台灣產扁平多囊蟹奴的生命週期及幼體性別比

Abstract

刺針幼體型蟹奴(Kentrogonid-type Rhizocephala)的生命週期仰賴雌性幼體感染宿主，以及雄性幼體進入virgin externa這兩個環節來完成，其中需要蟹奴與宿主生命週期之間的同步。這種同步性可從感染率、外體發育階段及幼體性別比的季節變化中觀察到。幼體性別比具有季節性變化亦可能反映出性別決定受到環境因子，如溫度與光照週期的影響。本研究旨在探討蟹奴的生命週期、宿主與寄生者互動以及蟹奴的性別決定。這個研究的假說為感染盛行率、外體發育階段及幼體性別比的季節變化可反映蟹奴的生命週期與牠和宿主的互動，且蟹奴的性別受到溫度與光照週期的影響。本研究於2022年3月至2023年6月於基隆與宜蘭採集寄生在土夸大額蟹（Metopograpsus thukuhar）上的扁平多囊蟹奴（Polyascus planus）。為了解蟹奴的生命週期即與宿主的互動關係，每月紀錄感染率、外體發育階段、宿主背甲寬與性別，以及第一批釋放的幼體性別比。為探討溫度與光週期對性別決定的影響，將帶有外體的宿主飼養於四種組合處理下（28°C/18°C × 13:11/11:14 hr光週期），並收集這些蟹奴釋放的幼體。由於雌雄幼體的甲殼長度有重疊（59 μm），因此以觸角（antennules）上的嗅覺剛毛（aesthetascs）來判定幼體性別。結果顯示扁平多囊蟹奴的感染率偏高（29.82%-86.36%），可能與漁港封閉的環境有關。夏季出現偏高雌性幼體比例，與低感染率的時期重疊。然而，virgin externa出現的季節（夏季）與雄性比例偏高的季節（冬季）並未同步。所有外體發育階段與雌雄幼體全年皆可觀察到，且性別比變異大（25.15 ± 34.75% 至 79.68 ± 47.95%），顯示宿主與蟹奴之間的同步性並非扁平多囊蟹奴繁殖所必需。此外，四種溫度與光照處理下的幼體性別比並無顯著差異，顯示溫度與光照週期並非影響性別決定的因子。性別決定可能受到來自宿主或virgin externa釋放的化學訊號影響，或可能由遺傳機制所控制，但皆尚未有足夠研究。本研究有助於理解蟹奴的繁殖策略與宿主寄生蟲之間的互動，並指出扁平多囊蟹奴的幼體性別決定並非單純由溫度或光週期控制，可能涉及其他調控機制。

The life cycle of kentrogonid-type Rhizocephala depends on female larvae infecting the host and male larvae entering the virgin externa, requiring synchronization with both the host and conspecifics. This synchrony is reflected in seasonal patterns of infection prevalence, externa developmental stages, and larval sex ratio. Seasonal shifts in larval sex ratio may also indicate that sex determination is influenced by environmental factors such as temperature and photoperiod. This study aims to explore the life cycle, host-parasite interaction, and sex determination of rhizocephalans. I hypothesize that the seasonality of infection prevalence, proportion of externa stages, and larval sex ratio reflects life cycle dynamics and host-parasite interactions, and that sex is influenced by temperature and photoperiod. From March 2022 to June 2023, Polyascus planus parasitizing Metopograpsus thukuhar were collected from Keelung and Yilan. To investigate the life cycle and host-parasite interaction, monthly infection prevalence, externa stages, host size and sex, and the larval sex ratio of the first broods were recorded. To test the effects of temperature and photoperiod on sex determination, crabs bearing externa were cultured under four treatments combining summer/winter temperatures (28°C or 18°C) and light-dark cycles (13:11 or 11:14 h). Larval sex was identified by the presence of aesthetascs on the antennules, as carapace length showed substantial overlap (59 μm) between sexes. P. planus exhibited high infection prevalence (29.82%–86.36%) in M. thukuhar, likely due to the enclosed harbor environment. Female-biased larval ratios coincided with low infection prevalence in summer, but no synchrony was found between virgin externa emergence (summer) and male-biased ratios (winter). All externa stages and both larval sexes occurred year-round, with highly variable sex ratios (25.15 ± 34.75% to 79.68 ± 47.95%), suggesting that reproductive synchronization may not be essential for P. planus. Additionally, larval sex ratios did not differ across temperature and photoperiod treatments, indicating that sex determination is not solely controlled by these factors. It may instead be influenced by other environmental cues, such as chemical signals from hosts or externa, or through a genetic mechanism. This study provides insight into the reproductive strategy and host-parasite interaction of rhizocephalans, and suggests that larval sex determination in P. planus is not solely regulated by temperature or photoperiod, implying the involvement of other regulatory mechanisms.