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Abstract The intensive fishing and lack of data for squid fisheries in Belitung waters have increased the risk of stock population decline, which could lead to large economic losses. Comprehensive studies on the biological aspects and stock status of squid were crucial to developing some management recommendations. This study investigated several population ecological characteristics (size distribution, length-weight relationship, condition factors, maturity stages, length at maturity, and growth parameters), the stock status, and the biological reference points of mitre squid (Uroteuthis chinensis Gray, 1849) in Belitung waters. The squid’s biological data were collected from August 2023 to March 2024. The stock status was investigated by several approaches, including length-converted catch-curve (LCC), yield per recruit (YPR) analysis, and length-based spawning potential ratio (LB-SPR). This study revealed that the mean size of male and female squid captured by squid jig was 16.22 ± 6.72 cm and 14.23 ± 3.65 cm (Mean of mantle length ± standard deviation), with the length at first maturity (Lm50) of 19 cm for males and 12 cm for females. Female gonadally mature squid were found throughout the year, with the largest proportion in January (93%). The overfishing condition of U. chinensis has occurred based on the current exploitation level for male (Ecur: 0.63; Fcur: 2.61 year-1) and female (Ecur: 0.72; Fcur: 4.39 year-1) that exceeds both the optimal exploitation rate (Eopt=0.5) and the fishing mortality given the maximum yield per recruit (Male Fmax: 2.08 year-1; Female Fmax: 3.35 year-1). Recruitment overfishing has also occurred based on the current spawning potential ratio of 18%. Reducing the fishing trips by 24%, applying a season closure in January, and enlarging the selectivity to 15 cm were recommended to raise the squid stock and ensure the sustainability of squid fisheries in Belitung waters. Resumo A pesca intensiva e a falta de dados sobre a pesca de lula nas águas de Belitung aumentaram o risco de declínio da população dos estoques, o que poderia levar a grandes perdas econômicas. Estudos abrangentes sobre os aspectos biológicos e o status dos estoques de lula foram cruciais para a formulação de algumas recomendações de manejo. Este estudo investigou várias características ecológicas da população (distribuição de tamanho, relação comprimento-peso, fatores de condição, estágios de maturidade, comprimento na maturidade e parâmetros de crescimento), o status do estoque e os pontos de referência biológicos da lula-mitra (Uroteuthis chinensis Gray, 1849) nas águas de Belitung. Os dados biológicos da lula foram coletados de agosto de 2023 a março de 2024. O status do estoque foi avaliado por meio de diversas abordagens, incluindo curva de captura convertida pelo comprimento (LCC), análise de rendimento por recruta (YPR) e razão potencial de desova baseada em comprimento (LB-SPR). Esse estudo revelou que o tamanho médio das lulas machos e fêmeas capturadas pelo gabarito de lula foi de 16,22 ± 6,72 cm e 14,23 ± 3,65 cm (média do comprimento do manto ± desvio-padrão), com o comprimento na primeira maturidade (Lm50) de 19 cm para os machos e 12 cm para as fêmeas. Lulas fêmeas com gônadas maduras foram encontradas durante todo o ano, com maior proporção registrada em janeiro (93%). A condição de sobrepesca da U. chinensis ocorreu com base no nível de exploração atual para machos (Ecur: 0,63; Fcur: 2,61 ano-1) e fêmeas (Ecur: 0,72; Fcur: 4,39 ano-1) que excede a taxa de exploração ideal (Eopt=0,5) e a mortalidade por pesca, considerando o rendimento máximo por recruta (Fmax masculino: 2,08 ano-1; Fmax feminino: 3,35 ano-1). A sobrepesca de recrutamento também ocorreu com base na atual taxa de potencial de desova de 18%. A redução das viagens de pesca em 24%, a aplicação de um defeso em janeiro e a ampliação da seletividade para 15 cm foram recomendadas para aumentar o estoque de lula e garantir a sustentabilidade da pesca de lula nas águas de Belitung.

