Contributions to Zoology, 86 (3) – 2017Daniel Martin; Miguel A. Meca; João Gil; Pilar Drake; Arne Nygren: Another brick in the wall: population dynamics of a symbiotic species of Oxydromus (Annelida, Hesionidae), described as new based on morphometry
Results

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Population size-structure

A total of 275 worms were collected during this study, 246 infesting S. plana (241 during the seasonal monitoring, five in January 2013), 26 in M. pellucida (plus three outside the host, but in the same container) (Table 9). Sizes ranged from 1.8 to 3.6 mm. Small-sized worms seem to be better represented during late autumn, but also in winter, while large worms were more or less constantly present throughout the study period, except during late winter and early spring (Fig. 4).

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Table 9. Oxydromus okupa sp. nov. Synthesis of the monthly captures, total and infested number of hosts, percentage of ripe females, and prevalence during the study period.

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Fig. 4. Oxydromus okupa sp. nov. Size-frequencies of the specimens hosted by Scrobicularia plana from April 2011 to May 2012, and by Macomopsis pellucida in January 2013. 2011: April, n = 16; June, n = 22; July, n = 15; August, n = 10; September, n = 17; October, n = 11; November, n = 28; December, n = 49. 2012: January, n = 23; February, n = 20; March, n = 8; April, n = 7; May, n = 15; 2013: January, n = 26.

Ripe females occurred during the whole period except in April 2012, and were always among the largest size-classes (2.2 to 3.6 mm) (Fig. 4). The highest percentages occurred in mid spring and summer, being August 2011 the single month during which the proportion ripe females vs. non-sexed adults sexes was 1:1 (Fig. 5; Table 9). The lowest percentages occurred in autumn and winter (Fig. 5; Table 9).

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Fig. 5. Oxydromus okupa sp. nov. Monthly prevalence of ripe females vs. total prevalence in the infested Scrobicularia plana.

A total of 6,917 specimens of S. plana and 39 of M. pellucida were collected (Table 9). Length in S. plana ranged from 20 to 40 mm, with the exception of one infested host measuring 40.7 mm. However, bivalves with intermediate lengths (i.e., 26-36 mm) occurred during the whole year, being always the most abundant and also the most infested ones (Fig. 6). The most balanced size class frequency distribution occurred in September 2011. In M. pellucida, the size-class range was restricted to 20-30 mm length, and the most infested ones were slightly smaller (20-28 mm) than in S. plana (Fig. 6).

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Fig. 6. Size-frequencies of the total vs. infested Scrobicularia plana from April 2011 to May 2012 and Macomopsis pellucida in January 2013 (n = 39). Monthly total number of individuals of S. plana: 2011: April, n = 300; June, n = 200; July, n = 94; August, n = 100; September, n = 108; October, n = 99; November, n = 99; December, n = 124. 2012: January, n = 100; February, n = 98; March, n = 150; April, n = 102; May, n = 100.

Overall, there was a non-significant size correlation between O. okupa sp. nov. and the host S. plana (Pearson coefficient = 0.123, p = 0.067). The monthly trends were also non-significant (Pearson coefficient = -0.375 to 0.351, p = 0.103 to 0.818), except for a positive correlation in April 2012 (Pearson coefficient = 0.870, p = 0.011). Conversely, O. okupa sp. nov. / M. pellucida symbiont-host pairs (26) collected in January 2013 showed a significant, positive size correlation (Pearson coefficient = 0.400, p = 0.021).