A gymnoblastic hydroid (Cnidaria: Hydrozoa) from Egypt .

 

Presented by Dr. Fayez A. M. Shoukr, Professor of Invertebrates, Zoology Department, Faculty of Science, Tanta University, Tanta 31527, Egypt. Shoukr, F. A. and Abdel – Hamid, M. E. (1987): Morphological variations of Tubularia crocea (Agassiz, 1862) (Hydrozoa: Tubulariidae) in Egyptian harbours. Arab Gulf J. Scient. Agric. Biol. Sci., B5 (3): 455 – 462.

 

ABSTRACT. The gymnoblastic hydroid Tubularia crocea (Agassiz, 1862) was prominent among fouling communities in the Egyptian harbors at Alexandria, Port-Said and Port-Taufiq. This hydroid showed morphological variation in the number of oral and aboral tentacles, and gonophores. These variables were significantly and positively correlated with the length of the hydroid. In spite of the morphological variation of this species, its identification is reliable and there is no need to construct a new species.

Hydroids are frequently observed among the fouling communities on ship's bottoms and harbor installations in Egypt. Thirteen species of gymnoblastic and calyptoblastic hydroids have been recorded among the fouling organisms (Abdel-Hamid et al., 1984 and Shoukr et al., 1984). The gymnoblastic hydroid Tubularia crocea is widespread in Egyptian seas (Shoukr, 1982). Morphological variation in this hydroid throughout the year suggests that more than one species might be present. T. crocea, like T. individsa, develops a single polyp from one actinula larva. Therefore, it is a solitary hydroid living in clumps and not a modular (colonial) hydroid. Fenchel (1905) made a very careful study on the variability of specific criteria in the genus Tubularia. He stated that twenty two forms of Tubularia described previously as separate species were all referable to Tubularia larynx. Rees (1963) and Brinckmann-Voss (1970) considered that Tubularia polycarpa and T. mesembryanthemum were synonymous with T. crocea.

The aim of this paper was to investigate morphological variation in T. crocea in Egypt and to show that insufficient study of variation in specific characters of a hydroid may lead to errors in identification and unnecessary description of new species.

Materials and Methods

Samples of the hydroid T. crocea were collected from the Egyptian harbors at Alexandria, Port-Said and Port-Taufiq from Feb. 1980 to Jan. 1981. The hydroids were narcotized with magnesium sulphate to prevent shrinkage following fixation and preservation in 5% sea-water formalin. Clumps of this species were separated into single individuals and the stem length of the isolated hydroids were measured. The oral and aboral tentacles on 53 hydroids and the gonophores on 44 hydroids, randomly selected from Alexandria harbor, were counted and the relationship between these variables and hydroid length examined by linear and non-linear regression analysis (Spiegel, 1972; Bishop 1974 and Walpole 1976).

Results

Description of Tubularia crocea

The hydranth of Tubularia crocea has filiform tentacles arranged in two whorls, one oral and one aboral. The polyps appear as aggregate individuals forming clumps with occasionally false ramification. The perisarc of the hydrocaulus is slightly annulated at its origin from the stolon and at irregular intervals along the stem. The coenosarc forms a collar under the hydranth base. The aboral tentacles are more numerous than oral ones. The sexes are separate and gonophores grow in long unbranched racemes. The male gonophores are round or oval and devoid of apical tentacles. The female gonophores are elongate and characterized by 4 to 8 laterally compressed apical tentacles which surround the spadix (PI. I A and B).

Morphological Variables

The hydroid Tubularia crocea exhibits much morphological variation in Egyptian water. The variation included the stem length of polyps, the number of oral and aboral tentacles and the number of gonophores. The relationships between these variables, are shown in Figs. 1, 2 and 3. The equations for the regression of hydroid length (x) on tentacles and gonophores numbers (y) and of tentacles and gonophores numbers (y) on hydroid length (x) are calculated and regression lines are drawn on the figures. There is an upper limit to all variables. Oral & aboral tentacles and gonophores numbers reaching to 22, 28 and 16 respectively. The individuals length is up to 120 mm. The correlation coefficients (0.755, 0.751, 0.752 respectively) indicate that as Tubularia crocea grows, there is a linear increase in the number of tentacles and gonophores. The hypothesis that the data show a non-linear trend was tested by non-linear regression and rejected.

