor Seamount the shells of at least three species of Atlanta were found (Atlanta inclinata, Atlanta inflata and Atlanta peroni).

3.2.2. Spatial Density Distribution near Senghor Seamount

When looking at the distribution of Heteropods at Senghor Seamount (figures 21), only Atlanta species were abundant enough to make a comparison between open ocean and seamount top. It can be seen that Atlanta species have their highest densities over the peak of the seamount, while they are most scarce on its flanks. Getting into the open ocean the abundance increases again. The high abundance of Atlanta over the top of the seamount is caused by the pumping effect of the taylor column. Many of the Atlanta drifting from the

17 open ocean near to the flank of the seamount they probably get into the ring current of the taylor column and are transported to the center of the ring current over the top of the seamount.

Figure 7. Atlanta at Senghor Seamount

3.2.3. Diel Vertical Migration

As in the pteropods the vertical distribution of the two most abundant heteropod taxa was investigated. In the figures the dots represent the abundances of the taxa at the single stations, while the bars are the average for all stations. The white line indicates the position of the pycnocline that was measured to be in around 50 to 60m depth.

Figure 22 Atlanta at Senghor Seamount Figure 23. Oxygyrus keraudreni at Senghor Seamount The average abundance of Atlanta sp. (fig. 22) at night is highest in the upper 50m of the water column, varying between 153 and 165 per 1000m³. Highest locally observed densities were in 25 to 50m with 275 per 1000m³. Below 50 m only low densities were observed, 18 varying between 0 and 46 per 1000m³. During the day the general pattern is similar, although the average surface values are lower (95 to 119 per 1000m³) with locally observed highest values of 304 per 1000m³. Below 50m the values are on average a little higher than during the night, with locally observed values varying between 0 and 83 per 1000m³. As nearly all Atlantids are epipelagic species (Richter & Seapy, 1999) their presence above the pycnocline fits to observations done by former researchers.

Oxygyrus keraudreni (fig. 23) was only found in waters shallower than 100m. During the night the highest average densities were observed. They were 35 per 1000m³, with highest locally observed values of 63 per 1000m³ in 25 to 50m depth. During the day the highest average densities, being less than a half of those in the night, were observed in the near surface waters with 13 per 1000m³ and locally observed densities of up to 52 per 1000m³. No specimens were found deeper than 75m.

Summary and directions for future research

The pelagic molluscan community at the Cape Verde Islands is quite diverse. A total 25 pteropod and at least 5 heteropod species were found, although the heteropod fauna probably is more diverse. The most common pteropod species are Limacina inflata, Limacina lesueuri and Clio pyramidata f. lanceolata. The most common heteropods are species of the genus Atlanta, which could not be identified further. Nothing can be said of janthinids and nudibranchs, that were not found, probably due to the collecting methods used.

According to the IKMT-stations the pelagic molluscan communities at Brava Island and Senghor Seamount are different. There is also an indication for upwelling near Brava Island and the influence of Central Waters at Senghor Seamount. The vertical distribution patterns at Senghor Seamount demonstrate that pelagic molluscan species accumulate in different water layers. It is also shown that many species migrate to shallower waters during the night.

More detailed studies are necessary to resolve fine-scale distribution patterns of zooplankton/nekton assemblages at Cape Verde Island seamounts and islands. Especially the influence of seamounts on the distribution patterns of pelagic gastropods and their influence on the vertical water layers preferred at night and daytime would be proposed targets for further detailed studies. 19 References

Bontes, B. and S. van der Spoel. 1998. Variation in the Diacria trispinosa groups, new interpretation of colour patterns and description of D. rubecula n. sp. (). Bulletin Zoölogisch Museum Universiteit van Amsterdam 16: 77-84 Fiekas, V., Elken, J., Muller, T. J., Aitsam, A., & Zenk, W. 1992. A view of the canary basin thermocline circulation in winter. Journal of Geophysical Research - Oceans 97: p

12495-12510.