Reproductive investment generally involves a trade-off between somatic growth and energy allocation for reproduction. Previous studies have inferred that jumbo squid Dosidicus gigas support growth during maturation through continuous feeding (an “income” source). However, our recent work suggests possible remobilization of soma during maturation (a “capital” source). We used fatty acids as biochemical indicators to investigate energy acquisition and allocation to reproduction for female D. gigas . We compared the fatty acid profiles of the ovary to those of the mantle muscle (slow turnover rate tissue, representing an energy reserve) and the digestive gland (fast turnover rate organ, reflecting recent consumption). For each tissue, the overall fatty acids among maturity stages overlapped and were similar. The changes with maturation in fatty acid composition in the ovary consistently resembled those of the digestive gland, with the similarity of fatty acids in the mantle muscle and the ovary increasing during maturation, indicating some energy reserves were utilized. Additionally, squid maintained body condition during maturation regardless of increasing investment in reproduction and a decline in feeding intensity. Cumulatively, D. gigas adopt a mixed income-capital breeding strategy in that energy for reproduction is mainly derived from direct food intake, but there is limited somatic reserve remobilization.

Background Cephalopods are widely distributed in oceans worldwide and are important fishery resources. Most species have a lifespan of approximately one year and die after reproduction. In cephalopods, gonadal development may be influenced by seawater temperature; however, the endocrine mechanisms underlying reproductive maturity remain unclear. In recent years, gonadotropin-releasing hormone (GnRH)-like peptide has been identified in invertebrates, including cephalopods, as a possible endocrine regulator, similar to their role in vertebrates. Nevertheless, knowledge of its specific functions in cephalopod reproduction remains limited. This study aimed to clarify the involvement of the endogenous peptide in gonadal development in cephalopods in the bigfin reef squid ( Sepioteuthis lessoniana ). We performed histological observations of gonadal maturation and analyzed brain expression levels and localization of the peptide throughout sexual maturation. Additionally, we examined the relationship between annual gonadal maturation and the seawater temperature cycle. Results We identified coding sequences for peptides with conserved functional regions similar to those of other mollusks. Quantitative analysis revealed that brain expression significantly increased during the spermatid stage of testicular development, whereas no association with ovarian development was observed. Immunoreactivity was primarily localized in the optic lobe and around the optic gland, a central site of reproductive regulation in cephalopods. Although ovarian development progressed with increasing seawater temperature, testicular development showed no clear association with the temperature cycle. Conclusions These findings suggest that GnRH-like peptides may contribute to early testicular development in S. lessoniana through optic gland signaling or direct neural pathways. In contrast, ovarian maturation appears to be strongly influenced by seawater temperature. This study provides foundational insight into the reproductive physiology of cephalopods and highlights regulatory mechanisms governing male and female gonadal development.

Anthropogenic carbon dioxide (CO2) is being absorbed into the ocean, altering seawater chemistry, with potentially negative impacts on a wide range of marine organisms. The early life stages of invertebrates with internal and external aragonite structures may be particularly vulnerable to this ocean acidification. Impacts to cephalopods, which form aragonite cuttlebones and statoliths, are of concern because of the central role they play in many ocean ecosystems and because of their importance to global fisheries. Atlantic longfin squid (Doryteuthis pealeii), an ecologically and economically valuable taxon, were reared from eggs to hatchlings (paralarvae) under ambient and elevated CO2 concentrations in replicated experimental trials. Animals raised under elevated pCO2 demonstrated significant developmental changes including increased time to hatching and shorter mantle lengths, although differences were small. Aragonite statoliths, critical for balance and detecting movement, had significantly reduced surface area and were abnormally shaped with increased porosity and altered crystal structure in elevated pCO2-reared paralarvae. These developmental and physiological effects could alter squid paralarvae behavior and survival in the wild, directly and indirectly impacting marine food webs and commercial fisheries.

