In a preserved state of specimens, the C. bigelowi were examined under stereomicroscope, photographed with Olympus Camera. In order to identify the captured species, the morphological characteristics depicted in detail. The identification was confirmed with regard to main features and measurements as indicated in the literature using different keys 5101516.

Through 48 catches from the northern Red Sea coast off Hurghada at 2m depth during August 2014-July 2015, six mature medusae of C. bigelowi were obtained. Of them four specimens were observed at station 3 in December 2014 and two specimens recorded at station 1 in February 2015.

Order ANTHOATHECATA Cornelius, 1992

Suborder CAPITATA Kühn, 1913

Family CORYMORPHIDAE Allman, 1872

Genus Corymorpha Maas, 1905

Corymorpha bigelowi Maas, 1905 (Figure 2a,b)

Since the erection of C. bigelowi by Maas in 1905 as Euphysora and up to the study of Sassaman and Rees 17, this genus had a complex taxonomic history wavering among Euphysora, Euphysa or Corymorpha (as Steenstrupia) 5101819. The polyp of C. bigelowi was reared in the laboratory and its nomenclature was revised on the basis of life cycle, to be a species of Corymorpha 17. Later on, the living hydroid bearing medusa buds was first collected in the field from coastal region of Akajima Island, Kerama Islands, Okinawa, Japan 20. In samples collected from the sea of Japan at high depths (2545–2555 m and 3340–3347 m), a larva in the form of young polyps (1 mm height) was identified as Euphysora bigelowi 21. All recorded medusae in the present study were adults. However, the immature medusa of this species was recorded in Japan 89.

For examined specimens in the present study, the umbrella measured up to 5.5 mm high and 2.5 mm width. The umbrella was dome shaped with apical pointed projection terminating in a patch of small papillae (Figure 2). There were four spherical radial bulbs on the bell end with four tentacles at the bell margin, one long and three short. On the tentacles: the principle one was as long as the bell and differs from others not only in size, but also in structure. It has as many as 10 (possibly nine) unilateral, adaxial nematocyst bulbs along its length and a large distinct terminal bulb. Although the remaining three tentacles were short, pointed and rudimentary, without nematocyst bulbs, the two tentacles adjacent to the principle tentacle, the lateral tentacles, were approximately twice as long as the one opposite (Figure 2a, b). Manubrium is cylindrical and approximately as long as bell cavity, with simple circular mouth. C. bigelowi feed on copepods as shown in Figure 2b.

In comparison, the morphological characteristics of C. bigelowi medusae collected from the Egyptian water of the northern Red Sea fitted with the range of variations stated in some studies and varied with others as shown in Table 1. Previous studies proved that there are three quite variable characters in C. bigelowi medusae; i.e the presence or absence of the apical canal, the relative lengths of the three short tentacles, and the relationship between bell height and the number of nematocyst bulbs on the principle tentacle. For the first distinguished character, it would be confused that specimens of C. bigelowi recorded from the Malay Archipelago 1, and from the Sunda Strait 22 included animals with and without apical canals. While other descriptions for specimens from the Indian Ocean and off Japan 1923 coincided with the Egyptian material in the complete absence of apical canals. In contrast, all reared California specimens, had well-developed apical canals 17.

Secondly, on the tentacles and the relative length, specimens from Sunda Strait showed that the opposite tentacle is shorter than the other two in small specimens (medusae height: 1.5 mm), but its relative length increases with medusa size and may eventually exceed the lateral tentacles in length (medusae height: 2.25-3 mm) 22. Whereas those noted in small specimens from Indian Ocean off Japan were of equivalent length 23, but that in the larger specimen (4 mm high) the opposite tentacle was much shorter than the lateral tentacles. In the California 17 and Egyptian specimens (present study), the opposite tentacle was substantially shorter than the other two, even in the largest medusa (>5 mm high).

Concerning the third morphological feature, most observations concluded that the number of nematocyst bulbs on the principal tentacle often increase with decreasing medusa height. For example, the highest medusa recorded from the Indian Ocean (4 mm) 23, California (5 mm) 17, and the present study (5.5 mm), had 11 nematocyst bulbs, whereas a 2.25 mm medusa from the Sunda Strait had 21 nematocyst bulbs, and one individual of 1.5 mm high had 31 22. In other study, a 3.5 mm medusa with 26 subterminal bulbs was recorded 19.