John, H. C. & Zelck, C. (1997). Features, boundaries and connecting mechanisms of the Mauritanian Province exemplified by oceanic fish larvae. Helgoländer

Meeresuntersuchungen 51: p 213-240.

Jung, P. (1973). Pleistocene pteropods - Leg 15, site 147, Deep Sea Drilling Project. Initial Reports

Deep Sea Drilling Project, 15: 753-767

Leyen, A. van and S. van der Spoel, S. 1982. A new taxonomic and zoogeographic interpretation of the Diacria quadridentata group (Mollusca, Pteropoda). Bulletin Zoölogisch Museum Universiteit van Amsterdam 8: 101-119. Meisenheimer, J., 1905. Pteropoda. Wissenschaftliche Ergebnisse der deutschen Tiefsee-Expedition auf dem. Dampfer'Valdivia', 9, 1-314 Rampal J. 2002. - Biodiversité et biogéographie chez les Cavoliniidae (Mollusca, Gastropoda, , ). Régions faunistiques marines. Zoosystema 24 (2) : 209- 258. Richter, G. and Seapy, R.S., 1999. Heteropoda. In: Boltovskoy D. (ed) South Atlantic zooplankton. Backhhuys, Leiden, The. Netherlands, pp 621–647 . Rolán, E., 2005. Malacological fauna from the Cape Verde Archipelago: 1. Polyplacophora and Gastropoda. ConchBooks: Hackenheim, Germany. ISBN 3-325319-73-2. 455 pp. Seapy, R.R., C.M. Lalli and F.E. Wells 2003. Heteropoda from Western Australian waters. The Marine Flora and Fauna of Dampier, Western Australia. Western Australian Museum, Perth., 513-546. Spoel, S van der and Dadon J.R., 1999. Pteropoda. In: Boltovskoy D. (ed) South Atlantic zooplankton. Backhhuys, Leiden, The. Netherlands, pp 649–706 . Spoel, S. van der, 1967. Euthecosomata: A Group with Remarkable Developmental Stages (Gastropoda, Pteropoda). J. Noorduijn en Zoon N.V.,Gorinchem. 1-375. Spoel, S. van der, 1973. Growth, reproduction and vertical migration in Clio pyramidata Linnaeus, 1767 forma lanceolata (Lesueur, 1813) with notes on some other Cavoliniidae (Mollsuca, Pteropoda). Beaufortia 21 (281): 117-134. Spoel, S. van der, 1976. Pseudothecosomata, Gymnosomata and Heteropoda (Gastropoda) 20 Bohn, Scheltema & Holkema, Utrecht. Spoel, S. van der, J. Bleeker and H. Kobayasi. 1993. From Cavolinia longirostris to twenty-four Diacavolinia taxa, with a phylogenetic discussion (Mollusca, Gastropoda). Bijdragen tot de Dierkunde 62: 127-166. Tesch, J.J., 1906. Heteropoden der Siboga Expedition. Siboga Reports 51: 1–112 Tesch, J.J., 1946. The thecosomatous pteropods. I. The Atlantic. Dana Report. 28, 1-81. . Tesch, J.J. (1949). Heteropoda. Dana Report 34: 1–53.

Acknowledgements

My thanks to the participants of Poseidon Cruise Pos 320-2 that carried out the sampling of the material used in this study.

Special thanks to Dr. Uwe Piatkowski for guidance and help whenever needed.

