Early life history and recruitment of the tropical eel Anguilla bicolor pacifica, as revealed by otolith microstructure and microchemistry

Otolith microstructure and microchemistry of the tropical eel Anguilla bicolor pacifica Schmidt were examined in glass eels collected at the mouth of the Dumoga River, North Sulawesi Island, Indonesia. Ages of the glass eels examined (age at recruitment) ranged from 124 to 202 d (167 ± 19.3 d; mean ± SD), hatching being estimated as having occurred between November 1995 and March 1996. Otolith increment widths markedly increased from age 101 to 172 d (135 ± 18.2 d; mean ± SD), coincident with a drastic decrease in otolith Sr:Ca ratios, suggesting that metamorphosis began during that period. The duration of metamorphosis was estimated as 20 to 40 d, on the basis of otolith microstructural characteristics. The fluctuation patterns in otolith increment widths and Sr:Ca ratios were similar to those of the temperate Japanese eel A. japonica. Received: 20 May 1998 / Accepted: 7 October 1998     

Differences in the early life history of the Australasian shortfinned eel Anguilla australis from Australia and New Zealand, as revealed by otolith microstructure and microchemistry

Otolith microstructure and microchemistry were examined in juveniles of Australasian shortfinned eel, Anguilla australis, from Australia and New Zealand. Patterns in otolith increment widths were similar in these two stocks. A marked increase in otolith increment width was found from age 138 to 198 d (mean ± SD: 164 ± 18.6 d) from Australia, 161 to 208 d (185 ± 17.3 d) from the western coast of New Zealand and 161 to 211 d (187 ± 18.9 d) from the eastern coast. These changes coincided with a drastic decrease in otolith Sr:Ca ratios, suggesting that metamorphosis began at these ages in each species. The duration of metamorphosis estimated from otolith microstructure was the same in the different stocks, 17 to 41 d (mean ± SD: 27 ± 5.4 d). Ages at recruitment were 186 to 239 d (mean ± SD: 208 ± 17.4 d) from Australia, 214 to 263 d (232 ± 19.8 d) from the western coast of New Zealand and 208 to 266 d (237 ± 20.0 d) from the eastern coast. There was a significant difference in the age between the Australian stock and the two stocks from New Zealand. No significant difference was found in the age between the latter two stocks. The difference in the period for the recruitment between Australian and New Zealand stocks seems to reflect their geographical separation. Received: 1 March 1999 / Accepted: 19 July 1999     

RAPD-PCR investigations of systematic relationships among four species of eels (Teleostei: Anguillidae), particularly Anguilla anguilla and A. rostrata

 We examined the genetic identities of 49 individuals of four species of eels in the genus Anguilla Shaw, A. anguilla, A. japonica, A. reinhardti and A. rostrata, using the random(ly) amplified polymorphic DNA (RAPD)-PCR technique. We used 15 random decamer primers. Out of the 22 148 amplification products detected, 454 markers were evaluated using Nei's distance coefficient, two numerical methods (neighbor-joining, UPGMA), and phylogenetic analysis using parsimony (PAUP). Both numerical and parsimony methods were congruent and confirmed the specific status of all four examined Anguilla species. In all dendrograms, all individuals of each of the species clustered together within four closed groups with highly significant bootstrap values (between 97 and 100%). Additionally 1 to 18 species-specific monomorphic diagnostic fragments were detected in three of the species; none were detected in A. anguilla. Mean interspecific genetic distances ranged from 0.384 to 0.559, with the minimum between A. anguilla and A. rostrata and the maximum between A. anguilla and A. reinhardti. Individuals collected at one locality and belonging to a single species did not cluster together, and neither were any diagnostic monomorphic fragments found for individuals of single localities. This probably reflects a random dispersal of larvae in ocean currents before they reach coastal waters. Among all possible species pairs, only the two Atlantic species were consistently clustered in all dendrograms, with highly significant bootstrap values (100%). Additionally, we detected ten diagnostic markers for this pair of eel species. From RAPD data, we suggest a phylogram which was routed by the descending analysis method: A. reinhardti appears to have been the first species to diverge from a putative common ancestor of the four eel species. Later, A. japonica separated from the hypothetical ancestor of the Atlantic species, which then split recently into A. anguilla and A. rostrata. The close relationship of A. anguilla and A. rostrata is indicated by their low genetic distance (GD = 0.384) and a high degree of shared RAPD fragments (SF = 71.2%); this compares with the following means for all other species pairs: ( = 0.531,  = 44.2%). The prerequisites for using RAPD data for the reconstruction of phylogenies are discussed. Received: 2 September 1999 / Accepted: 8 May 2000     

