The relationship between seaweed diet and purple ink production in Aplysia dactylomela rang, 1828 from Northeastern Brazil

Journal of Shellfisheries Research, August, 2004 by Luis Ernesto Arruda Bezerra, Ana Fontenele Urano Carvalho, Luciana Aires Barreira, Vanessa Lucia Rodrigues Nogueira, Jose Roberto Feitosa Silva, Ilka Maria Vasconselos, Vania Maria Maciel Melo

ABSTRACT Aplysia dactylomela is a large marine opisthobranch gastropod, which inhabits shallow tropical shoreline regions, eats red and green algae, and releases a purple ink when disturbed. Many functions have been proposed for this secretion and although there is no consensus on this, some of its constituents are believed to be derived from the red algal diet, it may provide the snail with a substantial survival advantage. A. californica only produces ink when it ingests red seaweeds. In some locations of the Northeastern Coast of Brazil, A. dactylomela is seen feeding only on green seaweeds, and yet it releases the ink. The aim of this work is to investigate this contradiction by studying the feeding habits of A. dactylomela and assessing the relationship between the algal diet and the purple ink. Feeding habits were investigated by field observation and by analysis of gut contents. Purple ink production was monitored by histologic analysis of the ink gland from sea hares kept in water tanks, fed with either red or green seaweeds. Composition and protein profile of the purple ink also were studied. Homologies between seaweed components and the purple ink were sought for by immunodiffusion techniques. Our findings are that the sea hare A. dactylomela, likewise other Aplysia species, needs to consume red seaweeds to be able to secrete the purple ink. The proteins of the ink seem to be synthesized by the sea hare itself and are not obtained directly from the diet, as is the case for the ink pigments.

KEY WORDS: Aplysia dactylomela, purple ink, ink gland, seaweed diet

INTRODUCTION

Aplysia dactylomela is a large marine opisthobranch gastropod which inhabits shallow tropical shoreline regions, eats red and green algae, and lays large numbers of fertilized eggs in string-like gelatinous masses close to the sea surface. It is hermaphroditic and nocturnally active, but may be exposed to sunlight as it rests during the day (Carefoot 1987). Sea hares probably are best known because of the purple ink they release when disturbed. This ink is secreted from the ink gland located on the edge of the mantle shelf, and the ability to produce purple ink is reported to be associated with a red seaweed- containing diet (Coelho et al 1998). Of the 37 species of sea hare from the Aplysia genus, 30 can secrete purple ink (Nolen et al. 1995). Many functions have been proposed for this secretion, such as camouflage, alarm signal, pheromone, aposematism (use of color patterns by prey animals to signal their distastefulness to predators), bile excretion, predator deterrent, and cue of danger (Johnson & Willows 1999). Probably the ink has more than one role in the biology of sea hares. Although there is no consensus on the biologic function of the ink, it is known that some of its constituents are derived from red algae in the diet (Prince et al 1998), and it provides the snail with a substantial survival advantage. Recent work by Prince et al. (1998) on the ink glands of A. californica has improved our understanding of ink gland structure and processing and secretion of purple ink.

In addition to studies of the ecologic and biologic aspects of chemical defense in sea hares, special attention also has been paid to the isolation and characterization of new bioactive substances from the purple ink. Thus, many bioactive substances have been isolated from the purple ink, including proteins with antibacterial activity in A. punctata (Nistratova et al. 1992), antitumor and citolytic activities in Dolabella auricularia, (Kisugi et al 1989, Yamazaki et al. 1989a) and citolytic and antibacterial activities in A. kurodai (Yamazaki et al. 1989b, Yamazaki et al. 1990). In Brazil, antibacterial and hemagglutinating activities were described for the purple ink of A. dactylomela (Melo et al. 1998; Melo et al. 2000).

On the northeastern Coast of Brazil, there is an abundance of seaweed species that support the great biodiversity of this tropical region. On some beaches there is a predominance of green seaweeds (mainly Ulva fasciata), whilst on others, red seaweeds prevail. On the beaches dominated by green seaweed, A. dactyomela is seen feeding mainly on them and yet releases the purple ink when disturbed. This fact could be intriguing, considering that other Aplysia species need to consume red seaweeds to be able to produce the ink. To solve this apparent contradiction this study investigates the feeding habits of A. dactylomela and assesses the relationship between the algal diet and the purple ink.

MATERIALS AND METHODS

Determination of Feeding Habits

Feeding habits of A. dactylomela were observed in 2 beaches of Ceara State, Northeast of Brazil, between August 2000 and July 2001. One beach was particularly rich in the green seaweed Ulva fasciata (beach 1, 38[degrees]38'48"W and 3[degrees]41'24"S); whereas, the other was densely rich in the red species, particularly Hypnea musciformis and Gracilaria spp. (beach 2,39[degrees]25'45"W and 3[degrees]22'18"S). Feeding habits of sea hare were examined monthly during daytime at low tide by observing the seaweed species consumed in the field and by dissecting under a stereomicroscope the gut contents of the sea hares and capturing the images with a digital camera (Sony-Mavica, Japan).