21

Attachment

Station No. Position begin and end time [UTC] sampling depth [m] bottom depth [m] Sampling gear 05/84 17°00.17'N, 22°10.17'W 15:48 250‐0 3350 Multinet 17°01.46'N, 22°09.00'W 16:26 3315 06/86 17°11.62'N, 21°58.14'W 19:38 250‐0 169 Multinet 17°09.89'N, 21°58.81'W 20:11 622 07/90 17°11.86'N, 21°57.52'W 09:57 100‐0 110 Multinet 17°12.30'N, 21°56.74'W 10:13 348 08/96 17°14.77'N, 21°53.41'W 14:11 250‐0 2055 Multinet 17°13.27'N, 21°54.79'W 14:45 1134 09/99 17°23.69'N, 21°44.92'W 17:47 250‐0 3397 Multinet 17°25.19'N, 21°44.56'W 18:20 3391 10/105 17°25.58'N, 22°08.88'W 08:05 250‐0 3347 Multinet 17°24.00'N, 22°10.03'W 08:35 3343 11/108 17°13.46'N, 21°59.75'W 11:06 250‐0 565 Multinet 17°15.37'N, 21°59.37'W 11:44 1100 12/131 17°10.26'N, 21°56.66'W 11:03 250‐0 135 Multinet 17°11.19'N, 21°55.49'W 11:40 426 05/83 17°00.33'N, 22°10.12'W 14:09 200‐0 3342 IKMT 17°02.12'N, 22°08.39'W 14:59 3295 07/91 17°12.40'N, 21°56.42'W 10:29 130‐0 417 IKMT 17°11.60'N, 21°58.18'W 11:04 296 08/95 17°12.80'N, 21°55.40'W 13:06 200‐0 626 IKMT 17°14.74'N, 21°53.34'W 14:04 2066 09/100 17°25.28'N, 21°44.70'W 18:35 200‐0 3391 IKMT 17°22.53'N, 21°46.36'W 19:30 3386 10/104 17°23.90'N, 22°10.07'W 07:00 200‐0 3346 IKMT 17°25.70'N, 22°08.76'W 07:53 3347 11/109 17°13.46'N, 21°59.83'W 12:07 200‐0 603 IKMT 17°15.93'N, 21°59.13'W 12:57 1509 12/112 17°10.64'N, 21°56.41'W 14:41 200‐0 151 IKMT 17°09.25'N, 21°57.50'W 15:22 725 06/130 17°11.04'N, 21°58.63'W 09:23 200‐0 463 IKMT 17°08.87'N, 22°00.42'W 10:10 1296 146 14°48.57'N, 24°33.28'W 21:45 500‐0 2721 IKMT 14°51.95'N, 24°34.35'W 22:54 1590 156 14°40.68'N, 24°51.29'W 20:34 500‐0 2927 IKMT 14°42.31'N, 24°53.55'W 21:44 2591 162 14°48.25'N, 24°44.18'W 15:05 500‐0 381 IKMT 14°49.16'N, 24°48.31'W 16:18 2138 165 14°47.37'N, 24°46.46'W 10:10 500‐0 1853 IKMT 14°49.97'N, 24°48.59'W 11:24 2415 167 14°45.82'N, 24°49.06'W 13:35 500‐0 2946 IKMT 14°48.18'N, 24°48.41'W 14:42 2352 170 14°44.61'N, 24°50.62'W 16:33 500‐0 2921 IKMT 14°48.10'N, 24°49.82'W 17:55 3025