Changes in otolith microstructure and microchemistry during larval development of the European conger eel (<Emphasis Type="Italic">Conger conger</Emphasis>)

Otolith microstructure and chemical composition (Sr:Ca ratios) of the European conger eel (Conger conger) were examined during the larval developmental stages by scanning electron microscopy and wavelength dispersive spectrometer. Back-calculated hatching dates from the otolith microstructure of the developing leptocephali indicate a protracted spawning season from December to June. The early age of our developing specimens captured south of the Azores Islands suggests that the conger eel has another spawning area closer to Azores than the Mediterranean. Otolith increment width, which was relatively constant and narrow in the developing leptocephalus stage, increased sharply at age 170-250 days. Sr:Ca ratios in the otolith, which increased during the developing leptocephalus stage, showed a rapid drop coinciding with the increase in increment width. These coincidental changes were regarded as the onset of metamorphosis for this species. A close linear relationship between the age at metamorphosis and otolith growth rate indicates that the faster-growing larvae metamorphose earlier, suggesting that somatic growth should play an important role in the timing of metamorphosis. As shown in earlier work, the existence of an otolith marginal zone with unclear rings during metamorphosis prevents an accurate estimate of the larval stage duration of this species.     

Microstructure of settlement-marks in the otoliths of tropical reef fishes

The morphology and ultrastructure of the otolith settlement-mark was examined in 44 tropical reef-fish species spanning nine families. A classification scheme based on similar otolith characteristics is presented. Three major categories are identified based on changes in increment width and optical qualities of the settlement-mark. Of the 44 species examined, 39 possessed “abrupt” settlement-marks (Type I) characterised by a rapid decrease in increment width (up to 50% reduction) over settlement. Type I settlement-marks were found in all nine families examined. The 39 species spanned the whole range of possible larval durations (Pomacentrus moluccensis, 15 d ± 0 SE; Naso hexacanthus, 91.2 d ± 2.97 SE). Four of the 44 species possessed “zonal” settlement-marks (Type II), featuring a band of increments that are wider than pre-settlement increments. Species in this category are the labrids Corisaygula, Thalassoma bifasciatum, T. lunare and an unidentified acanthurid (Acanthurus sp. 2). One species of acanthurid (N. brevirostris) possessed a “gradual” settlement-mark (Type III), manifest as a gradual decrease in increment width during the settlement period. A possible fourth type was identified from the literature. Gnatholepis thompsoni and Coryphopterus glaucofraenum possessed a settlement-mark with increment widths that increased post-settlement. Available data suggest a poor relationship between the structure of the settlement-mark and the magnitude of metamorphosis (previously reported as internal and external morphological change). Evidence suggests that the increment profile over early development and the increment transitions associated with the settlement event are taxon-specific and may enable late-larval stage fishes to be identified to species level. Received: 21 May 1997 / Accepted: 3 February 1999     

Occurrence of the semi-catadromous European eel Anguilla anguilla in the Baltic Sea

Strontium (Sr) and calcium (Ca) contents in the otoliths of yellow and silver European eels [Anguilla anguilla (L.)] collected from coastal waters of the Baltic Sea and a freshwater lake in Sweden were examined by wavelength dispersive X-ray spectrometry with an electron microprobe. The mean Sr/Ca ratios from elver check to otolith edge were significantly higher for the eels from coastal waters (5.39 ± 1.09‰) than for those from the lake (0.71 ± 0.89‰). The evidence indicates that European eels in the Baltic Sea do not necessarily migrate into freshwater streams during the growth phase. Received: 30 September 1999 / Accepted: 6 April 2000     

