Ontogenesis of the digestive gland through the planktotrophic stages of Strombus gigas

ABSTRACT The queen conch, Strombus gigas (Linnaeus, 1758), is a marine mollusc of ecological and economic importance in the Caribbean. Its populations are declining due to overexploitation. We describe ontogenesis of the digestive gland in S. gigas during the larval stages. Larvae were studied over a period of 42 d in laboratory culture, from eggs to crawling stage. Experiments were conducted at 28 ± 1°C. Veligers were reared at a density of 100 larvae l−1 in 10-l containers. Larvae were fed with the microalgae Nannochloropsis oculata at a concentration of 1,000 cells l−1. In this study, we analysed ultrastructural ontogenesis of the digestive gland in strombid larvae using light and electron microscopy. Examination for Coccidia (Apicomplexa) symbionts in the digestive gland was done by viewing sections with light and scanning electron microscopes at different larval development stages during a 42-d period. In early veligers (9 d after hatching), only digestive cells were observed in the digestive gland. By the late veliger stages (17 d old), both digestive and crypt cells were apparent in the digestive gland. Within crypt cells, spherocrystals were detected and the presence of Ca, Cl, Cu, P and Zn was identified by energy-dispersive X-ray spectroscopy. In late pediveligers (35 d old), the digestive gland still exhibited only digestive and crypt cells. Vacuolated cells (i.e. harbour the coccidian symbionts in adults) were only observed in newly settled juveniles (42 d old) and were devoid of apicomplexan structures. While coccidian symbionts were found in the digestive gland of adult S. gigas, they were not observed in the digestive gland of S. gigas larvae from hatching to settlement under laboratory conditions. This suggests that this symbiont is not vertically transmitted to new host generations in this marine gastropod species.

The complete mitochondrial genome of Harpago chiragra and Lambis lambis (Gastropoda: Stromboidea): implications on the Littorinimorpha phylogeny

The complete mitochondrial genomes of Harpago chiragra and Lambis lambis (Strombidae) were determined with the size of 15,460 bp and 15,481 bp, respectively, and both sequences contained 13 protein-coding genes, 22 tRNAs, and two rRNAs. H. chiragra and L. lambis have similar mitochondrial features, corresponding to typical gastropod mitochondrial genomes, such as the conserved gene order, a high A + T content (66.22% for H. chiragra and 66.10% for L. lambis), and preference for A + T-rich codons. The start or termination codon of same protein-coding gene in H. chiragra was consistent with that in L. lambis, except for the termination codon of cox1 gene (TAG for H. chiragra and TAA for L. lambis) and the start codon of nad4 (GTG for H. chiragra and ATG for L. lambis). Pairwise sequence alignments detected different degrees of variations in H. chiragra and L. lambis mitochondrial genomes; and the two species had lower levels of genetic distance (0.202 for nucleotide sequence) and closest relationships as compared to Strombus gigas and Oncomelania hupensis. The 13 partitioned nucleotide sequences of protein coding genes of H. chiragra and L. lambis were aligned with representatives of the main lineages of gastropods and their phylogenetic relationships were inferred. H. chiragra and L. lambis share the same gene order as Littorinimorpha species, except Vermetoidea, which demonstrate a gene rearrangement in species. The reconstructed phylogeny supports three major clades within Littorinimorpha: 1) Stromboidea, Tonnoidea, Littorinoidea, and Naticoidea, 2) Rissooidea and Truncatelloidea, and 3) Vermetoidea. In addition, a relaxed molecular clock calibrated with fossils dated the diversification of Strombidae near 112 (44–206) Mya and a possible radiation is detected to occur between 45–75 Mya, providing implications to understand the Cenozoic replacement event (65–135 Mya) of Aporrhaidae by Strombidae.