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Chart 1. Station List

Taxa Station 05/83 Station 07/91 Station 08/95 Station 09/100 Station 10/104

N % N % N % N % N %

Cavolinia uncinata uncinata 0 0,00 0 0,00 0 0,00 5 6,67 0 0,00

Diacavolinia deshayesi 0 0,00 10 16,95 1 50,00 0 0,00 1 2,70

Diacavolinia sp. indet. 1 50,00 1 1,69 0 0,00 1 1,33 0 0,00

Diacria major 0 0,00 4 6,78 0 0,00 0 0,00 3 8,11

Diacria rampali 0 0,00 1 1,69 0 0,00 0 0,00 0 0,00

Diacria sp. indet. (trispinosa group) 0 0,00 0 0,00 0 0,00 1 1,33 0 0,00

Diacria trispinosa 0 0,00 1 1,69 0 0,00 0 0,00 0 0,00

Clio pyramidata f. lanceolata 0 0,00 39 66,10 1 50,00 68 90,67 3389,19

Clio recurva 0 0,00 3 5,08 0 0,00 0 0,00 0 0,00

Cavoliniinae sp. indet. 1 50,00 0 0,00 0 0,00 0 0,00 0 0,00

Thecosomata sp. indet. 0 0,00 0 0,00 0 0,00 0 0,00 0 0,00

Gymnosomata sp. indet. 0 0 0 1 0

Total Thecosomata 2 59 2 75 37

Taxa Station 11/109 Station 12/112 Station 06/130 Total

N % N % N % N %

Cavolinia uncinata uncinata 0 0,00 0 0,00 0 0,00 5 2,58

Diacavolinia deshayesi 4 66,67 0 0,00 0 0,00 16 8,25

Diacavolinia sp. indet. 0 0,00 0 0,00 2 33,33 5 2,58

Diacria major 0 0,00 0 0,00 0 0,00 7 3,61

Diacria rampali 2 33,33 0 0,00 1 16,67 4 2,06

Diacria sp. indet. (trispinosa group) 0 0,00 0 0,00 0 0,00 1 0,52

Diacria trispinosa 0 0,00 0 0,00 0 0,00 1 0,52

Clio pyramidata f. lanceolata 0 0,00 5 71,43 3 50,00 149 76,80

Clio recurva 0 0,00 1 14,29 0 0,00 4 2,06

Cavoliniinae sp. indet. 0 0,00 0 0,00 0 0,00 1 0,52

Thecosomata sp. indet. 0 0,00 1 14,29 0 0,00 1 0,52

Gymnosomata sp. indet. 0 0 0 1

Total Thecosomata 6 7 6 194

Chart 2. Pteropods in the IKMT-Hauls at Sengor Seamount

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Taxa Station 146 Station 156 Station 162 Station 165 Station 167 N % N % N % N % N % Cavolinia gibbosa f. flava 1 4,17 2 7,69 0 0,00 0 0,00 0 0,00 Cavolinia uncinata uncinata 0 0,00 0 0,00 0 0,00 1 7,14 1 8,33 Diacavolinia deshayesi 6 25,00 1 3,85 0 0,00 0 0,00 0 0,00 Diacavolinia limbata 2 8,33 0 0,00 0 0,00 0 0,00 0 0,00 Diacavolinia sp. indet. 0 0,00 0 0,00 3 42,86 2 14,29 0 0,00 Diacria atlantica 0 0,00 0 0,00 0 0,00 6 42,86 0 0,00 Diacria trispinosa 2 8,33 2 7,69 3 42,86 2 14,29 0 0,00 Clio cuspidata 1 4,17 0 0,00 0 0,00 0 0,00 0 0,00 Clio pyramidata f. lanceolata 12 50,00 21 80,77 1 14,29 2 14,29 2 16,67 Clio recurva 0 0,00 0 0,00 0 0,00 0 0,00 1 8,33 Cuvierina columnella f. atlantica 0 0,00 0 0,00 0 0,00 1 7,14 0 0,00 Cavoliniinae sp. indet. 0 0,00 0 0,00 0 0,00 0 0,00 8 66,67 Total 24 26 7 14 12

Taxa Total N % Cavolinia gibbosa f. flava 3 3,61 Cavolinia uncinata uncinata 2 2,41 Diacavolinia deshayesi 7 8,43 Diacavolinia limbata 2 2,41 Diacavolinia sp. indet. 5 6,02 Diacria atlantica 6 7,23 Diacria trispinosa 9 10,84 Clio cuspidata 1 1,20 Clio pyramidata f. lanceolata 38 45,78 Clio recurva 1 1,20 Cuvierina columnella f. atlantica 1 1,20 Cavoliniinae sp. indet. 8 9,64 Total 83