Chemical composition and growth indices in leptocephalus larvae

 Leptocephali grow at extremely high rates (>1 mm d⁻¹), but, unlike most fish larvae, leptocephali may remain in the plankton as larvae for several months before metamorphosing into the juvenile form. During their planktonic phase, leptocephali accumulate energy reserves in the form of glycosaminoglycans which are then expended along with lipid reserves to fuel metamorphosis. Otolith growth rates were determined using scanning electron microscopy for four species of leptocephali common in the Gulf of Mexico, Paraconger caudilimbatus (Poey, 1867), Ariosoma balearicum (Delaroche, 1809), Gymnothorax saxicola (Jordan and Davis, 1891), and Ophichthus gomesii (Castelnou, 1855). Proximate composition, RNA:DNA ratios and protein growth rates were examined with respect to mass, length and age. The leptocephalus growth strategy was strongly reflected in the growth indices. Mass (Y) in all four species increased with increasing age (X) according to the equation Y = aX ^b , where a is a species-specific constant and 1.05 < b < 2.40. The accumulation of acellular mass was evident in protein growth rates and RNA:DNA ratios, and was observed as a shift in increasing size from rapid growth in length to a greater increase in mass with age. These results suggest that the proportion of actively metabolizing tissue declines with size and is replaced by the metabolically inert energy depot: the glycosaminoglycans. Leptocephali can thus grow to large size very rapidly with minimal metabolic penalty, an unusual and successful developmental strategy. Received: 27 December 1999 / Accepted: 8 June 2000     

Microstructural growth in otoliths of conger eel (Conger myriaster) leptocephali during the metamorphic stage

Otolith growth during metamorphosis and some aspects of the early life history of conger eel (Conger myriaster) were determined as indicated from microstructure in otoliths of the leptocephali collected from Cheonsu Bay, Korea during May and June 1988. The leptocephali occurred from early May to late June in the study area. Larvae collected in early May were in the late leptocephalus stage, and the proportion of the metamorphosing leptocephali increased over time. Otoliths in the late leptocephalus stage showed a translucent zone only. Although the fish did not feed and the body length diminished during metamorphosis, the otolith continued to grow and, consequently, the opaque zone was formed outside the translucent zone. The inner translucent zone can be considered a leptocephalus zone, and the outer opaque zone a metamorphic zone. Assuming that the growth increments were deposited daily from hatching, the conger eel can be considered to have hatched between September and February. The number of increments in the inner hyaline zone ranged from 124 to 239, and was assumed to represent the number of days from hatching to the onset of metamorphosis. The duration of metamorphosis was estimated as 51 to 75 d based on the number of increments in the opaque zone at the end of the metamorphic stage.     

Growth history and age at recruitment of European glass eels (Anguilla anguilla) as revealed by otolith microstructure

The growth history and age at recruitment of Anguilla anguilla Linnaeus, 1758 were studied, based on growth increments in sagittal otoliths of glass eels and elvers collected from the eastern Atlantic coast in 1989 and in 1990. The maximum otolith radius varied with pigmentation stage. Deposition of the transition ring was complete at Stage VIA₀. The size of the leptocephalus growth zone varied as a function of site, increasing from south to north. The oceanic migration of the leptocephali required less than one year.     

Validation of otolith growth-increment periodicity in tropical gobies

 We assessed the efficacy of tetracycline, calcein and strontium chloride for validating the periodicity of otolith growth-increments in eight species of tropical marine gobies (family Gobiidae). We compared the number of fishes in which the otoliths were successfully marked when each of these chemicals was administered by intraperitoneal injection or immersion bath at a range of doses and immersion times. All three chemicals could produce a detectable mark in the otoliths of five reef-flat gobies, Asterropteryx semipunctatus, Amblygobius bynoensis, Istigobius goldmanni, Valenciennea muralis and Amblygobius phalaena; however, tetracycline injection at 50 mg kg⁻¹ is recommended because it produced a brighter otolith mark than calcein and is cheaper and quicker to detect than strontium chloride. Calcein immersion treatment of 125 mg l⁻¹ for 24 h was the most successful treatment for two estuarine gobiids, Favonigobius reichei and Glossogobius biocellatus. No treatment produced a detectable mark in the otoliths of the coral-dwelling goby Gobiodon histrio. Adequate care in the preparation of otoliths was found to be essential for detection of the validation mark produced by tetracycline and calcein. Otolith growth-increments were deposited daily in the seven species of goby for which validation was possible. Received: 15 October 1999 / Accepted: 15 June 2000     

