Ascophyllum nodosum

Chock & Mathieson (1979) demonstrated the physiological responses of Ascophyllum nodosum and its detached ecad scorpioides were similar under varying conditions of light intensity, temperature and salinity.

• frequent alternation of high and low salinities so a supply of freshwater is of primary importance;
• good shelter from wave action because of the unattached state of the ecad;
• absence of fast moving water, whether caused by freshwater streams or tidal conditions;
• flat, undulating or slightly sloping shore profile where stability is high, and
• substratum type, the porosity of which affects the conditions of salinity and also influences, to some extent, the development of the ecad.

• The species is very long lived and has low recruitment. Growth rate is very slow in germlings but increases as the plant ages. During the first year growth takes place at 0.2 cm per year, rising to 1.5 cm per year in the second year (Sundene, 1973). The first air bladder is formed in the fifth year, after which they are produced annually. The holdfasts of Ascophyllum nodosum are thought to persist for several decades from which new fronds regenerate.
• Growth is apical. 90% of the apical elongation takes place in the 0 to 5mm zone behind the apex. Growth rate is maximal in the morning, followed by a continuous decline throughout the day (Strömgren & Nielsen, 1986). In Strangford Lough in Northern Ireland, Stengel & Dring (1997) observed growth to be highly seasonal with low growth rates during November and December, and highest growth rates in late spring and early summer. A decline in growth in mid-summer was observed at all shore levels.
• Ascophyllum nodosum repeatedly sloughs its entire outer epidermis, a phenomenon not exhibited by other related seaweeds. (Filion-Myclebust & Norton, 1981). Despite its longevity Ascophyllum nodosum is remarkably free of epiphytes even when adjacent plants of other species of fucoid algae are heavily infested. Shedding activity appears to contribute to this difference. The authors frequently observed that when the outer layers are shed, potential epiphytes including spores and germlings of other algae that had settled on the surface were discarded with the epidermis. Only those epiphytes with deeply penetrating rhizoids, such as Polysiphonia lanosa (see below), are able to maintain a hold.
• Polysiphona lanosa is an obligate epiphyte that occurs primarily on Ascophyllum nodosum. The rhizoids of Polysiphonia lanosa penetrate the host and obtain some nutrition from Ascophyllum nodosum. However, the quantity of carbon obtained is minimal and Polysiphonia is pigmented and can photosynthesize itself (Levin & Mathieson, 1991).
• The thalli of Ascophyllum nodosum contain an endophytic fungus, the ascomycete Mycosphaerella ascophylli Cotton, that penetrates throughout the thallus (Fries, 1988). Garbary & MacDonald (1995) provided experimental evidence for an obligate mutualistic symbiosis where infected thalli were longer, had greater apical diameters and more apical hairs than non-infected thalli. Garbary & London (1995) also suggest that the fungus may protect Ascophyllum nodosum from desiccation.

• Life span.
  Ascophyllum nodosum fronds can become up to 15 years old before breakage. The holdfasts, from which new fronds regenerate, are observed to exist for much longer so whole plants may live to be several decades old. Sundene (1973) found Ascophyllum nodosum needed five years to develop into fertile plants.
• Time of first gametes.
  Mainly March and April, sometimes as early as January and February (D. Stengel pers. comm.).
• Ascophyllum nodosum is dioecious and like all other species of fucoids has only a sexual generation. Receptacles are initiated in April and may take one year to become fertile. Thus, receptacles are present on the plant for 12-14 months and ripen in April to June of the following year. Gametes are released from April onwards and the release of gametes is triggered by the exposure of ripe receptacles to air overnight. Fertilization takes place externally and zygotes settle and form a rhizoid within ten days. The receptacles are then shed during June.
• Recruitment.
  Recruitment in Ascophyllum nodosum is very poor with few germlings found on the shore. The reason for this poor recruitment is unclear, because the species invests the same high level of energy in reproduction as other fucoids and is extremely fertile every year (Printz, 1959). However, the reproductive period lasts about two months, much shorter than for other fucoids. Printz (1959) suggests that it must be assumed that some special combination of climatic or environmental conditions is needed for an effective recolonization of Ascophyllum nodosum. The slow growth rate of germlings, which increases the chance of their being covered by diatoms or grazed by Littorina, may also help to explain the scarcity of germlings (Baardseth, 1970).
• Ascophyllum nodosum var. mackaii
  In Europe, direct reproduction of the ecad mackaii is vegetative and sexual reproduction gives rise to attached Ascophyllum nodosum. The formation of receptacles on intermediate ecad stages appears to be a frequent phenomenon, although abnormalities in receptacle shape and position usually accompany this (South & Hill, 1970). Sexual reproduction of intermediate and advanced forms of ecad mackaii in Newfoundland is relatively rare (South & Hill, 1970).