Chart 3. Pteropoda in IKMT-Hauls at Fogo-Brava Channel

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Taxa Station 05/84 Station 06/86 Station 07/90 Station 08/96 Station 09/99 N % N % N % N % N % Cavolinia gibbosa f. flava 2 0,21 0 0,00 2 0,16 0 0,00 0 0,00 Cavolinia inflexa f. imitans 0 0,00 0 0,00 0 0,00 6 1,01 0 0,00 Cavolinia sp. indet. 3 0,31 37 3,11 39 3,09 0 0,00 12 0,30 Cavolinia uncinata uncinata 17 1,76 3 0,25 0 0,00 1 0,17 1 0,02 Diacavolinia sp. 2 0,21 2 0,17 0 0,00 1 0,17 0 0,00 Diacria danae 0 0,00 0 0,00 0 0,00 1 0,17 0 0,00 Diacria atlantica 0 0,00 0 0,00 0 0,00 0 0,00 0 0,00 Diacria spec. (trispinosa group) 11 1,14 12 1,01 19 1,50 2 0,34 24 0,59 Hyalocylis striata 0 0,00 0 0,00 0 0,00 0 0,00 0 0,00 Styliola subula 0 0,00 0 0,00 0 0,00 3 0,51 0 0,00 Clio pyramidata f. lanceolata 348 35,99 232 19,50 213 16,86 55 9,29 340 8,36 Creseis acicula f. acicula 0 0,00 1 0,08 0 0,00 0 0,00 0 0,00 Creseis virgula 0 0,00 2 0,17 3 0,24 20 3,38 0 0,00 Cavoliniinae indet. 0 0,00 0 0,00 0 0,00 0 0,00 0 0,00 Limacina bulimoides 0 0,00 0 0,00 6 0,48 1 0,17 1 0,02 Limacina inflata 460 47,57 460 38,66 898 71,10 308 52,03 3495 85,96 Limacina lesueurii 90 9,31 411 34,54 59 4,67 178 30,07 129 3,17 Limacina trochiformis 7 0,72 13 1,09 2 0,16 2 0,34 4 0,10 Peraclis apicifulva 0 0,00 0 0,00 0 0,00 0 0,00 0 0,00 Peraclis reticulata 0 0,00 0 0,00 0 0,00 0 0,00 0 0,00 Peraclis sp. indet. 18 1,86 15 1,26 6 0,48 10 1,69 6 0,15 Cymbuliidae sp. indet. 2 0,21 1 0,08 1 0,08 2 0,34 1 0,02 Desmopterus papilio 3 0,31 0 0,00 1 0,08 0 0,00 0 0,00 Thecosomata sp. indet. 4 0,41 1 0,08 14 1,11 2 0,34 53 1,30 Gymnosomata sp. indet. 2 0 0 0 0 Total Thecosomata 967 1190 1263 592 4066 Filtered Volume [m³] 1177 1402 592 1402 1203

Taxa Station 11/108 Station 10/105 Station 12/131 Total N % N % N % N % Cavolinia gibbosa f. flava 0 0,00 0 0,00 0 0,00 4 0,04 Cavolinia inflexa f. imitans 6 1,19 0 0,00 0 0,00 12 0,12 Cavolinia sp. indet. 2 0,40 19 2,48 0 0,00 112 1,16 Cavolinia uncinata uncinata 0 0,00 0 0,00 0 0,00 22 0,23 Diacavolinia sp. 1 0,20 0 0,00 0 0,00 6 0,06 Diacria danae 3 0,59 0 0,00 0 0,00 4 0,04 Diacria atlantica 2 0,40 0 0,00 2 0,66 4 0,04 Diacria spec. (trispinosa group) 2 0,40 7 0,91 0 0,00 77 0,80 Hyalocylis striata 0 0,00 1 0,13 0 0,00 1 0,01 Styliola subula 1 0,20 0 0,00 0 0,00 4 0,04 Clio pyramidata f. lanceolata 55 10,87 81 10,57 41 13,44 1365 14,14 Creseis acicula f. acicula 0 0,00 0 0,00 0 0,00 1 0,01 Creseis virgula 10 1,98 27 3,52 7 2,30 69 0,71 Cavoliniinae indet. 0 0,00 1 0,13 0 0,00 1 0,01 Limacina bulimoides 4 0,79 2 0,26 4 1,31 18 0,19

