The endobenthic deposit-feeder polychaetes belonging to the Capitella capitata (Fabricius, 1780) species complex have been reported worldwide in polluted and organically enriched marine and estuarine sediments, often being referred to as bioindicators (e.g. Pearson and Rosenberg 1978, Méndez et al. 1997,2000). The Capitella capitata complex consists of at least 50 sibling species, with about 13 being described from laboratory cultures (Grassle and Grasle 1976, Blake 2009). The non-interbreeding, morphologically similar cryptic species of this complex are distinguishable mainly by protein variation and ecophysiological, developmental and reproductive features (Wu et al. 1991, Linke-Gamenick et al. 2000a, Adkins and Schulze 2011). However, only a few have been described in detail (Méndez 2016).

The genus Capitella mostly includes opportunistic r-selected species able to produce either free-swimming planktotrophic or benthic lecithotrophic larvae (Grassle and Grassle 1974, Méndez 2002, Adkins and Schulze 2011) and showing a wide geographical and interspecific variety of reproductive strategies (Méndez et al. 2000). The latter include planktotrophy, lecithotrophy and direct development, as well as hermaphroditism and poecilogony, which give rise to differences in size and duration of the life stages, number of brooded embryos, and ciliation patterns in metatrochophore larvae (Méndez et al. 2000, 2019, Méndez 2016).

Nevertheless, the life cycle of the different species follows a similar pattern. With faecal materials, substrate and potential food, females construct brood tubes that are open at both ends to allow oxygenation. The fertilized eggs are brooded until the trochophore stage (Reish 1980). In species with planktotrophic development they are then directly hatched (then progressing to the metatrochophore stage in the water column), whereas in lecithotrophic species they stay inside the tube until they reach the metatrochophore stage (George 1984). The juvenile stage can be reached inside the tube (direct development) or in the sediment after settling of metatrochophores. Juveniles are already vermiform, with a complete segmentation, a clear distinction between thorax and abdomen and a chaetal arrangement consisting of capillaries in the first three segments and hooded hooks in the subsequent ones (Méndez 1995). Juveniles are first transparent, and then become pink and red because of haemoglobin production until they become immature adults (Méndez 2002, 2006); a change from intense red to greyish red indicates that specimens are becoming old (Méndez 2006). Adult males bear genital spines between chaetigers 8 and 9 (Reish 1980). Immature females bear paired, yellowish, sac-like ovaries suspended by mesenteries in the ventral coelomic cavity throughout the mid-body segments (Bellan et al. 1972, Eckelbarger and Grassle 1983), which become visibly white when mature (Méndez 2002, 2006, Méndez and Barata 2015).

Field and laboratory studies have demonstrated that growth depends strongly on environmental food availability (Forbes and Lopez 1990, Qian and Chia 1992, Linton and Taghun 2000, among others) and quality (Qian and Chia 1991, Hu et al. 2003, Ramskov and Forbes 2008). The presence of high amounts of food accelerated growth and offspring production in a species of Capitella and increased fecundity (because their generation time became shorter), while limiting food-induced growth delays (thus increasing generation time), and egg size became food-quality mediated (Qian and Chia 1992). Such a key role of organic matter in the physiology of the species of Capitella allows us to focus our study on determining the effects of two different organic content conditions on mortality, growth and development of two populations of Capitella in laboratory conditions.

The two populations of Capitella studied here came from different localities and living conditions. Capitella teleta, an opportunistic species originally inhabiting very polluted environments (Blake et al. 2009), came from a laboratory culture held for more than 40 years (Ramskov et al. 2009). Capitella sp. came from natural population inhabiting a low organically enriched environment and was acclimatized to laboratory conditions for a brief period just before the experiments started. Capitella teleta showed females, males and hermaphrodites, while all Capitella sp. were females, hindering developmental comparisons. Capitella teleta, as described by Blake et al. (2009), was smaller than Capitella sp., a prostomium flattened dorsoventrally (conical in Capitella sp.), eyespots in adults (absent in Capitella sp.), capillary chaetae in chaetigers 1-7 (1-4/7 in Capitella sp.) and a pygidium with two rounded lobes and a narrow dorsal slit-like notch (with a large simple rounded and dark lobe in Capitella sp.).

Capitella sp. did not produce mature males and females or brooding females, and did not develop reproductive structures in laboratory conditions, which prevented both its formal description and proper comparisons with other morphologically described sibling species from close locations, such as Capitella sp. A from Les Cases d’Alcanar, Tarragona, a town less than 20 km from the Trabucador Beach (Méndez 2016) or Capitella capitata (Méndez 1995) and Capitella sp. B (Méndez 2002), both from Barcelona. Our Capitella sp. is probably the same species as that of a field population from disturbed sands (OM <1%) from the Trabucador Beach showing females with brooding tubes and males with modified chaetae, with continuous reproduction over one year studied by Martin and Grémare (1997). These authors used the width of the fourth thoracic setiger to infer biomass (instead of total body length), which prevented proper comparisons with our Capitella sp. However, compared with the populations from nearby (i.e. less than 3 km away) organically enriched muds, those in disturbed sands showed more stable and less productive populations, as well as much lower biomasses and densities (<0.4 vs. >15 g of dry weight m^-2 and <5000 vs 170000. ind m^-2, respectively) and smaller individuals (0.2 µg dry weight ind^-1 vs. 0.5 µg dry weight ind^-1) (Martin and Grémare 1997), which strongly supports not only the key role of OM in structuring the populations but also its influence in their responses at individual level.

