The opening of the Suez Canal in 1869 enabled a large number of Indo-Pacific marine species to expand their ranges into the Mediterranean Sea (Galil et al. 2017), entering the Gulf of Suez from the Red Sea and migrating northward toward the easternmost basin of the Mediterranean. These species have been called “Lessepsian migrants” (Yonow 2015), “Lessepsian immigrants” (Kleitou et al. 2019), or “Lessepsian invaders” (Ivkić et al. 2019), named after Ferdinand de Lesseps who planned the Suez Canal’s construction, or “Erythraean non-indigenous species” (Galil et al. 2017), after the Erythraean Sea, an earlier maritime designation including both the Red Sea and the Gulf of Aden.

Among approximately 750 Lessepsian migrants (Galil et al. 2017), there are 30 heterobranchs that constitute ˜6% of the total Mediterranean sea slug fauna (Crocetta et al. 2013). Most are colorful and therefore popular subjects for underwater photographers who have shared numerous observations on internet sites, such as Sea Slug Forum and Mediterranean Slug Site (examples in Appendix S1: Table S1), or directly with sea slug taxonomists (Yonow 2015). This stimulated the use of sea slugs as model organisms in the monitoring of invasive species, in which recreational divers volunteered as citizen scientists (Fernández-Vilert et al. 2018, Kleitou et al. 2019, Paz-Sedano et al. 2019).

The nudibranch Plocamopherus ocellatus Rüppell & Leuckart, 1828 received increased scientific interest as a Lessepsian migrant when diver observations since the 1980s became the basis for publications (Appendix S1: Table S1). The recent discovery of two small specimens in the Red Sea (Fig. 1) drew attention to the question of whether it was more frequently encountered in the Mediterranean than in its native range (Yonow 2008), and what precisely constitutes this native range.

Indeed, based on records produced by underwater photographers in Israel, Rothman and Galil (2015) concluded that the reputedly rare Erythraean nudibranch P. ocellatus (see e.g., Nithyanandan 2012) appeared to be not so rare in the eastern Mediterranean. Israel was the first country with a Mediterranean record (1977–2015), followed by observations in Turkey (1998–2019), Lebanon (2000–2015), Cyprus (2015), and Greece (2020) (Fig. 2). The last record is also the westernmost locality: Kastellorizo Island off the Turkish coastline (Ragkousis et al. 2020). Altogether, 23 unique Lessepsian records from 16 localities were found, including two additional museum specimens from the Suez Canal (Appendix S1: Table S1). There are no records from the Mediterranean coast of Egypt or other North African countries.

Only 15 records are known from the native range of P. ocellatus (Fig. 2). Before the construction of the Suez Canal, the species had only been observed in 1825 at its type locality in the Gulf of Suez. Since then, 14 additional observations at eight localities have been reported in its native range constituting the Gulf of Suez, the Red Sea, the Arabian Gulf, and the Gulf of Oman (Fig. 2). This includes Kuwait in the northern Arabian Gulf, where it appears to be common in March and April (M. Nithyanandan, personal communication). No specimens are recorded from the Gulf of Eilat (northeastern Red Sea), despite its well-developed diving tourism. A specimen record from India (Appendix S1: Table S1) was not included since it is based on a misidentification: its color description does not match with that of P. ocellatus, which is brownish to dark red or reddish black, with yellow spots that have dark-colored flecks in the center and a brighter yellow margin followed by a dark ocellation, hence the name (Fig. 1). In poor light conditions, the color is quite cryptic (Rothman and Galil 2015: Fig 1D).

