锯缘青蟹嗅觉器官的保护性形态特征

MORPHOLOGICAL CHARACTERISTICS FACILITATING PROTECTION OF THE OLFACTORY ORGAN OF THE MUD CRAB (SCYLLA SERRATA)

用光镜和扫描电镜对锯缘青蟹嗅觉器官保护性的形态学结构进行了研究。内鞭基部具一个半圆锥形凹面 ,它使内、外鞭间形成约 4 0°的基本夹角。化感刚毛与着生面呈 30°～ 5 0°角 ,其小于两鞭间 4 0°～ 90°的可变张角。这种空间关系能有效地避免内鞭折断化感刚毛的可能 ,因而内鞭在化感刚毛前方形成了天然保护屏障。化感刚毛良好的柔韧性、基部的念珠状结构以及中部分节等特征可能在避免自身折断或变形方面具有作用。另外 ,外鞭顶段迎水面的杆状刚毛和刺状刚毛、内鞭迎水面上以及底节远侧端的刺状刚毛所处的位置 ,均是化感刚毛的重要门户 ,因此推测这些触觉性的刚毛可能有利于化感刚毛避开来自上。

A large amount of evidence has proven that antennules in decapod crustaceans are an important olfactory organ. However, reports on defense problem of the aesthetasc scesilla in the antennules being vulnerable to damage themselves are very rare. In this paper we report on morphological characteristics facilitating protection of aesthetascs in the antennules of the mud crab, Scylla serrata, which we observed with light and scanning electron microscopy (HITACH TSM T300). Our results showed that, the antennules of S. serrata were symmetrical in shape and location, and had no significant sexual dimorphism (Table 1). The mean weight of an antennule (left antennules removed at the subcoxa; 0 0103±0 0012 g, n =21) was only the 9×10 -5 of the average weight of the intact animal (114 4123±16 9153 g, n =21). An antennule of S. serrata consisted of three segments, which were subcoxa, coxa and basis, the most distal bearing the inner and outer flagella (Plate Ⅰ:1). All of 600～900 aesthetascs adhered to an area of only 0 4 mm 2 approximately on the outer flagellum of the antennule (Plate Ⅰ:1～2, 5～6). Each aesthetasc was approximately 8μm in diameter and about 500～800 μm in length (Plate Ⅰ:1～2) and had considerable mechanical flexibility. The basal section of each aesthetasc had 4～5 expansible parts (Plate Ⅱ:7～8), and the middle section had some annuli (Plate Ⅱ:9). The aesthetasc had an angle of about 30°～50 ° with its adhered surface of the outer flagellum in distally (Plate Ⅰ:1～4). Examination of the aesthetasc tufts of all animals showed them to be almost free from any floccules or breakages. Our results revealed also that, the basal segment of the inner flagellum had a half tapered concavity (Plate Ⅰ:1, Ⅱ:11) which caused there to be a basic angle of about 40° between the inner and the outer flagella. Since the outer flagellum could move independently the angle between the inner and outer flagella varied from 40°to 90°. The results show that each outer flagellum bore some rod shape setae at the distal edge of the tuft, and that distal segments had some thorn shape setae and 2～3 apical setae on the outer flagellum (Plate Ⅰ:3～4). In addition, some thorn shape setae also occur on the inner flagellum (Plate Ⅰ:1, Ⅱ:11) and on the distal region of the basal segment of the antennule (PlateⅡ:12). How do the aesthetasc tufts of the antennules of \%S.serrta\% maintain clean or intact? We believe this can be deduced from certain morphological features. One of these is, as previously mentioned, the concavity of the inner flagellum which results in a basic angle of about 40 between the inner and the outer flagella that can be increased to 90°under natural conditions. The second is the angle of inclination of 30°～ 50° between the each of aesthetascs and their adhering surface, towards the outer flagellum distally, and their inclination is in the same direction as the inner flagellum. Therefore, the angle of inclination of each aesthetasc is obviously less than that of the two flagella. Thus it is possible that the space created between the aesthetascs and the inner and outer flagella can avoid damage of aesthetascs from the inner flagellum. Moreover, the inner flagellum is anterior to the outer flagellum during antennular flicking. On account of these features, we propose that the inner flagellum constitutes a natural defense for the aesthetascs of the outer flagellum in S. serrata . In addition, the basic expansible parts and the middle annuli of the each of aesthetascs, and the pliability of aesthetascs permit bending of the aesthetascs without breakage or distortion of their cylindrical form. Besides the above morphological characteristics, rod shape setae, thorn shape setae and apical setae on the antennules may play a role in the protection of aesthetascs against floccules. These setae are presumed to be tactile, and their spacial configuration is such as to form a protective barrier around each aesthetasc tuft. The above observations and analysi