Growth history and inshore migration of the tropical eel, Anguilla marmorata, in the Pacific

A comparative study of the otolith microstructure and microchemistry of Anguilla marmorata glass eels in the western North Pacific (Japan, Taiwan, the Philippines, Indonesia) determined the timing of metamorphosis and age at recruitment to freshwater habitats with a view to learning about the early life history and recruitment of this species of tropical anguillid eel, which has a wide range throughout much of the western Pacific and parts of the Indian Ocean. Three new samples (from Japan, Taiwan, Indonesia) were analyzed and statistically compared along with two other previously published samples that were analyzed using the same techniques. Ages at metamorphosis and recruitment, respectively, were 123ᆡ.4 days (mean-SD) and 154ᆥ.0 days in specimens from Japan, 116ᆢ.6 days and 145ᆣ.6 days in those from Taiwan, 120ᆡ.0 days and 154ᆡ.5 days in the Philippines stock and 132Nj.7 days and 159ᆟ.7 days, and 120ᆣ.6 days and 152ᆣ.2 days in the Indonesian stock. The average duration of the period of metamorphosis estimated from otolith microstructure was very similar (15-17 days) in the specimens from all locations. A close linear relationship was found between the ages at metamorphosis and recruitment at all locations, suggesting that individuals that metamorphosed earlier were recruited to freshwater habitats at a younger age. Back-calculated hatching dates ranged over about 6 months of the year, suggesting that this species may spawn throughout much of the year. It is hypothesized that specimens from all four sites are from the same spawning population originating in a spawning area in the North Equatorial Current of the western North Pacific.     

Use of the bomb radiocarbon chronometer for age validation in the blue grenadier Macruronus novaezelandiae

Accelerator mass-spectrometry was used to measure radiocarbon in the earliest formed portions of selected blue grenadier, Macruronus novaezelandiae, otoliths to provide a validation of fish-age estimates based on the quantification of opaque and translucent zones in otolith thin-sections. Δ¹⁴C data from blue grenadier otoliths were compared with previous estimates of Δ¹⁴C in seawater-dissolved inorganic carbon at similar latitutes, longitudes, and depths to link variation in otolith Δ¹⁴C to time. Minimum otolith Δ¹⁴C was −76.9 ± 7.7‰, indicative of pre-bomb radiocarbon levels below the surface mixed-layer at latitudes where juvenile blue grenadier are found. When plotted versus fish age estimated from otolith sections, the majority of the Δ¹⁴C data combined to define a curve reflecting the increase in bomb radiocarbon in temperate oceans of the Southern Hemisphere, indicating that age-estimation procedures based on otolith thin-sections are satisfactory for determining blue grenadier age. If otolith-section age estimates were correct, peak otolith Δ¹⁴C of 106.8 ± 7.9‰ occurred during the late 1960s, i.e. earlier than expected. This may be a manifestation of an increase in the mixed-layer depth associated with increased frequency of zonal westerly winds at this time. Received: 3 December 1996 / Accepted: 17 December 1996     

Role of protein in larval swimming and metamorphosis of Bugula neritina (Bryozoa: Cheilostomatida)

Larvae of the marine cheilostomatid bryozoan Bugula neritina (L.) were prevented from settling for 1, 4 and 8 h by mechanical agitation, following which settlement and metamorphosis success were examined. Settlement rates were significantly affected by swimming time, which decreased from 100% after 2 h to 93.7 ± 4.3% after 8 h. Similarly, metamorphosis to the feeding ancestrula was significantly impaired following a swimming time of 8 h, declining from 93.7 ± 4.3% after 1 h to 65.9 ± 7.0% after 8 h. The resultant colonies grew well for the first 3 wk, following which time, growth patterns became erratic. Growth rate was in all cases highly variable, and did not correlate with enforced swimming times. Larval protein composition was examined after 1, 4 and 8 h swimming time, and post-larval composition 1, 2, 5, 24 and 48 h after settlement using sodium-dodecyl-sulphate polyacrylamide-gel electrophoresis (SDS-PAGE). Individual protein content was measured using a densitometer. Larvae did not consume protein during swimming, however a protein measuring 170 kdaltons was consumed during metamorphosis. These results are discussed in the context of larval settlement and energetics. Received: 19 July 1998 / Accepted: 3 December 1998     