Despite the involvement of citizen scientists, many sea slug species are still rarely encountered. In sea slug ecology, “rarity” is frequently used as a relative and intuitive concept (Schubert and Smith 2020), which has been applied at population or assemblage level (usually expressed as densities) but also in a biogeographical setting (number of locality records) as in the case of P. ocellatus (e.g., Nithyanandan 2012, Rothman and Galil 2015). Since the opening of the Suez Canal, there are more records (23 from 16 sites) from the Mediterranean than from the much larger native range (14 from eight sites), while underwater photography since the 1980s contributed to 17 records from 14 sites in the non-native range and 10 records from five sites in the native range (Appendix S1: Table S1). An absence of potential predators in the Mediterranean may play a role, although there is no information on natural predators in the native range. Perhaps attention for invasiveness has encouraged a chase for records in its non-native range but not in its native range, causing an observer-expectancy effect. A similar but much more recent case concerns the tiny (<15 mm long) sea slug Haminoea cyanomarginata Heller & Thompson, 1983, now synonymized and considered the purple color morph of Lamprohaminoea ovalis (Pease, 1868; see Oskars and Malaquias 2020). This minute species has only seven records from its native range (Red Sea, Gulf of Eilat, Gulf of Oman) since its description in 1980 and at least 20 locality records from around the Mediterranean since 2005 (Fernández-Vilert et al. 2018, Rizgalla et al. 2018). Its maximum length is much smaller than the 60 mm recorded for P. ocellatus (Ragkousis et al. 2020), which supports the hypothesis that increased awareness can stimulate a quest for new records of introduced species. Nine P. ocellatus sightings in the Mediterranean were reported from shipwrecks, which are popular among recreational divers and may have contributed to the observer bias mentioned earlier. The diet of P. ocellatus consists of bryozoans (Yonow 2008, Rothman and Galil 2015), which need a solid substrate and shipwrecks fit in that category, although why some individuals are found on sand is unclear (Fig. 1; Yonow 2008, Rothman and Galil 2015). Plocamopherus ocellatus occurs at depths of 1.5–50 m (Appendix S1: Table S1) and produces large egg ribbons attached to solid substrates (Nicolaidou et al. 2012: Fig. 19, Rothman and Galil 2015: Fig. 1C). Although the life history and dispersal mechanisms of P. ocellatus are not known, spawning and new records indicate its establishment in the Mediterranean. The increasing range and abundance of P. ocellatus in the Mediterranean could therefore be fact.

On the other hand, among the dense populations of residents and tourists along the Mediterranean shores, there is a large potential of underwater observers. Although the lack of records from the southern Mediterranean and the Gulf of Eilat cannot be explained and needs further investigation, it appears that the rarity in the native range of P. ocellatus is most likely an artifact.

1869 年苏伊士运河的开通使大量印太海洋物种得以将活动范围扩大到地中海（Galil et al. 2017），从红海进入苏伊士湾，向北迁徙至最东端的盆地地中海的。这些物种被称为“莱塞普斯移民”（Yonow 2015）、“莱塞普斯移民”（Kleitou et al. 2019）或“莱塞普斯入侵者”（Ivkić et al. 2019），以规划苏伊士运河建设的费迪南德·德·莱塞普斯命名，或“Erythraean 非本土物种”（Galil 等人，2017 年），在 Erythraean Sea 之后，这是一个较早的海洋名称，包括红海和亚丁湾。

在大约 750 名莱塞普斯移民中（Galil 等人2017 年），有 30 个异种分支占地中海蛞蝓动物群总数的约 6%（Crocetta 等人2013 年）。大多数都是丰富多彩的主题，因此对于水下摄影师来说很受欢迎，他们在互联网网站上分享了许多观察结果，例如海蛞蝓论坛和地中海蛞蝓网站（附录 S1 中的示例：表 S1），或者直接与海蛞蝓分类学家（Yonow 2015）分享。这刺激了将海蛞蝓作为模型生物用于监测入侵物种，其中休闲潜水员自愿成为公民科学家（Fernández-Vilert 等人2018 年，Kleitou 等人2019 年，Paz-Sedano 等人2019 年）。

当 1980 年代以来的潜水员观察成为出版物的基础时，1828 年的裸鳃海蛞蝓 Plocamopherus ocellatus Rüppell & Leuckart 作为莱塞普斯移民获得了越来越多的科学兴趣（附录 S1：表 S1）。最近在红海发现的两个小标本（图 1）引起了人们的关注，即它在地中海是否比在其原生地区更常见（Yonow 2008），以及究竟是什么构成了这个原生范围。

事实上，根据以色列水下摄影师的记录，Rothman 和 Galil ( 2015 ) 得出结论认为，号称稀有的Erythraean nudibranch P. ocellatus（参见例如 Nithyanandan 2012）在地中海东部似乎并不罕见。以色列是第一个有地中海记录的国家（1977-2015），其次是土耳其（1998-2019）、黎巴嫩（2000-2015）、塞浦路斯（2015）和希腊（2020）（图2）。最后的记录也是最西端的地点：土耳其海岸线外的 Kastellorizo​​ 岛 (Ragkousis et al. 2020）。总共发现了来自 16 个地区的 23 份独特的莱塞普斯记录，其中包括来自苏伊士运河的另外两个博物馆标本（附录 S1：表 S1）。没有来自埃及地中海沿岸或其他北非国家的记录。

