Morphological and anatomical observations on the hairy eyes of males and females of the marine amphipod dulichia porrecta (crustacea, amphipoda, podoceridae)

Abstract

Male and female individuals of the Baltic Sea, stalk‐inhabiting amphipod Dulichia porrecta posses eyes that differ in size and organization. Ommatidial numbers also vary, and in adult male and female specimens amount to approximately 80 and 90, respectively. Ommatidial diameters are larger in males (40–45) μm) than in females (approx. 30 μ.m). Externally, female eyes are covered with a dense pelt of approximately 3–4 μm long and 0.01–0.15 μm thick cuticular hairs, interspersed by up to 8 μm long, filament‐containing sensilla of unknown function. An identical structure occurs in males, but there it is less frequent. Eyes of male individuals also have considerably fewer and shorter (0.5 μm) cuticular hairs.

Internally, the main difference is the somewhat broader (2.7 versus 2.4 μm) and longer (40–50 versus 32–40 μm) rhabdom in male animals and the significantly greater amount of screening pigment granules in the eyes of females. Anatomical features shared between males and females consist of bipartite crystalline cones, five equally‐contributing retinula cells and their rhabdomeres per ommatidium, a perirhabdomal space and two kinds of screening pigment granules. Ultrastructural derangements of rhabdom or microvilli following several hours of exposure to very bright spotlights are not apparent. The rhabdom length/width ratio for both male and female individuals is very high and, despite their essentially sedentary biology, allies D. porrecta most closely with pelagic amphipod species such as Hyperia and Phronima. It is concluded that from a sensory‐ecological viewpoint D. porrecta is a species that does not require vision for most of its vital functions, but that it does make use of vision in shallower water to evade predators. Males are likely to have more sensitive eyes than females, but those of the latter may posses greater resolution.

Key words:

• Vision
• Compound eye
• Photoreceptor sexual dimorphism
• Crustacea
• Amphipoda

Notes

From June 1991 on leave from the Department of Zoology, University of West Indies, Mona, Jamaica at the Regional Sophisticated Instrumentation Centre, North Eastern Hill University, Bijni Complex, Bhagyakul, Shillong ‐ 793003, Meghalaya, India.