Tubularia crocea in Alexandria harbor are larger and have more gonophores than hydroids from Port-Said and Port-Taufiq (Table 1). These differences seem to be related to seasonal variation. The actinula larvae have 0-4 oral tentacles and 5-8 aboral tentacles. These larvae often settle on the stems and hydrorhiza of their parents and the subsequent growth produces a sort of false branching.

 

Table 1. Morphological variations of Tubularia crocea from

different Egyptian harbors.

Variables	Alexan- dria	Port- Said	Port- Taufiq	Total variation
Polyps length :	0.3 – 120	7- 46	0.7- 60	0.3 – 120
Tentacles no.: Oral	3-22	8- 16	3-20	3-22
Aboral	7-28	12-24	7-22	7-28
Gonophores no. Mature polyps length:   Immature Polyps length:   No. examined :	4- 16   15 – 120   0.3- 100       917	4-6   18- 46   7- 25       54	4-6   18- 60   0.7 – 53       58	4- 16   15 – 120   0.3 – 100       1029
Date of Collection:	Feb. 1980- Jan. 1981	Sept. 1980	Oct. 1980	Feb. 1980- Jan. 1981

N.B. All measurements are in millimeters.

Fig. 1. The relationship between the length of individuals and the

number of oral tentacles in T. crocea.

Fig. 2. The relationship between the length of individuals and

the number of aboral tentacles in T. crocea.

Fig. 3. The relationship between the length of individuals and

the number of gonophores in T. crocea.

Plate 1

A. Mature female hydranth of Tubularia crocea. X 32

a.t.. aboral tentacle; c., collar; f.g,, female gonophore; o.t., oral tentacle.

B. Female gonophore of Tubularia crocea showing laterally compressed tentacles

(longitudinal section). X 160

f.g.. female gonophore; L, tentacle.

Discussion

Tubularia crocea is a tubularian hydroid with a cosmopolitan distribution in warmer waters (Rees ,1963). There is much variation in the morphology of this hydroid in Egyptian harbors throughout the year. Many authors have been confused in their identification of hydroids for lack of detailed studies of such variation. Brink (1925) stated that the weak point of hydroid systematics was based on the fact that the study of the variability of external characters has not been taken into consideration in the discovery and description of new species. Fenchel (1905) showed that some forms of Tubularia that were described as new species (e.g. T. coronata, T. bellis, T. humilis and T. polycarpa) were in fact, Tubularia larynx. The same author pronounced that such error in identification occurred as result of insufficient notice been given to variability in specific characters. Hawes (1955) mentioned that the hydroid Tubularia bellis differs from T. larynx only in the height of the colony. Furthermore, Hughes (1983) showed that the hydroid Tubularia ceratogyne is merely a form of Tubularia indivisa.

Taxonomy of the genus Tubularia is based on colony branching, number of oral and aboral tentacles and annulations of the perisarc (Fraser, 1937). Specific characters of hydroids are subject to much variation within the same species. Many Tubularia species apparently correspond to T. crocea when morphological variations are taken into consideration.

The colonies of T. crocea in Egypt appear unbranched or with false branching resulting from settlement and growth of actinula larvae on the parent stems and hydrorhizae. In such a condition the infertile hydroid may be confused with T. larynx. The ranges in the numbers of the oral (3-22) and aboral (7-28) tentacles of T. crocea encompass those found in other species such as T. marina, T. polycarpa and T. mesembryanthemum. Other specific characters, such as the presence of a collar under the hydranth and shape of male and female gonophores, are also similar in these species. Thus, the species T. marina (described by Fraser, 1937), in addition to T. polycarpa and T. mesembryanthemum (described by Allman 1871), is synonymous with T. crocea (Rees, 1963 and Brinkmann-Voss ,1970).

It is obvious that the morphological differences in T. crocea in the Egyptian marine water reflect seasonal and ecological variation in growth and gonophores development (Abdel- Hamid et al. ,1983). With respect to the morphological variations of this hydroid, its identification is constant and does not lead to the erection of new species.

 

References

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Correspondence To :

 

Prof. Dr. Fayez Shoukr, Professor of Invertebrates, Zoology Department, Faculty of Science, Tanta University, Tanta 31527,Egypt.

 

Shoukr, F.A. and Abdel – Hamid, M. E. (1987): Morphological variations of Tubularia crocea (Agassiz, 1862) (Hydrozoa: Tubulariidae) in Egyptian harbours. Arab Gulf J. Scient. Agric. Biol. Sci., B5 (3): 455 – 462.

 

 

E-mail: fayez_shoukr@hotmail.com

 

Web site: http://www.fayezshoukr.5u.com/