从P的原生范围中只知道 15 条记录。ocellatus（图 2）。在苏伊士运河建造之前，该物种仅在 1825 年在其位于苏伊士湾的典型地点观察到。从那时起，在苏伊士湾、红海、阿拉伯湾和阿曼湾等 8 个地方又报告了 14 次观测（图 2）。这包括阿拉伯湾北部的科威特，在 3 月和 4 月似乎很常见（M. Nithyanandan，个人通讯）。埃拉特湾（红海东北部）尽管潜水旅游发达，但没有记录到任何标本。不包括来自印度的样本记录（附录 S1：表 S1），因为它是基于错误识别：它的颜色描述与P的颜色描述不匹配。ocellatus，呈褐色至深红色或红黑色，黄色斑点在中心有深色斑点，边缘较亮的黄色，然后是深色的ocellation，因此得名（图1）。在光线不足的情况下，颜色非常神秘（Rothman 和 Galil 2015：图 1D）。

尽管有公民科学家的参与，但仍然很少遇到许多海蛞蝓。在海蛞蝓生态学中，“稀有”经常被用作一个相对和直观的概念（Schubert 和 Smith 2020），它已应用于种群或组合水平（通常表示为密度），也应用于生物地理环境（地点记录的数量） ) 与P的情况一样。 ocellatus（例如，Nithyanandan 2012，罗斯曼和加利尔 2015）。自苏伊士运河开通以来，来自地中海的记录（来自 16 个站点的 23 个）比来自更大的原生范围（来自 8 个站点的 14 个）更多，而自 1980 年代以来的水下摄影贡献了来自 14 个站点的 17 个记录。非本地范围和来自本地范围内五个站点的 10 条记录（附录 S1：表 S1）。尽管没有关于本地范围内天敌的信息，但地中海没有潜在的捕食者可能会起作用。也许对侵入性的关注鼓励了对其非本地范围内但不在其本地范围内的记录的追逐，从而导致观察者期望效应。一个类似但更近期的案例涉及微小（<15 毫米长）海蛞蝓Haminoea cyanomarginataHeller 和 Thompson，1983 年，现在同义并考虑了Lamprohaminoea ovalis的紫色变形（Pease，1868 年；参见 Oskars 和 Malaquias 2020 年）。自 1980 年被描述以来，这种微小的物种在其原生区域（红海、埃拉特湾、阿曼湾）中只有 7 个记录，而自 2005 年以来，在地中海周边地区至少有 20 个地点记录（Fernández-Vilert 等人，2018 年，Rizgalla等人，2018 年）。它的最大长度远小于P记录的 60 毫米。ocellatus (Ragkousis et al. 2020 )，它支持这样的假设，即提高认识可以刺激对引入物种的新记录的追求。九个P。 _据报道，在地中海的ocellatus目击事件来自沉船，这在休闲潜水员中很受欢迎，并且可能导致了前面提到的观察者偏见。P的饮食。ocellatus由苔藓虫组成（Yonow 2008，Rothman 和 Galil 2015），它们需要固体基质，并且沉船属于该类别，尽管尚不清楚为什么在沙滩上发现了一些个体（图 1；Yonow 2008，Rothman 和 Galil 2015）。Plocamopherus ocellatus发生在 1.5-50 m 的深度（附录 S1：表 S1）并产生附着在固体基质上的大卵带（Nicolaidou 等人，2012 年：图 19，Rothman 和 Galil2015 年：图 1C）。虽然P的生活史和传播机制。ocellatus未知，产卵和新记录表明它在地中海建立。P的增加范围和丰度。因此，地中海的ocellatus可能是事实。

另一方面，在地中海沿岸居民和游客密集的人群中，水下观察者的潜力很大。尽管无法解释缺乏来自地中海南部和埃拉特湾的记录并需要进一步调查，但P的原生范围似乎很稀有。ocellatus很可能是人工制品。