Summary Outer membrane vesicles (OMV) are critical elements in many host‐cell/microbe interactions. Previous studies of the symbiotic association between Euprymna scolopes and Vibrio fischeri had shown that within 12 h of colonizing crypts deep within the squid's light organ, the symbionts trigger an irreversible programme of tissue development in the host. Here, we report that OMV produced by V. fischeri are powerful contributors to this process. The first detectable host response to the OMV is an increased trafficking of macrophage‐like cells called haemocytes into surface epithelial tissues. We showed that exposing the squid to other Vibrio species fails to induce this trafficking; however, addition of a high concentration of their OMV, which can diffuse into the crypts, does. We also provide evidence that tracheal cytotoxin released by the symbionts, which can induce haemocyte trafficking, is not part of the OMV cargo, suggesting two distinct mechanisms to induce the same morphogenesis event. By manipulating the timing and localization of OMV signal delivery, we showed that haemocyte trafficking is fully induced only when V. fischeri, the sole species able to reach and grow in the crypts, succeeds in establishing a sustained colonization. Further, our data suggest that the host's detection of OMV serves as a symbiotic checkpoint prior to inducing irreversible morphogenesis.

Tracheal cytotoxin (TCT), a fragment of the bacterial surface molecule peptidoglycan (PGN), is the factor responsible for the extensive tissue damage characteristic of whooping cough and gonorrhea infections. Here, we report that Vibrio fischeri also releases TCT, which acts in synergy with lipopolysaccharide (LPS) to trigger tissue development in its mutualistic symbiosis with the squid Euprymna scolopes. As components of PGN and LPS have commonly been linked with pathogenesis in animals, these findings demonstrate that host interpretation of these bacterial signal molecules is context dependent. Therefore, such differences in interpretation can lead to either inflammation and disease or to the establishment of a mutually beneficial animal-microbe association.

Globally, small-scale fisheries are influenced by dynamic climate, governance, and market drivers, which present social and ecological challenges and opportunities. It is difficult to manage fisheries adaptively for fluctuating drivers, except to allow participants to shift effort among multiple fisheries. Adapting to changing conditions allows small-scale fishery participants to survive economic and environmental disturbances and benefit from optimal conditions. This study explores the relative influence of large-scale drivers on shifts in effort and outcomes among three closely linked fisheries in Monterey Bay since the Magnuson-Stevens Fisheries Conservation and Management Act of 1976. In this region, Pacific sardine (Sardinops sagax), northern anchovy (Engraulis mordax), and market squid (Loligo opalescens) fisheries comprise a tightly linked system where shifting focus among fisheries is a key element to adaptive capacity and reduced social and ecological vulnerability. Using a cluster analysis of landings, we identify four modes from 1974 to 2012 that are dominated (i.e., a given species accounting for the plurality of landings) by squid, sardine, anchovy, or lack any dominance, and seven points of transition among these periods. This approach enables us to determine which drivers are associated with each mode and each transition. Overall, we show that market and climate drivers are predominantly attributed to dominance transitions. Model selection of external drivers indicates that governance phases, reflected as perceived abundance, dictate long-term outcomes. Our findings suggest that globally, small-scale fishery managers should consider enabling shifts in effort among fisheries and retaining existing flexibility, as adaptive capacity is a critical determinant for social and ecological resilience.