The distribution of large floating seagrass (Zostera marina) clumps in northern temperate zones of Bohai Bay in the Bohai Sea, China

Seagrass meadows (Zostera marina) are important coastal ecosystems with high levels of productivity and biodiversity. They are subject to considerable natural and anthropogenic threats in China, such as oyster and snail aquaculture, wastewater discharge, electro-fishing, shellfish collection, typhoons and floods. When seagrass communities are disturbed, they can become removed from the sediment and converted into floating clumps, which then serve as marine hot spots attracting a variety of marine organisms that then inhabit them. They are important nursery habitats for many economic fish such as red drum (Sciaenops ocellatus), Atlantic cod (Gadus morhua), queen conch (Strombus gigas), and blue crab (Callinectes sapidus). Thus, it is necessary to study the distribution and biological characteristics of these floating seagrass clumps. In September 2016 we observed large scale floating Z. marina clumps in the northernmost area of Bohai Bay (38°57’1.14”−39° 0’41.28” N, 118°45’23.22”−118°47’6.96” E), in the Bohai Sea, China. We observed characteristics that precluded their origination from the nearby Caofeidian seagrass meadows. Two research cruises were undertaken, during which we did not observe other marine organisms accompanying these floating Z. marina clumps. The dominant frond lengths were 40–50 cm, with less than 5% of the total number of fronds found in larger size categories (80–90 and 90–100 cm). We aim to pursue future research into the breakdown and dislodgement characteristics of Z. marina clumps and the processes whereby they sink and integrate with the sediment.

Growth parameters and density variation of a queen conch, Strombus gigas (Neotaenioglossa: Strombidae), population from Xel-Ha park, a marine protected area

This contribution used mark-recapture data to estimate the von Bertalanffy equation parameters and population number variation within a non harvest population from a protected area, to clarify the biometric parameters that better suit for the whole population, or for the juvenile and adult fractions. Conchs from XelHa Park were monthly sampled from November 2001 to August 2005. Every conch found was measured and marked with a numbered tag that identified month and locality; and monthly abundance was estimated with Jolly’s method. Length, lip thickness and weight increments were used to estimate the von Bertalanffy growth equation parameters with Appeldoorn’s subroutine of FISAT program. The population numbervaried through the study, with a minimum of 49 in April 2003 and maximum of 9 848 during June 2005. Conchs make only temporary use of Xel-Ha cove. Shell length gavethe best fit for the juvenile fraction: L∞=251, K=0.3, C=0.8 Wp=0.3; and lip thickness for adults: L∞=47.78, K=0.17, C=0.1, Wp=0.86, while, the whole population was better represented by weight: L∞=3850, K=0.36, C=0.8, Wp=0.3. A maximum age of 19 years was estimated from the population. Natural mortality was 0.49/year for juveniles and 0.29/year for adults. There were two pulses of recruitment: fall-winter and summer. It is concluded that population studies from length frequency data, should be analyzed independently in two groups, shell for the juvenile fraction and lip thickness for the adult fraction, or if it is not possible to analyze the population fractions separately,weight should be used to avoid miss calculation of the age structure.

Efecto de la temperatura y la acidificación en larvas de Strombus gigas (Mesogastropoda: Strombidae)

The increase in CO2 emissions produces heating and reduced pH in the oceans, which may have negative effects on many marine organisms. This is particularly important for those with calcified structures such as the molluscs and their larval stages. We studied Strombus gigas larvae, a gastropod of commercial importance in the Caribbean Sea, in order to know the effect of water temperature and acidification on their development, growth, mortality and calcification during the larval period. A larval culture with triplicate samples was carried out employing four treatments of temperature and pH (Control = 28 °C - pH 8.1, T1 = 28 °C - pH 7.6, T2 = 31 °C - pH 8.1 and T3 = 31 °C - pH 7.6) in August 2015. We registered hatching (No. of eggs – No. of larvae hatched) and organs development, while shell growth and mortality ratio were evaluated over time. Shell calcification was studied in 30 days old larvae using EDX and RAMAN analysis. Our results showed that organs development and shell growth were higher at 31 °C treatments (initial size of 230 ± 4.12 to 313.27 ± 11.34 µm, and final size from 829.50 ± 11.33 to 1 054.50 ± 11.13 µm; from T1 to T2 respectively), and the same pattern was recorded for hatching time (18 hr) and mortality rate (~ 57 %). The Calcium proportion (% wt) was similar between treatments (34.37 ± 10.05 to 37.29 ± 16.81 % wt). Shell Raman analysis showed aragonite in all experimental treatments, with the highest values in the control (1 039.54 ± 780.26 a.u.). Calcite was detected only in 31 °C treatments (174.56 ± 127.19 a.u.), while less intensity of aragonite and calcite were registered at pH 7.6. In conclusion, S. gigas could be adapted to ocean future predictions, however, shell biomineralization processes can be affected.