Predation on Euphausia pacifica by demersal fishes: predation impact and influence of physical variability

The importance of Euphausia pacifica Hansen to the demersal fish community off Sendai Bay, northern Japan, was studied based on a total of 256 trawl samples collected during May, October and November, from 1989 to 1992. The samples were classified into 16 assemblages according to year, month and depth. The importance of E. pacifica to the total diet of each of the assemblages was measured using an index considering both fish diet and species composition. A total of 24 fish species out of 87 were found to ingest E. pacifica. Of these, Gadus macrocephaus and Theragra chalcogramma had the highest predation impact due to their dominance in the fish assemblages and the high proportion of E. pacifica in their diets. The contribution of E. pacifica to the total diet of the fish assemblage was highest in the shallow (≤300 m) regions during May, accounting for an average of 38.5% for the 4 years. However, during May 1990, when the warm Kuroshio Extension prevailed, the contribution was higher in the deep (>300 m; 22.6%) region, reflecting active and/or passive movements of E. pacifica. The average predation impact was maximal in the May/shallow assemblage (mean ± SE; 4.6 ± 1.4 kg wet wt km² d⁻¹) and was minimal in the November/shallow assemblage (0.4 ± 0.3). Annual estimates of consumption by demersal fish fishes ranged from 43 to 128 metric tons, representing 15 to 64% of the annual commercial catch of E. pacifica by local fisheries. It is therefore suggested that the fisheries on E. pacifica have the potential to considerably impact the demersal fish assemblages. Received: 3 December 1997 / Accepted: 10 June 1998     

New clues for freshwater eels (<Emphasis Type="Italic">Anguilla</Emphasis> spp.) migration routes to eastern Madagascar and surrounding islands

A total of 4,172 freshwater eels have been collected by electrofishing in upper estuaries from Madagascar (East coast), Mascarene (Réunion and Mauritius Is.), Comoros (Mayotte Is.) and Seychelles (Mahé and Praslin Is.) Archipelagos, between October 2003 and February 2006. Eel species composition in the sampling stations was contrasted between eastern Madagascar (Anguilla mossambica 96.0%, A. marmorata 3.9% and A. bicolor bicolor 0.2%), the Comoros (A. marmorata 56.1% and A. bicolor bicolor 43.9%), the Mascarene (A. marmorata 91.4%, A. bicolor bicolor 5.4% and A. mossambica 3.2%) and the Seychelles Archipelagos (A. bicolor bicolor 100.0%). This gradient in species composition, even concerning the short time-range of our sampling, argued for separate migration routes between species. A total of 168 eels were aged by reading their otolith microstructure, and otolith growth rates were calculated from pre-leptocephalus stage (post-hatching) to metamorphosis, until freshwater check. For all species, mean otolith growth rate (OGR) was related to specific migration routes: A. bicolor bicolor is distributed in the lowest latitudes and showed the highest OGR during leptocephalus stage, whereas A. mossambica, endemic of the Malagasy area, has the most southern distribution and showed the lowest OGR. OGR during leptocephalus stage was negatively correlated to the leptocephalus stage duration, showing a decrease of global metabolism with time, classical in leptocephali. This relationship was found significant for A. marmorata and A. mossambica, probably because all these larvae crossed successively the same environments, but not for A. bicolor bicolor, probably because their larvae crossed different pelagic environments, opening the hypothesis of larvae from different origins.     

Contrasts between spawning times of <Emphasis Type="Italic">Anguilla</Emphasis> species estimated from larval sampling at sea and from otolith analysis of recruiting glass eels