Bacterial flagella mediate host–microbe interactions through tissue tropism during colonization, as well as by activating immune responses. The flagellar shaft of some bacteria, including several human pathogens, is encased in a membranous sheath of unknown function. While it has been hypothesized that the sheath may allow these bacteria to evade host responses to the immunogenic flagellin subunit, this unusual structural feature has remained an enigma. Here we demonstrate that the rotation of the sheathed flagellum in both the mutualist Vibrio fischeri and the pathogen Vibrio cholerae promotes release of a potent bacteria-derived immunogen, lipopolysaccharide, found in the flagellar sheath. We further present a new role for the flagellar sheath in triggering, rather than circumventing, host immune responses in the model squid-vibrio symbiosis. Such an observation not only has implications for the study of bacterial pathogens with sheathed flagella, but also raises important biophysical questions of sheathed-flagellum function. While a few of the bacteria that live in and on the bodies of humans and other animals are harmful and can cause disease, most others can offer benefits to their hosts. Many bacteria—including some important human pathogens—have tails called flagella that rotate to move the bacteria inside its host. However, the immune system can detect parts of these flagella and eliminate the pathogen. Bacterial flagella are made from filaments of proteins, and some flagella are also enclosed by a sheath that is similar to the outer membrane that encloses certain bacteria. The function of this sheath is unclear, although some researchers have suggested that it might prevent the immune system from detecting the proteins in the flagellum. Now, by studying the interactions between the Hawaiian bobtail squid and a marine bacterium, Brennan et al. show that the sheath can actually alert the host that the bacteria are around. The Hawaiian bobtail squid collects bioluminescent bacteria within a so-called ‘light organ’. This organ undergoes a number of developmental changes to house the bacteria, and the squid then uses the light from the bacteria to mask its own shadow, which helps it to avoid being detected by predators. Brennan et al. compared how wild-type bacteria and mutant bacteria that either had no flagella, or had flagella that did not rotate, interacted with young squid. Only bacteria with working flagella were able to trigger the normal development of the squid’s light organ, which suggests that the rotating flagella are releasing the signal that tells the squid that the beneficial bacteria are present. Brennan et al. demonstrated that the rotation of sheathed flagella led to the release of a molecule called lipopolysaccharide. This molecule is known to activate the immune system in animals, and it is one of the bacterial signals that the squid responds to. Moreover, when the flagella of other bacteria with sheaths—such as those that cause cholera—are rotating, there is also an increase in the release of lipopolysaccharide. However, rotation of the flagella of bacteria without sheaths has no such effect. The next challenge will be to test the importance of this release of lipopolysaccharide from rotating flagella on the outcome of bacterial diseases of humans and other animals.

Sthenoteuthis oualaniensis (SA) is an important squid species in the South China Sea. Based on SA samples collected in 2016, SA was divided into the \"dwarf\" form (DF) and \"medium\" form (MF). To understand the changes in gastrointestinal function in SA during sexual maturation, we undertook transcriptomic analyses of the stomach and intestine tissues of the mature and immature DF and MF of SA using the deep-sequencing platform Illumina HiSeq™. We exploited a high-throughput method to delineate differentially expressed genes (DEGs) in the DF and MF of SA. A total of 135464 unigenes (68627 unigenes of the DG and 66837 unigenes of the MF) were generated. We identified 7965 and 4051 relative DEGs in the intestine and stomach tissues of the mature DF of SA compared with those of the immature DF of SA; and 22138 and 18460 DEGs in the intestine and stomach of the mature MF of SA compared with those of the immature MF of SA. Gastrointestinal function related to the metabolism of lipids, amino acids, glucose, and energy were changed in SA during sexual maturation. This work is the first to identify a set of genes associated with gastrointestinal function during sexual maturation in SA.

The cranchiid Teuthowenia pellucida, like many deep-sea squid species, possesses large eyes that maximise light sensitivity in a nearly aphotic environment. To assess ontogenetic changes in the visual system, we conducted morphometric and histological analyses of the eyes using specimens from New Zealand collections. While the ratio between eye diameter and mantle length maintained a linear relationship throughout development, histological sections of the retina revealed that the outer photoreceptor layer became proportionally longer as the animal aged, coincident with a habitat shift into deeper, darker ocean strata. Other retinal layers maintained the same absolute thickness as was observed in paralarvae. Granules of the pigment ommin, normally located in the screening layer positioned at the base of the photoreceptors, were also observed at the outer end of the photoreceptor segments throughout the retina in young and mid-sized specimens. Early developmental stages of this species, dwelling in shallow waters, may therefore rely on migratory ommin to help shield photoreceptors from excess light and prevent over-stimulation. The oldest, deeper-dwelling specimens of T. pellucida examined had longer photoreceptors, and little or no migrated ommin was observed; we suggest therefore that short-term adaptive mechanisms for bright light conditions may be used primarily during epipelagic, early life stages in this species.