During the Netherlands Indian Ocean Program (NIOP) 1990-1995, benthic sampling was carried out to assess among other things, meiofauna densities, sediment community oxygen consumption and nematode assemblages. Four transects were sampled along the Kenyan coast in the Western Indian Ocean (WIO), from north to south: Kiwayu, Tana, Sabaki and Gazi. Samples were taken during two seasons: South East monsoon in June/July and the onset of the North East monsoon in November/December at depths: 20m, 50m, 500m, 1000m and 2000m. The samples were taken using a box core or lander. From each box two subsamples were taken to a depth of 5 cm using a plastic core 2.6 cm internal diameter. The two sub-samples from each box were mixed and fixed in hot, 4 % formaldehyde. In the laboratory, they were centrifuged in Ludox and nematodes were separated and enumerated. Between 100-200 nematodes were picked and processed using standard methods for slide preparation. They were then identified to genera level. Genus composition and distribution in the transects, in the two seasons and in depth were studied. Genus diversity was estimated using Hills diversity numbers (N0, N, N2, H’). From some selected families (Chromadoridae, Comesomatidae, Microlaimidae and Molgolaimus), the nematodes were identified to species level and where well represented they were described. For the selected families (Chromadoridae, Comesomatidae, Microlaimidae and Molgolaimus), genera and species composition and distribution were studied in the two periods, four transects and in depth. Species diversity was estimated using Hills diversity numbers and Hierarchical diversity. Environmental data from the same area (WIO) were obtained from published work or unpublished data analysed in other institutes. This included sediment community oxygen consumption (SCOC), DNA:RNA ratio, oxygen concentration and saturation, organic carbon, C:N ratio and sediment composition. Species description: from the three families, 42 new species were described and from 13 known species additional information provided. Of the 55 species described an average of 75 % of the nematodes were new. From Chromadoridae, 68 % were new species, Molgolaimus had 100 % new species, from Microlaimidae, 89 % were new species and from Comesomatidae, 75 % were new. This means that a lot of nematodes especially from the tropics are still not described both from the sublittoral and deep sea. Nematode density: nematode densities were highest at the shallowest stations (20m) which recorded 927 and 1350 ind/10cm2. At 50-2000m depth the density was 112 -669 ind/10 cm2. The low nematode density compared to the temperate region was associated to low food availability. The general trend in all transects was high nematode density at shallow depth which decreased upto 1000m, then increased slightly or decreased slightly upto 2000m. This trend was similar to the trend in oxygen concentration, and therefore oxygen was thought to be influencing nematode density. Nematode composition (in Transects and two periods): a total of 36 families were identified in June/ July and 38 families in Nov/Dec. The most dominant families were Xyalidae, Monhysteridae and Comesomatidae. Some temporal variation in family composition was observed in that Comesomatidae and Linhomoeidae had a higher relative abundance in June/ July compared to Nov/Dec, and Microlaimidae and Chromadoridae had a higher relative abundance in Nov/Dec compared to June/July. In June/July a total of 199 nematode genera were identified while in Nov/Dec 217 genera were identified. In June/July, 19 genera had at least 1.0 % which contributed to 50 % of the community. In Nov/Dec, 20 genera had at least 1.0 % and this contributed to 50 % of the community. The most dominant genera in both periods were Monhystera (7 %, 8 % respectively), Halalaimus (6 %, 5%), Sabatieria (5 %, 4%), Daptonema (4 %, 6 %) and Acantholaimus (3 %, 5 %). There was some temporal variation in genera distribution between the two periods in that during June/July Terschellingia had a higher dominance compared to Nov/Dec and in Nov/Dec Acantholaimus and Molgolaimus had a higher dominance compared to June/July. In the transects, the most dominant genera were similar, although the relative abundance varied from transect to transect. Some of the most dominant genera common in all transects were Monhystera (4-10%) and Halalaimus (4-6 %). Some temporal variation was observed in that again Terschellingia was the most dominant genus in only Sabaki in June/July and in the Training transect in Nov/Dec, Microlaimus was the second most dominant genus. Species (genus) diversity was estimated using Hills diversity numbers. Average diversity (Hill’s Ni) was lower in June/July (24) compared to Nov/Dec (26). In the transects, diversity was lowest in Sabaki in both periods (19 and 20 respectively). In Jule/July, genus diversity was highest in Tana (27) while in Nov/Dec it was highest in Training transect (29). Ecological groups (Depth): on the basis of nematode genera composition, TWINSPAN and DCA analysis identified four groups of stations. The four groups were A- stations at 20m, 50m and 200m depth, B- stations at 20m and 50m C- stations at 500m and 1000m and D- stations at 1000m and 2000m This shows that depth had a lot of influence on structuring the nematodes composition in the area. Besides depth, nematode composition may have been influenced by sediment composition. Station groups A and B were at nearly the same depths but group A stations had sediment with high sand content and very low silt content. Further analysis of the nematodes was therefore done with the groups separated on the basis of depth but group A and B were kept separate. Therefore, the following groups were considered for further analysis; 1 -A 2-B 3 -500m stations 4 -1000m stations 5 -2000m stations. Nematode composition in the ecological groups: in all five ecological groups, the most dominant families were Xyalidae (8-21 %), Monhysteridae (3-32 %), Chromadoridae (6-10 %), Comesomatidae (4-17 %) and Oxystomatidae (6-9 %). The most notable trend in family distribution in the groups was the increase in relative abundance of Monhysteridae and the decrease of Xyalidae from group 1 to 5. The two shallow station groups had also high relative abundances of Linhomoeidae (12 %) in group 1 and Microlaimidae (13 %) in group 2. Some of the most dominant genera were common in all the five groups of stations, although their relative abundance differed from group to