25 Limacina inflata 56 11,07 465 60,70 14 4,59 6156 63,76 Limacina lesueurii 318 62,85 99 12,92 232 76,07 1516 15,70 Limacina trochiformis 4 0,79 6 0,78 0 0,00 38 0,39 Peraclis apicifulva 0 0,00 0 0,00 1 0,33 1 0,01 Peraclis reticulata 0 0,00 0 0,00 1 0,33 1 0,01 Peraclis sp. indet. 28 5,53 41 5,35 0 0,00 124 1,28 Cymbuliidae sp. indet. 8 1,58 11 1,44 0 0,00 26 0,27 Desmopterus papilio 3 0,59 1 0,13 0 0,00 8 0,08 Thecosomata sp. indet. 3 0,59 5 0,65 3 0,98 85 0,88 Gymnosomata sp. indet. 0 1 0 3 Total Thecosomata 506 766 305 9655 Filtered Volume [m³] 1652 1431 1099 9958

Chart 4. Pteropods in Multinet Hauls of the Senghor Seamount

Station Station Taxa Station 05/83 Station 07/91 Station 08/95 09/100 10/104 N % N % N % N % N % Atlanta sp. 0 0,00 3 100,00 0 0,00 8 72,73 3 50,00 Oxygyrus keraudreni 2 18,18 0 0,00 1 50,00 0 0,00 0 0,00 Carinaria lamarcki 1 9,09 0 0,00 1 50,00 1 9,09 3 50,00 Pterotrachea sp. 0 0,00 0 0,00 0 0,00 2 18,18 0 0,00 Total Thecosomata 0 3 0 8 3

Taxa Station 11/109 Station 12/112 Station 06/130 Total N % N% N% N % Atlanta sp. 0 0,00 2 66,67 2 100,00 18 60,00 Oxygyrus keraudreni 0 0,00 1 33,33 0 0,00 4 13,33 Carinaria lamarcki 0 0,00 0 0,00 0 0,00 6 20,00 Pterotrachea sp. 0 0,00 0 0,00 0 0,00 2 6,67 Total Thecosomata 0 3 2 30

Chart 5. Heteropods in IKMT-Hauls of Senghor Seamount

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Taxa Station 146 Station 156 Station 162 Station 165 Station 167 N % N % N % N % N % Oxygyrus keraudreni 0 0,00 1 100,00 0 0,00 0 0,00 0 0,00 Carinaria lamarcki 0 0,00 0 0,00 0 0,00 0 0,00 1 50,00 Pterotrachea sp. 0 0,00 0 0,00 0 0,00 0 0,00 1 50,00 Total 0 1 0 0 2

Taxa Station 170 Total N % N % Oxygyrus keraudreni 1 100,00 2 50,00 Carinaria lamarcki 0 0,00 1 25,00 Pterotrachea sp. 0 0,00 1 25,00 Total 1 4

Chart 6. Heteropoda in the IKMT-Hauls at the Fogo Brava Channel

Taxa Station 05/84 Station 06/86 Station 07/90 Station 08/96 Station 09/99 N % N % N % N % N % Atlanta sp. 15 93,75 31 93,94 67 87,01 19 95,00 82 85,42 Oxygyrus keraudreni 0 0,00 1 3,03 10 12,99 0 0,00 14 14,58 Carinaria lamarcki 0 0,00 1 3,03 0 0,00 0 0,00 0 0,00 Pterotrachea sp. 1 6,25 0 0,00 0 0,00 1 5,00 0 0,00 Total Heteropoda 16 33 77 20 96 Filtered Volume [m³] 1177 1402 592 1402 1203

Taxa Station 11/108 Station 10/105 Station 12/131 Total N % N % N % N % Atlanta sp. 54 94,74 56 94,92 10 100,00 334 90,76 Oxygyrus keraudreni 1 1,75 2 3,39 0 0,00 28 7,61 Carinaria lamarcki 0 0,00 1 1,69 0 0,00 2 0,54 Pterotrachea sp. 2 3,51 0 0,00 0 0,00 4 1,09 Total Heteropoda 57 59 10 368 Filtered Volume [m³] 1652 1431 1099 9958

Chart 7. Heteropods in Multinet-Hauls at Senghor Seamount

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