This study reviewed literature on spawning times for three north temperate species of anguillid eels estimated by sampling for small leptocephali (larvae) at sea and for several temperate and tropical species by back-calculating from putative daily ages derived from otolith increment analysis of glass eels that recruited to coastal waters. Estimates from otoliths of European eels, Anguilla anguilla, American eels, Anguilla rostrata, and Japanese eels, Anguilla japonica, imply much more protracted spawning seasons than are indicated by sampling at sea during various times of year. European eels are inferred to spawn year-round from otolith analysis, but the smallest, recently hatched leptocephali are found only in late winter and spring. From otoliths, the spawning times of these three species are all estimated to occur much later in the year than when small leptocephali are found at sea, indicating that ages appear to be underestimated. For these and other temperate and tropical eels, there are inconsistencies in assigned ages among various studies, which are most extreme for the European eel. This species has the longest larval migration and often has an opaque zone in the glass eels’ otoliths where it is difficult to discern growth increments. These inconsistencies suggest that interpretation of otolith growth increments is incorrect at least in some studies, and the apparently consistent mismatch between otolith and sea-sampling studies suggests that increments may not always be formed at some period during the unusual early life history of anguillids. Because daily increments may be formed in eels during most of their early life history, future research is needed to determine the cause of the mismatch of glass eel aging studies and the apparent spawning times of eels offshore. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

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Halogenated metabolites in two marine polychaetes and their planktotrophic and lecithotrophic larvae

This study investigated the occurrence and ontogenetic changes of halogenated secondary metabolites in planktotrophic and lecithotrophic larvae and adults of two common, infaunal polychaetes, Streblospio benedicti (Spionidae) and Capitella sp. I (Capitellidae), with different life-history traits. S. benedicti contains at least 11 chlorinated and brominated hydrocarbons (alkyl halides) while Capitella sp. I contains 3 brominated aromatic compounds. These halogenated metabolites are potential defense compounds benefiting both larvae and adults. We hypothesized that: (1) planktotrophic larvae contain halogenated metabolites because they are not protected by adult defenses, (2) quantitative and qualitative variation of planktotrophic larval halogenated metabolites parallels that of adults, and (3) brooded lecithotrophic larvae initiate the production of halogenated metabolites only after metamorphosis. To address these hypotheses, volatile halogenated compounds from polychaete extracts were separated using capillary gas chromatography and identified and quantified using mass spectrometry with selected ion monitoring. All four life stages (pre- and post-release larvae, new recruits, adults) of both S. benedicti and Capitella sp. I contained the halogenated metabolites previously identified from adults. This is the first report of halocompounds identified and quantified in polychaete larvae. Allocation of potential defense compounds to offspring varied as a function of species, feeding type and developmental stage. Pre-release larvae of S. benedicti with planktotrophic development contained the lowest concentration of total halogenated metabolites (1.75 ± 0.65 ng mm⁻³), post-release and new recruits contained intermediate concentrations (8.29 ± 1.72 and 4.73 ± 2.63 ng mm⁻³, respectively), and planktotrophic adults contained significantly greater amounts (28.9 ± 9.7 ng mm⁻³). This pattern of increasing concentrations with increasing stage of development suggests synthesis of metabolites during development. Lecithotrophic S. benedicti post-release larvae contained the greatest concentrations of halometabolites (71.1 ± 10.6 ng mm⁻³) of all S. benedicti life stages and developmental types examined, while the amount was significantly lower in new recruits (34.0 ± 15.4 ng mm⁻³). This pattern is consistent with a previously proposed hypothesis suggesting a strategy of reducing potential autotoxicity during developmental transitions. Pre-release lecithotrophic larvae of Capitella sp. I contained the highest concentration of total halogenated metabolites (1150 ± 681 ng mm⁻³), whereas the adults contained significantly lower total amounts (126 ± 68 ng mm⁻³). All concentrations of these haloaromatics are above those known to deter predation in previously conducted laboratory and field trials. As a means of conferring higher larval survivorship, lecithotrophic females of both species examined may be expending more energy on chemical defenses than their planktotrophic counterparts by supplying their lecithotrophic embryos with more of these compounds, their precursors, or with energy for their synthesis. This strategy appears common among marine lecithotrophic larval forms. Received: 14 July 1999 / Accepted: 20 January 2000     

Selective feeding in shellfish: size-dependent rejection of large particles within pseudofaeces from Mytilus edulis, Ruditapes philippinarum and Tapes decussatus