group. The dominant genera were Monhystera, Sabatieria, Halalaimus, and Daptonema. Acantholaimus was also dominant but it was completely absent in group 1. The shallow station groups, 1 and 2, were distinctly different from the deeper stations in some of their dominant genera, while the deeper station groups 3, 4 and 5 had nematode genera composition that shifted gradually with depth from 3 to 5. In group 1, the most dominant genera were Daptonema (11 %), Terschellingia (9 %), Dorylaimopsis (7 %) and Halalaimus (4 %). In group 2, the most dominant genera were Microlaimus (6 %), Halalaimus (6 %), Daptonema (5 %) and Sabatieria (4 %). Terschellingia and Dorylaimopsis were typical for group 1 and Microlaimus was one of the typical genera for group 2, where they had higher relative abundance than in any other. The deeper station groups were characterised by varying proportions of Sabatieria (6 %, 7 %, 2 % respectively), Monhystera (5, 9, and 23 %), Acantholaimus (4, 5, 8 %) and Molgolaimus (4, 4, 2 %). Halalaimus had a high relative abundance in all the five groups (4, 6, 6, 6, 5 %). Genus diversity in the station groups was compared using Hills diversity numbers. It was highest in group 2 and 3 intermediate in group 1 and 4 and lowest in group 5. All indices indicated a low diversity at group 1, increased to a maximum at group 2 or 3 and then decreased to a minimum in group 5 stations. This trend in diversity followed the same trend as the average sediment sand content in the station groups. It appears like sediment sand content had an influence on nematode diversity. Besides, Spearman Ranked Order correlation (SROC) showed a significant positive correlation between genus diversity and sand and a negative correlation with silt content in the sediment. Genus diversity was a function of high number of genera combined with low dominance. In group 2 and 3, with the most diverse nematode communities, at least 129 and 121 genera (respectively) had 0.1 % relative abundance, and 50 % of the population was made up by 22 genera in group 2 and 19 genera in group 3. In group 1 and 4, at least 110 genera had relative abundance of 0.1 %, and 50 % of the community was made up by 15 genera in both groups. In group 5, only 94 genera had 0.1 % relative abundance and 50 % of the population was made up by only 10 genera. Temporal variation was observed in nematode composition and diversity in the deep station groups. In the 500m stations higher genus diversity (using Hills Ni) was observed in June/July (32) compared to Nov/Dec (28). In the 2000m stations, higher genus diversity was observed in Nov/Dec (19) compared to June/July (18). Trophic composition Wieser’s classification of nematodegenera into, selective deposit feeders (la), non-selective deposit feeders (lb) epistrate feeders (2a) and predator/omnivore feeders (2b) were analysed in the ecological groups for trophic composition. Ecological groups 1 and 2 had almost similar trophic composition (la, lb and 2a: 28-33 %), ecological groups 3 and 4 had almost similar (la was dominant with 38 %) and group 5 was close to the last two albeit some minor differences (lb was dominant with 41 %). In all the ecological groups, trophic category 2b contributed to less than 10% of the nematode population. Species distribution: Chromadoridae :19 genera and 81 species were identified. The most dominant genera were Acantholaimus (49%) followed by Dichromadora (14 %) and Actinonema (10 %). Except for Acantholaimus that increased in relative abundance with increase in depth, most other genera of the Chromadorids were more dominant at the shelf than on the slope stations. A high number of morphospecies (species identified on the basis of qualitative morphological characters alone without measurements because sometimes only juveniles were available) of Acantholaimus (38) were recorded leading to a high species richness in the slope stations. However, the number of genera was higher in the shelf than the slope stations. All transects recorded higher diversity in Nov/Dec compared to June/July period. The Northern most transect had higher species diversity compared to the southern one, although a south-north trend was not quite clear. Generally, species diversity seemed to increase with increase in depth while genera diversity showed the opposite trend. This kind of a trend may have been mainly due to the distribution of Acantholaimus suggesting that Acantholaimus is very important in this area. Comesomatidae 12 genera and 44 species were identified in the family Comesomatidae. The family was dominated by the genus Sabatieria (40%) followed by Cervonema (20 %) and Dorylaimopsis (15 %). The first two were the most dominant in the deeper station groups while the last one was dominant at the shallow station group 1. The number of genera was highest in station groups 1 and 3 while the number of species was highest in group 3 and 4 stations. Hills diversity (Ni) index showed that November/ December period had a higher species diversity compared to June/July period. Species diversity was highest in group 3 station and decreased with increase in depth. Among the two shelf stations, group 1 had a higher diversity than group 2. Thus, most Comesomatidae had a preference for fine sand with high silt content rather than course sand. In November/December higher diversity was observed in Kiwayu compared to Gazi while in June/July, Gazi had a higher diversity compared to Kiwayu. Microlaimidae + Molgolaimus 9 genera and 41 species were identified. The most dominant genera were Microlaimus (35 %) and Molgolaimus (35 %). Molgolaimus had a higher relative abundance during the first campaign (in June/ July) than in the second campaign while Microlaimus had a higher relative abundance in November/December compared to June/July period. Microlaimus dominated the two shallow station groups 1 and 2 while Molgolaimus dominated station groups 3 and 4. Overall diversity was highest during November /December compared to June/July. In June/July campaign, Kiwayu had the highest diversity index while in November/December, the diversity index increased from North to South. The diversity index N0 was highest in group 3 stations and decreased with depth while Ni was highest in group 2 stations and decreased with increase in depth. Among the Microlaimidae, most genera and species appear to prefer coarse sands rather than sand with high silt content. This work presents the first comprehensive study of free living nematodes of the Indian Ocean. |