During our experiments, the weekly replacement of sediments provided enough food for the two species to feed, as demonstrated during their acclimation and in previous stock cultures of C. teleta (Méndez et al. 2013) and Capitella sp. A (Méndez 2016). Also, the sediment in the original stock culture of C. teleta had enough food to feed all worms (OM=3.00±0.21%) (Ramskov et al. 2009), a similar value to that used in their experiments (maximum OM=3%) (Ramskov and Forbes 2008).

Capitella teleta and Capitella sp. showed physiological differences in our two treatments, which were reflected in different mortality, growth and development. The species of the C. capitata complex have high physiological and morphological variability, with even sibling offspring from a single female having different responses to identical environment conditions (Qian and Chia 1991). However, in general terms, juveniles and adults of C. teleta survived longer than those of Capitella sp., which agrees with previous observations leading C. teleta to be considered the most tolerant species among the C. capitata complex (Grassle and Grassle 1974, Tsutsumi 1987, Forbes et al. 1994). Its high tolerance to adverse conditions may explain its higher survivorship under NOES than under OES. Its generation time in cultures has been reported to vary, depending on food abundance and temperature, from 20 days (Tenore and Chesney 1985) to 30 to 40 days (Grassle 1984, Whitlatch and Zajac 1985, Tsutsumi et al. 1990), and can reach up to 147 days (Linke-Gamenick et al. 2000b), which agrees with the high juvenile and adult survivorship in our treatments.

Capitella teleta (like Capitella sp. I) may be found at low densities in organic-poor sediments, where they only produce small numbers of larvae (Grassle and Grassle 1974, 1976, Tsutsumi et al. 1990); this is in agreement with the scarcity of specimens in the studied field population of Capitella sp., with only 0.51% of OM. The high variability in mortality and growth of Capitella sp. could be attributed to a failure in acclimatizing to laboratory conditions, which may also explain why some worms survived long under NOES, where the organic content was similar to that of their natural habitat. However, the extremely high survivorship shown by two juveniles (164 and 254 days) and one adult (279 days) under OES suggests a higher longevity, comparable to C. capitata (one year lifespan; Warren 1976), C. capitata (nine months lifespan; Lardicci and Ceccherelli 1994), Capitella sp. L (one year lifespan; Gamenick and Giere 1994) and Capitella sp. Y, (six months on average, two years and a half of maximum survivorship; Méndez 2006).

Although juvenile and adult growth was highly variable and negative for both species, all juveniles showed higher shrinkage in NOES than in OES, as expected owing to relationships between growth and environmental food availability (Tenore 1977, Tsutsumi et al. 1990, Linton and Taghun 2000). Though food content in the experiments proved to be enough to feed adults, the higher size reduction of adults of C. teleta in OES could be attributed either to food quality (Quian and Chia 1991, Hu et al. 2003) or to investing energy in developing sexual structures rather than growing (Ramskov and Forbes 2008).

Our observations confirm the importance of organic enrichment for development and reproduction in the species of the Capitella complex (Grémare et al. 1988, 1989, Marsh et al. 1990), with juveniles of Capitella sp. under NOES not reaching the adult stage, and no adults under NOES developing new sexual structures, while juveniles of Capitella sp. under OES reached the immature adult stage and juveniles and immature adults of C. teleta under OES reached maturity.

The presence of ovaries and genital spines in an individual of C. teleta under OES confirmed its hermaphroditism (Petraitis 1990, Blake et al. 2009, Ramskov et al. 2009). Most species of the Capitella complex appear to be only protandrous (having genital spines first and then also white ovaries), while protogyny (having ovaries first and then also genital spines) seems to be rarer (Grassle and Grassle 1977, Petraitis 1990, Ramskov et al. 2009). However, protogynous specimens appeared when the species was exposed to fluoxetine, a beneficial result as protogyny could be an excellent strategy for ensuring the population survivorship in natural conditions (Méndez et al. 2013, Méndez and Barata 2015). However, the fact that one immature female developed genital spines during our experiments suggests that other factors (e.g. type of sediment or artificial food) could also trigger protogyny.

The scarcity of mature males and females and brooding females in the field population of Capitella sp. prevented progeny production and made it impossible to follow the development of the species in the laboratory. Although some juveniles under OES showed yellow ovaries (immature stage), none of them developed white ovaries (mature stage). In some species of the complex, the period of storage of coelomic oocytes in females is minimal when there are mature males in the culture (Eckelbarger and Grassle 1983), which could explain the females’ delay in or lack of maturity during our experiments. Knowing the life cycle of Capitella sp. could improve the design and interpretation of experiments testing environmental stress effects at individual and population levels, which would require further future laboratory studies.

Our results confirm the role of sediment organic content in the physiological responses of benthic detritivores, with juvenile and adult survival, growth and development of both C. teleta and Capitella sp. being impaired under NOES. Moreover, juveniles and adults of C. teleta under OES developed sexual structures, including two protandrous and one protogynous individuals. Finally, we suggest that the negative growth rates in worms under OES could be better attributed to food quality or to investing energy in developing sexual structures rather than in growing.