Mytilus edulis L., Ruditapes philippinarum (Adams & Reeve) and Tapes decussatus L. were fed particles of the same shape (spherical), the same density (2.1 g cm⁻³) and the same chemical composition (SiO₂), but which varied in diameter from 5 to 37 μm. Findings obtained at different particle concentrations (mean ± SD) of 51 ± 2, 105 ± 18 and 171 ± 17 mg l⁻¹ invariably indicate that significant proportions of all particles with diameters larger than from between 7.5 and 22.5 μm were preferentially rejected as pseudofaeces. We define the preferential ingestion index (PII) as the ratio between average particle volume in pseudofaeces and average particle volume in food. Whatever the particle concentration or the species, this PII was always statistically higher than 1. Irrespective of particle concentration, PII values in M. edulis were lower than in T. decussatus (averages of 1.2 and 2, respectively). PII values in M. edulis were also lower than in R. philippinarum maintained at particle concentrations above 171 ± 17 mg l⁻¹. We suggest that preferential size-dependent rejection of larger particles could be of significant adaptive value in the natural environment, either if there are large inorganic particles, or if the average organic content of smaller particles is higher. Received: 11 January 1997 / Accepted: 8 March 1997     

Validation of daily increment deposition in otoliths. Age and growth determination of Aphia minuta (Pisces: Gobiidae) from the northwest Mediterranean

The transparent goby Aphia minuta (Risso, 1810) is one of the main target species of the small-scale fishery off the Island of Majorca. Otolith microstructure and length-frequency analysis were used to study the age and growth of this species during the 1982/1983 and 1992/1993 fishing seasons. Daily periodicity of increment formation was determined by experiments with marked otoliths in individuals maintained in captivity. The length range of the catches during the 11 yr period was between 12 and 49 mm, with a main distribution (89%) between 24 and 40 mm. Otolith age-readings indicate that the population exploited in the commercial fishery consists of seven age-groups (2 to 8 mo old), with a very high proportion of individuals (95%) between 3 and 6 mo old. Population growth-curves revealed no differences between males and females. The growth parameters for the whole population are: asymptotic length, L _∞ = 53.69 mm; growth coefficient, K = 2.23 yr⁻¹; theoretical age at length zero, t ₀ = −0.005 yr. Those individuals of A. minuta caught in Majorca during the winter period reached a maximum age of 7 or 8 mo. Received: 30 December 1996 / Accepted: 16 April 1997     

Survival, growth and reproduction of the salt-marsh amphipod Uhlorchestia spartinophila reared on natural diets of senescent and dead Spartina alterniflora leaves

The talitrid amphipod Uhlorchestia spartinophila lives in close association with standing-dead leaves of the smooth cordgrass Spartina alterniflora Loisel in salt marshes along the Atlantic coast of North America. This study probed the strength of the trophic link between the amphipod population and the decomposition process in this detrital-based ecosystem. We measured survival, growth and reproductive output in groups of amphipods reared for 6 wk on five diets derived from sheath and blade portions of S. alterniflora leaves just prior to (senescent) and during (dead) decomposition. In unfed treatments, the daily specific mortality rate was 0.391 and starved amphipods survived no longer than 11 d. Among the fed treatments, a diet of senescent sheaths resulted in the lowest survival (20%) and yielded no offspring. Groups fed senescent blades, dead sheaths, dead blades and unwashed dead sheaths had survival rates of 56 to 84% and produced 5.0 to 12.5 offspring replicate⁻¹. Sex ratio usually favored females, but approached unity in treatments with high overall survival, suggesting that quality of available food resources may influence sex ratio in this species. Mean specific growth rates (mm mm⁻¹ d⁻¹) ranged from 0.013 to 0.016, and matched previous estimates of growth from field populations. Overall ecological performance (survival + growth + reproduction) was similar for all food treatments, except senescent sheaths, which yielded a final mean (±SD) dry biomass (0.4 ± 0.42 mg replicate⁻¹) of amphipods significantly lower than that of other diets (1.7 ± 0.81 to 2.6 ± 0.69 mg replicate⁻¹). Natural diets derived from decomposing cordgrass leaves can fulfill the nutritional requirements of U. spartinophila populations, but variation in initial amounts of living fungal biomass among the five experimental diets only partially explained the responses of amphipods in our experiment. Structural characteristics and variation in rates of fungal occupation within different portions of cordgrass leaves may affect the amphipod's ability to access plant production made available by decomposers. Received: 12 December 1996 / Accepted: 18 December 1996