In conclusion, a common view was that the three characters do not appear to be correlated. The observations revealed that the Egyptian specimens are more similar to those from the Indian Ocean and California with regard to the lengths of the secondary tentacles and the number of nematocyst bulbs on the primary tentacle, but resemble those from the Indian Ocean off Japan in lacking the apical canal.

This species is originally described from Malay Archipelago, Indonesia 1. It is widely distributed in the Pacific and Indo-Malayan region, also in coastal areas of Japan and China. Additional Indo-Pacific materials are originated from Hong Kong 24, Sunda strait, Philippines 22, Amakusa and Tanabe Bay 2526, China 2728, and off Pacific Ocean; Palao Islands 29, Chile and Australia 3031, Indo-Malayan region 5, Kerama Islands, Okinawa 20, Nansei Islands 32. Also, it was recorded from different localities off Indian Ocean such as; Alphonse Island 23, Bombay 33, Indian coasts 3435, Arabian Sea and Bay of Bengal 36. It is extended to Gulf of Aden and southern Red Sea at Dahlak Archipelago 12.

Although C. bigelowi is a widespread species in the Indo-Pacific, and was frequently recorded from many locations in the Pacific and Indian oceans since its first record in 1905 1 up to 2006 32, its occurrence in the Red Sea was restricted to southern end. Curiously, Schmidt 12 mentioned finding specimens of Hydromedusae in the Red Sea and Gulf of Aden. He divided the area of investigation into 6 regions (Bay of Eilat (E); Gulf of Aqaba (A); northern Red Sea proper (S); southern Red Sea (R); Dahlak Archipelago (D) and Gulf of Aden (N)). He reported five specimens of C. bigelowi at five sites in the southern Red Sea (R) (16° 32ˊ N, 41° 06ˊ E) in 1958 and 1964, and one specimen in Dahlak Archipelago (D) in 1962.

Nevertheless, C. bigelowi was not hitherto recorded from the northern Red Sea whereas it was recorded in the eastern Mediterranean 37. It is worth mentioned that the presence of C. bigelowi in the Mediterranean is solely based on two preserved medusae from Israeli coast, without any figure or description 37. Thus, this finding is somewhat doubtful and needs further confirmation since the identification lacks some credibility. Additionally, a limitation of Schmidt findings from samples collected in 1968 up to hitherto, no more records for this species was appeared in the Mediterranean. Despite suggestion of transporting C. bigelowi through the Suez Canal from the Red Sea to the Mediterranean 37, we could not confirm his suggestion due to the lack of information on the presence of this species in Suez Canal and no more findings in the Mediterranean. However, this work could help to figure the route of migration via Suez Canal, in case that his presence in the Mediterranean Sea is confirmed. Further research on tracing this species, which take genetic data into account as molecular evidence, needs to be undertaken to better understand of the introduction route of C. bigelowi to the Mediterranean.

The environmental factors are driving the geographical fauna distribution. In this attempt, the temperature and salinity were assessed. In the present study, C. bigelowi was recorded in the northern Red Sea in winter (December and February) when surface water temperature and salinity were ~18.4 °C and ~40 ‰, respectively. Earlier records indicated that C. bigelowi is a high temperature species being widely distributed in equatorial and tropical areas of the Pacific, Indo-Pacific and Indian Oceans 5, giving a tolerant temperature range between 24 and 28 °C 36. Moreover, C. bigelowi is considered a euryhaline species with an optimal salinity of ~ 33‰, and a tolerate range of 31.9-36‰ 36. The presence of C. bigelowi in the temperate-subtropical conditions of the northern Red Sea could be explained according to one of two assumptions. One of these could be that C. bigelowi broadened its tolerance range for temperature to the extent that it succeeded to occur in such lower temperature than its tolerance range. The second could be related to the climate change which makes the conditions in the northern Red Sea more suitable for C. bigelowi. Finally, it could likely reflect the scarcity of knowledge of Hydrozoa in the Red Sea due to lack of study rather than absence of species.